Troubleshooting the Fuel System

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Table of Contents

1. INTRODUCTION ...... 1 2. MAINTENANCE ...... 2 3. INITIAL TROUBLE SHOOTING...... 10 4. TROUBLESHOOTING THE IGNITION SYSTEM ...... 17 5. TROUBLESHOOTING THE FUEL SYSTEM...... 24 6. FAILURE ANALYSIS...... 28

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INTRODUCTION

INTRODUCTION Fasteners: 1. SHOP HANDBOOK INTENT • Most of the fasteners used on the engine are sized in metric. Some are fractional inches. For This Handbook is intended to provide service dealers this reason, wrench sizes are frequently identi- with an introduction to sound and proven troubleshoot- fied in the text, and measurements are given in ing and diagnostic procedures. U.S. and metric scales. Disclaimer: This handbook was written using a proto- • If a fastener has a locking feature that has worn, type engine. The information contained in this hand- replace the fastener or apply a small amount of book is correct at the time of writing. Both the product releasable thread locking compound such as and the information about the product are subject to Loctite® 242 (blue). change without notice. • Some fasteners like cotter pins are single-use About the text format: items that are not to be reused. NOTE: is used to point-out information that is Other fasteners such as lock washers, retaining relevant to the procedure, but does not fit as a rings, and internal cotter pins (hairpin clips) may step in the procedure. be reused if they do not show signs of wear or damage. This manual leaves that decision to CAUTION: is used to point-out potential danger the judgement of the technician. to the technician, operator, bystanders, or surrounding property. Assembly: Torque specifications may be noted in the part of the • Bullet points: indicate sub-steps or points. text that covers assembly, they may also be summa- Disclaimer: This handbook is intended for use by rized in tables along with special instructions regarding trained, professional technicians. locking or lubrication. Whichever method is more appropriate will be used. In many cases, both will be • Common sense in operation and safety is used so that the manual is handy as a quick-reference assumed. guide as well as a step-by-step procedure guide that • In no event shall MTD or Cub Cadet be liable for does not require the user to hunt for information. for poor text interpretation, or poor execution of The level of assembly instructions provided will be the procedures described in the text. determined by the complexity and of reassembly, and • If the person using this handbook is uncomfort- by the potential for unsafe conditions to arise from mis- able with any procedures they encounter, they takes made in assembly. should seek the help of a qualified technician. Some instructions may refer to other parts of the manual for subsidiary procedures. This avoids repeating the same procedure two or three times in the manual.

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The specifics in this manual are aimed at the MTD 2. Wear rate will vary somewhat with severity of engine however the contents can be used for most out- use. If the edges of the center electrode are door power equipment. rounded-off, or any other apparent wear / damage occurs, replace the spark plug before oper- As the saying goes “an ounce of prevention is worth a ating failure (no start) occurs. pound of cure” the same can be said about preventive maintenance on outdoor power equipment. By chang- 3. Cleaning the spark plug: ing the spark plug, air filter, and oil in annual intervals NOTE: It is not recommended to clean spark many failures can be avoided. Sometimes just clearing plugs. Use of a wire brush may leave metal off yard debris that was collected while in use can deposits on the insulator that causes the spark make the difference between a properly running piece plug to short out and fail to spark. Use of abra- of equipment or a failure. sive blast for cleaning may cause damage to Spark plugs ceramic insulator or leave blast media in the recesses of the spark plug. When the media 1. The spark plug used in the MTD engine is a comes loose during engine operation, severe Torch model F7RTC gapped to .024”-.032” (.60- and non-warrantable engine damage may result. .80 mm). See Figure 2.1. 4. Inspection of the spark plug can provide indications of the operating condition of the engine. • Light tan colored deposits on insulator and electrodes is normal. • Dry, black deposits on the insulator and electrodes indicate an over-rich fuel / air mixture (too much fuel or not enough air) • Wet, black deposits on the insulator and electrodes indicate the presence of oil in the combustion chamber. • Heat damaged (melted electrodes / cracked insulator / metal transfer deposits) may indicate detonation. • A spark plug that is wet with fuel indicates that Figure 2.1 fuel is present in the combustion chamber, but it is not being ignited. NOTE: Champion RN14YC or NGK BPR4ES are physically similar but may not match the F7RTC in heat range. This difference in heat ranges will effect performance and emissions. It is recommended that only the torch F7RTC plug be used in MTD engines.

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Air filters 4. Typically an air filter should be changed before every season. Generally air filters come in two different types, a pleated-paper element, or a foam plastic, sometimes a 5. If an air cleaner is dirty and not in to bad of con- combination of the two will be used like the one on the dition it can be cleaned and reused. The paper MTD engine. See Figure 2.2. pleated filters can be shaken or lightly tapped to free the debris from the filter. NOTE: Never use compressed air on a paper air filter. Compressed air will remove the tiny fibers Paper-Pleated that are used to catch the dirt in the air. Without Element these fibers the filter is useless. 6. Foam filters can be washer in warm soapy water. NOTE: When drying a foam filter either squeeze in side of a paper towel or let it air dry DO NOT wring because the filter will tear. 7. Before installing any foam filter, after it has been washed, it needs to be free of moisture. Foam Pre-Filter NOTE: Always check with factory specification prior to servicing/replacing any engine compo- Figure 2.2 nents.

1. The main function of the air filter is to trap air Oil type and capacity borne particles before they reach the carburetor 1. When checking the oil twist and remove the dip- that can cause catastrophic internal engine dam- stick from the engine. Clean the oil off of the tip age. of the dipstick. Re-insert the dipstick without 2. Air filters used on the MTD engine are designed threading it in to get the oil level reading. to prevent particles larger than 3-5 micron from See Figure 2.4. passing through into the engine. 3. The filter should be checked on a regular basis possibly several times in a season. See Figure 2.3.

Figure 2.4

• SAE 10W-30 oil with a SF/CD API rating is recommended for most operating conditions up 97° F(36° C.). Figure 2.3 • The oil capacity is 17.0- 20.3 fl.oz (0.5-0.6 liters).

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2. The oil level is determined by the lowest point on 1. Oil can be drained by removing the drain plug the dipstick that is completely covered with oil. located at the base of the filler tube / dipstick tube, using a 10mm wrench. See Figure 2.5. 3. If the oil is noticeable thin, or smells of gasoline, a carburetor repair may be needed before the engine can be safely run. • Check the oil level frequently and change the oil more frequently in severe operating conditions

such as high ambient temperature, dusty conditions, or high load use in exceptionally thick grass. • Synthetic oil is a suitable alternative, but it does not extend service intervals. Sealing Washer NOTE: It is recommended to use petroleum oil during the break in period to ensure the piston rings actually break in. • Synthetic vs. Petroleum based oil: To simply look at synthetic oil and to compare it with Petroleum based oil there is very little difference. However, Figure 2.5 when you look at the two through a microscope it is easy to see the difference. Synthetic is made NOTE: Replace the drain plug sealing washer up of smaller molecules which allows the oil to with a new one to ensure that it does not leak. get into areas that petroleum based oil cannot. NOTE: Tighten the drain plug to a torque of 84 • No oil additives or viscosity modifiers are recom- in. lbs. (10Nm) on installation. mended. The performance of a good oil meeting 2. An alternate (recommended) method for drain- the SF/ CD specifications will not be improved ing the mower engine oil would be to tip the by the addition of any oil additives. mower on its side, dipstick tube down and dip- NOTE: Some oil additives may cause severe stick removed, draining the oil into a waste oil and non warrantable engine damage, constitut- pan. ing a lubrication failure. 2a. Disconnect the spark plug lead and ground Changing the oil it to the engine block. NOTE: If the engine has been running allow the engine to cool before doing any maintenance work.

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2b. Lean the mower to the muffler side of the 1. Some of the early engines were equipped with a engine. See Figure 2.6. fuel filter placed between the fuel tank and the carburetor. See Figure 2.7.

Fuel Filter

Figure 2.6 Figure 2.7 Fuel 2. Currently the engines are being equipped with a 1. Use clean, fresh fuel with a pump octane rating fuel filter installed in the fuel tank where the feed of 87 or greater. line connects. See Figure 2.8. • Stale or out-of-date fuel is the leading cause of hard starting issues. • Pump octane ratings beyond 87 will not improve engine performance. Fuel filters Dirty fuel can clog the carburetor and introduce abrasive materials into the engine. To help prevent that, the MTD engine is equipped with a fuel filter.

Figure 2.8

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Valve lash 6. Confirm that the piston is at Top-Dead-Center on the compression stroke. See Figure 2.10. Valve lash can be checked and adjusted using the following steps: 1. If the engine has been run, allow it to cool thoroughly. Position the mower for easy access to the cylinder head. 2. Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole. 3. Remove the spark plug using a 13/16” or 21mm wrench. A flexible coupling or “wobbly” exten- Valves closed sion may help. See Figure 2.9. (push rods slack) Probe to confirm piston is at top of travel Spark plug hole (plug removed) Figure 2.10

• TDC can be identified using the probe. The keyway in the PTO end of the crankshaft also corre- sponds with the crank pin (and piston) position. • The compression stroke can be distinguished from the overlap stroke by the presence of air Valve cover pressure at the spark plug hole and the fact that High tension lead neither of the valves should move significantly Muffler on the compression stroke. • There is an automatic compression release Figure 2.9 mechanism that “bumps” the exhaust valve as the piston rises on the compression stroke. At 4. Remove the four bolts that secure the valve TDC, the exhaust valve should be fully closed. cover using a 10mm wrench, and remove the 7. Check valve lash between each valve stem and valve cover from the engine. rocker arm using a feeler gauge. NOTE: If care is used not to damage the valve See Figure 2.11. cover gasket, it can be re-used. Setting exhaust valve lash 5. Secure the safety bale with a spring clamp, and slowly pull the starter rope until air can be heard being expelled from the spark plug hole.

Figure 2.11

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8. Intake valve lash (top valve) should be .003”- 16. Install the spark plug. .005” (.10 + .02mm). 17. Release the spring clamp securing the safety 9. Exhaust valve lash (bottom valve) should be bail, start the engine and test run it long enough .005-.007” (.15 + .02mm). to confirm correct operation. 10. Use a 10mm wrench to loosen the jam nut, and Exhaust system a 14mm wrench to adjust the rocker arm fulcrum The exhaust system is a frequently overlooked compo- nut. See Figure 2.12. nent of an engine. it is important to make sure the muffle is in good condition and free of debris and/or insects. NOTE: A blocked muffler will result in poor performance. If a muffler is completely blocked the engine may not start. Cleaning the engine 1. To maintain a proper operating temperature and to keep the equipment looking good all debris should be removed from the engine. 2. It is recommended to use compressed air to blow all of the debris off of the engine. Setting intake valve lash NOTE: A pressure washer may be used to clean outdoor power equipment but only after the unit Figure 2.12 has been allowed to properly cool. NOTE: Debris can build up under the deck and • Tighten the rocker arm fulcrum nut to close-up cause the engine to operate under an unin- the clearance between the end of the valve stem tended load. and the contact point on the rocker arm. Blades • Loosen the rocker arm fulcrum nut to open-up the clearance between the end of the valve stem A sharp, balanced, and well secured blade is impor- and the contact point on the rocker arm. tant. 11. Hold the fulcrum nut with a 14mm wrench, NOTE: The blade also acts as a counter weight tighten the jam nut to a torque of 88.5 in-lb. for the engine. An engine should never be (10Nm) using a 10mm wrench. started without a balanced blade firmly attached. 12. Double-check the clearance after tightening the 1. A dull blade will bludgeon the grass rather than jam nut, to confirm that it did not shift. Re-adjust cutting it. if necessary. • This results in poor cut quality because the 13. Rotate the engine through several compression grass is torn rather than cut, and the additional cycles: drag can pull-down engine RPMs when the mower is near the limits of its capacity. • Observe the movement of the valve gear. • A dull blade makes the engine work harder, • Return the piston to TDC compression stroke increasing operating temperature and fuel con- and re-check the valve lash to confirm consistent sumption, while decreasing engine life. movement of the valve gear, including the slight bump to the exhaust valve from the automatic • A dull blade, in combination with other abuse compression release. issues may constitute abuse, voiding the war- ranty. 14. Clean-up any oil around the valve cover open- ing, clean the valve cover, replace the valve 2. An imbalanced or bent blade (sometimes cover gasket if necessary. accompanied by a bent crankshaft) creates vibration. 15. Install the valve cover, tightening the valve cover screws to a torque of 62 - 80 in-lbs (7-9 Nm).

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• This vibration makes the mower unpleasant to 5. When it is necessary to tilt the mower, tilt it so use and will damage the mower. that the carburetor is up. When tilting to the side, this will place the dipstick toward the ground. • Typical complaints include parts falling-off. The mower may also be positioned on the bench • In extreme cases the engine crankcase or as shown below, if the fuel tank is empty or the mower deck may crack or separate from each fuel cap is sealed. See Figure 2.13. other. CAUTION: Continued operation with a damaged blade or crankshaft will create safety issues.

CAUTION: Under NO CIRCUMSTANCES should a blade or crankshaft be straightened and re-used. Any dealer or operator who does Empty fuel so is subjecting themselves to extreme risk of tank, or seal fuel cap with injury and liability. plastic wrap between cap NOTE: Impact damage to the blade and/or and threads crankshaft is not warrantable except as a victim part of a specific failure of the damaged mower. Examples would be a baffle or axle from the damaged mower coming loose and contacting the blade. Figure 2.13 NOTE: Secondary vibration damage is not warrantable. A customer who continues to operate CAUTION: Disconnect the high tension lead a mower with a damaged blade or crankshaft from the spark plug and ground the lead before does so at their own risk. doing any work that exposes the blade. 3. Impact damage to the blade or crankshaft is often accompanied by impact damage to the fly- 6. If the blade is removed for any reason, make a wheel or key. Flywheel and flywheel key damage visual inspection of all blade mounting hardware. are not warrantable, and will be considered See Figure 2.14. abuse unless there is very specific evidence of a defect in material or workmanship on the part of MTD. 4. Use of non-OEM blades, blade adaptors, and Blade hardware is not recommended for MTD mowers. Belleville blade support Blade Blade bolt adaptor

Figure 2.14

CAUTION: At no time should any part of the body be placed in the path that the blade would follow if it were to rotate.

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7. When servicing the blade: 7a. Replace any parts that are damaged or suspect. 7b. Use a commercially available blade stop tool or a firmly positioned length of dimen- sional lumber to keep the blade from rotating during removal and installation. 7c. Tighten the blade bolt to a torque of 450-600 in-lbs. (51-68 Nm). 7d. Test run the mower in a safe area before returning it to service. NOTE: The rotating mass of the blade is important to the smooth operation of the engine. CAUTION: The engine will be difficult or impossible to start with the blade removed, and the resulting “kick-back” through the starter rope may cause painful hand injuries. Do not attempt to start the mower without a properly attached blade.

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INITIAL TROUBLE SHOOTING

Troubleshooting an engine is an art form that is built If the engine runs, but runs poorly start by performing upon several factors. First and most importantly is a the routine maintenance described in chapter 2. good understanding of how the engine works. The sec- Most gasoline engine diagnosis involves isolating prob- ond is skills that are honed by experience. Finally the lems in the four critical factors an engine needs to run use of visual observations and a structured, system- properly: atic approach to diagnosing a problem. • Ignition- sufficient spark to start combustion in When approaching an engine to troubleshoot it, keep the cylinder, occurring at the right time. your eyes open and keep your mind clear. What does this mean, well if there is a puddle of oil under the • Compression- enough pressure in the cylinder to engine their is no need to change the filters and spark convert combustion into kinetic motion. It also plug to get it running. address the oil issue first. Also be needs sufficient sealing to generate the vacuum aware of tunnel vision. Tunnel vision is when the mind needed to draw in and atomize the next intake is focused on one thing. A narrow focus can find one charge. problem, but maybe there are more than one problem. •Fuel- correct type and grade of fresh gasoline; in A narrow focus allows for other problems to be over- sufficient quantity, atomized (tiny droplets) and in look. An example could be focusing on a sheared fly- correct fuel/air proportions. wheel key and overlooking the cracked fins on the cylinder. The two are unrelated, but both require atten- •Flow- if all of the above conditions are met, but tion. the flow of air is constricted on the inlet or exhaust side it will cause the engine to run The first step in troubleshooting is to confirm the prob- poorly or not at all. This also includes the ensur- lem: ing the valves are timed to open at the proper • Does it start? Bad fuel, low compression, no time. spark, etc... 1. To isolate the ignition system and compression • Is it a hard start? Bad fuel, choke, fouled spark from the fuel system: plugs, etc... 1a. Prime the engine through the carburetor • Is the rope hard to pull? Engine seized, broken throat using a squirt bottle, filled with clean starter, etc... fresh gasoline. • Does the rope jerk back as you pull it? Timing, NOTE: Inspect the air filter while priming the bent crank shaft, etc... engine. Look for a dirty or plugged filter that could prevent air flow or a missing filter that • Does the engine surge when the engine is run- would indicate dirt ingestion. ning? Carburetor issue, governor, etc... 1b. Pull the safety bail back. • Does the engine lack power or running at a low speed? Bad fuel, carburetor, blocked muffler, 1c. Attempt to start the engine. etc... 1d. If the engine starts and runs long enough to Use these questions to narrow down causes of the burn the prime, the problem is effectively engine problem. isolated to the fuel system. proceed to the Troubleshooting the Fuel System section of CAUTION: The first rule in diagnosing is to this manual. cause no further harm to the engine and prevent injuries. Always make sure to check the oil for 1e. Check ignition system as described in the level and condition before starting an engine. Troubleshooting the Ignition System section Also check blade and/or attachments for dam- of this manual. age and make sure they are firmly mounted. 1f. If the ignition system is working, check the compression.

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INITIAL TROUBLE SHOOTING

2. To perform a compression test: 2f. Confirm that the gauge is “zeroed”, then hold the safety bail and pull the starter rope NOTE: Compression should be in the range of repeatedly, until the needle on the gauge 45-95 PSI (3.1-6.5 Bar). has risen as far as it is going to. 2a. If the engine has been run, allow it to cool See Figure 3.2. thoroughly. 2b. Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole. See Figure 3.1. Valve cover

Compression gauge Reading ~ 90 PSI Spark plug hole (plug removed) Figure 3.2

Muffle r High tension lead Interpreting compression readings: • Near Zero (< 20PSI [1.38 Bar]): most likely a Figure 3.1 stuck valve or too-tight valve lash, provided starter rope pulls with normal effort. 2c. Remove the spark plug using a 13/16” or 21mm wrench. A flexible coupling or “wob- • Moderately low (20-45 PSI [1.38-3.1Bar]): Valve bly” extension may help. seat damage or piston ring wear. Leak-down test or compressed-air test will help confirm if dam- 2d. Hold the safety bail and pull the starter rope age is isolated to valves or piston rings. several times to purge any fuel or oil from Oil smoke in exhaust on throttle increase tends the combustion chamber. to indicate piston ring wear. Oil smoke in NOTE: Air compresses readily, liquid does not. exhaust on over-run tends to indicate valve Liquid in the combustion chamber will result in guide wear. an artificially high compression reading. • Normal compression (45-95 PSI [3.1-6.5 Bar]) 2e. Install a compression gauge in the spark would indicate an ignition issue. Refer to the plug hole. chapter on ignition systems. • Too high compression (>95 PSI [>6.5 Bar]) most likely indicates excessive valve lash, negating the automatic compression release. It may also indicate a partial hydraulic lock or severe carbon deposits within the cylinder.

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INITIAL TROUBLE SHOOTING

PCV testing 2. When functioning properly, the PCV valve (Posi- tive Crankcase Ventilation) works with the inher- The PCV valve is located under the flywheel and ent pumping action of the piston in the bore to allows the crankcase pressure to escape. expel extra pressure from the crankcase. Leakage and blockage are the two failure modes for a See Figure 3.4. PCV system. Either mode will cause crankcase pressure to build-up, though the effects of a blocked PCV are generally more dramatic. Increased case pressure will result in oil entering the combustion chamber. 1. The PCV chamber is vented to the air filter through a molded rubber hose. The rubber hose directs crankcase fumes to a covered duct within the air filter housing. See Figure 3.3.

Figure 3.4

• Normally, small engines run with slightly nega- tive case pressure. • This case pressure can be measured using a slack-tube water manometer, or an electronic version of the same tool.

Figure 3.3 • Less than (between zero and -1”) ( -2.54cm) of water is a typical reading. NOTE: An adaptor can easily be made form an old or extra dipstick. See Figure 3.5.

Electronic manometer: Adaptor made by drilling a hole in a dipstick / oil cap

Figure 3.5

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3. An engine that fails to purge extra case pressure 7. The cover and gasket can be separated from the in a controlled manner will build case pressure. chamber. See Figure 3.7. The pressure will find it’s own way out of the engine in undesirable ways. • Oil will be forced by the rings and valve guides, being burnt in the combustion chamber. • The cause of this oil burning can be mistaken for Cover a worn-out engine, if proper diagnosis (compression, leak-down, and case pressure) is not per- Gasket formed. Baffle 4. Experimentation by MTD’s Training and Educa- tion Department has revealed the following char- Disc acteristics of MTD engines: • A leaky PCV system will not build-up substantial case pressure. • A leaky PCV system will allow the engine to ingest contaminants through the system, accel- Figure 3.7 erating engine wear. NOTE: The dimple in the cover helps locate the • A blocked PCV system will allow crankcase fiber disc over the port that leads into the crank- pressure to build very rapidly. Noticeable oil case. fumes will be evident in the exhaust within several minutes of normal operation. NOTE: the baffle in the cover helps separate the oil from the air in the chamber. It is desirable to 5. The PCV chamber is accessible by removing the allow the air out, but important to keep as much flywheel, as described in the IGNITION SYS- oil as possible in the engine. TEM chapter of this manual. See Figure 3.6. 8. The disc acts as a check valve: pressure pulses Dimple force it off of the port, but it falls back over the port when the pressure drops. PCV chamber 9. The folded wire mesh in the chamber also helps cover separate the oil from the air. See Figure 3.8.

Disc

Port to crankcase

PCV tube Port to PCV hose

Figure 3.6

Folded wire mesh 6. Remove the two screws that hold the PCV chamber cover to the engine block using a 10mm wrench. Figure 3.8

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10. The port to the PCV hose is near the top of the 12. With the cover re-installed and torqued to 27-35 chamber. The oil tends to settle out of suspen- in-lbs. (3-4 Nm), vigorous pumping with a hand sion, leaving mostly air to exit the chamber pump vacuum / pressure tester will result in a through the PCV hose. slight build-up of pressure within the chamber. See Figure 3.10. 11. The screen accumulates droplets of oil, which eventually drip down to the bottom of the chamber. Beneath the screen is a drain-back port, leading to the crankcase. the size of the port is small enough that significant pressure does not flow through it. See Figure 3.9.

Figure 3.10 Drain-back port • This 2-3 PSI (.14-.20 Bar) build-up will dissipate through the drain-back port over the course of 5- 10 seconds. Figure 3.9 • No vacuum should accumulate in the chamber

unless the drain-back port is blocked and the disc is not moving from over the crankcase port.

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Starter rope stops pulling 2c. To isolate the starter from the engine, with the spark plug removed turn the blade. A hard to pull starter rope is a fairly easy thing to diagnose, but is often misdiagnosed. There are a few rea- 2d. If the blade turns easily, the starter is the sons why a starter rope is hard to pull, don’t assume cause of the hard pull. the engine is seized without troubleshooting it first. 2e. If the blade does not turn, there is an inter- Trouble shooting a hard to pull starter rope. nal engine failure. Further examination of the engine will be required. 1. Check for hydraulic lock Starter rope is hard to pull (jerky) NOTE: Hydraulic lock or hydro-lock as it is more commonly called, it the collection of liquids in the A hard, jerky pull could be a couple of things, timing or combustion chamber. compression release. 1a. Remove the spark plug and pull the starter 1. Check the valve lash. rope. NOTE: A pull that gets hard then releases could 1b. If the rope can now be pulled, there was a be in the compression releases. In order to hydro lock in the engine. Start the engine make the engine easier to pull start a compres- and let it run. It will smoke until all the sion release (ACR) is employed in the MTD excess fluids are burned away. engines. The ACR is on the cam shaft and “bumps” the exhaust valve to bleed off compres- NOTE: Possible causes of hydro lock include: sion and automatically releases when the engine tipping the engine, overfilling the engine, leaking starts. If the valve lash is too loose, the exhaust carburetor or possibly bad piston rings. vale will not bleed off compression . NOTE: If an engine has been submerged, 2. Inspect the flywheel. change the fuel, oil and all filters. If the engine will start after that, start it and let it run to burn NOTE: If the rope is jerked away violently, that out the rest of the contaminates. Change the oil generally indicates timing. Inspect the flywheel every fifteen minutes or so, until all signs of con- for a sheared flywheel key. tamination are gone. 3. If the two above conditions were ok, then open 2. Turn the blade. the engine and look at the ACR on the cam shaft. See Figure 3.12. 2a. Check for and remove any material that maybe binding the blade. 2b. Check for any material that could be trapped in the blade brake that could be binding the flywheel. See Figure 3.11.

ACR bump Flyweight

Exhaust lobe

Look for debris trapped between Figure 3.12 the brake and the flywheel NOTE: If the flyweight is stuck in the out position, there will be no compression release. If the Figure 3.11 flyweight is stuck in he retracted position, the engine will have no power.

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INITIAL TROUBLE SHOOTING

4. Check the cam timing. NOTE: There are timing marks on the crank shaft and the cam. when the crank shaft is at TDC, the cam should be at TDC of the compression stroke as noted by having the intake and the exhaust lobes equally away from the tap- pets. See Figure 3.13.

Timing marks

Figure 3.13

NOTE: The cam gear may be a polymer gear. Use the same procedure for both. See Figure 3.14.

Polymer cam

Timing Marks

Figure 3.14

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TROUBLESHOOTING THE IGNITION SYSTEM

The purpose of the ignition system is to provide a spark Inspection of the spark plug in the combustion chamber at the proper time to ignite Inspection of the spark plug can provide indications of the fuel/air mixture. The first step in troubleshooting the the operating condition of the engine. ignition system is to examine the spark plug(s). • Light tan colored deposits on insulator and elec- About the spark plug: trodes is normal. • The spark plug is a Torch model F7RTC, gapped • Dry, black deposits on the insulator and elec- to .024”-.032” (.60-.80 mm). trodes indicate an over-rich fuel / air mixture (too • Champion RN14YC or NGK BPR4ES are physi- much fuel or not enough air) cally similar but may not match the F7RTC in • Wet, black deposits on the insulator and elec- heat range. This difference in heat ranges will trodes indicate the presence of oil in the com- effect performance and emissions. It is recom- bustion chamber. mended that the Torch F7RTC plug be used for service. • Heat damaged (melted electrodes / cracked insulator / metal transfer deposits) may indicate • Wear rate will vary somewhat with severity of detonation. use. If the edges of the center electrode are rounded-off, or any other apparent wear / dam- • A spark plug that is wet with fuel indicates that age occurs, replace the spark plug before oper- fuel is present in the combustion chamber, but it ating failure (no start) occurs. is not being ignited. Cleaning the spark plug: 1. Test the ignition system: • Cleaning the spark plug is not recommended. If After examining the spark plug reinstall it or a new one the plug needs to be cleaned, replace it. to ensure a good spark plug is being used. Now the ignition can be tested to verify spark is getting to the • Use of a wire brush may leave metal deposits on spark plug. To test the system: the insulator that cause the spark plug to short- out and fail to spark. 1a. Disconnect the spark plug wire. • Use of abrasive blast for cleaning may damage 1b. Connect a spark tester to the spark plug the ceramic insulator or leave blast media in the wire. recesses of the spark plug. When the media comes loose during engine operation, severe and non-warrantable engine damage may result.

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TROUBLESHOOTING THE IGNITION SYSTEM

1c. Connect the other end of the spark tester to To test the stop switch: the engine block. See Figure 4.1. 2a. locate the terminal that connects the stop switch wire to the primary windings of the ignition module. See Figure 4.2.

Spark tester Stop switch Wire to Figure 4.1 ignition model

CAUTION: Never remove the spark plug and Figure 4.2 hold it against the cylinder head to test for spark. The fuel/air mix coming out of the spark plug 2b. Connect an Ohm meter between the termi- hole will catch on fire. nal and a ground point. The reading should approach zero when the bail is released, NOTE: The spark should be a minimum of 10 Kv closing the contacts. See Figure 4.3. (10,000 volts) at pull over speed. 1d. Squeeze the safety bail and pull the starter Ground connection rope. Watch the spark tester. If sparks are Safety bail in seen in the spark tester, the ignition system OFF position is working. Replace the spark plug. 1e. If no sparks are seen in the spark tester further testing is required. 2. Test the stop switch: The stop switch for the MTD engine is located on the engine brake. It is designed to ground out the module when the brake is applied. Stop switch connection

Figure 4.3

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TROUBLESHOOTING THE IGNITION SYSTEM

2e. If further investigation is required, remove the recoil assembly. See Figure 4.5. NOTE: If the reading is high, the contacts may be burnt or there is a bad ground to the switch. Fan shroud This could prevent the engine from shutting down rapidly. Mounting posts for 2c. The reading should be high when the bail is engine cover pulled down, reflecting the resistance in the primary windings of the ignition module. See Figure 4.4.

Ground connection Safety bail in RUN position Studs for recoil assembly

Stop switch Figure 4.5 connection 2f. Lift the fan shroud off of the studs that locate it.

2g. Visually trace the wire from the stop switch to the spade terminal on the module, and inspect the wire for any damaged insulation Figure 4.4 or potential contact with ground. 2h. Unplug the wire from the spade terminal on NOTE: If the reading is low, the module is the module, and check continuity to ground shorted to ground and may not produce a spark. from the female spade terminal on the wire. The wire will need to be traced back to the mod- Resistance (Ohms) should be zero with the ule to find the short. bail released. See Figure 4.6. 2d. Alternatively, a jumper wire could be connected to the same locations. Use a commercially available spark checker to see if the ignition is working or not. • If the jumper disables the ignition, but releasing the bail does not, the problem lies in the switch. • If the jumper does not disable the ignition, then the wire that connects the switch to the ignition may have a fault, or the ignition module itself may be faulty. Further investigation is required. Safety bail in OFF • If the problem is a lack of spark when the bail is position, Module primary windings pressed against the upper handlebar, disconnect are grounded the wire from the switch using a 7mm wrench. Isolate the wire from incidental contact with ground, and test the ignition. Figure 4.6 If it fails to spark, the wire may be shorted or the ignition may be at fault. Further investigation is required.

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TROUBLESHOOTING THE IGNITION SYSTEM

2i. Resistance (Ohms) should be infinite (O.L) 3a. Normal performance of the coil is to produce with the bail released. See Figure 4.7. at least 10,000 volts at starter-rope pull- through speed. See Figure 4.8.

Safety bail in RUN position, Module primary windings Inexpensive spark gap tester not grounded provides usable information

Figure 4.7 Figure 4.8 2j. If the switch and wire work properly, and the connection is good at the spade terminal, 3b. Presence or absence of strong spark, with but releasing the bail fails to stop the the stop switch and wire known to be good, engine, then the problem lies within the is generally enough to identify the ignition module. coil as good or bad. Resistance readings may help confirm the source of the failure, 3. The module but are generally unnecessary. The ignition system is a capacitive discharge magneto, NOTE: Presence of a weak spark maybe the contained in a single module. result of an improper air gap. The air gap should • The magneto is a three leg design. be .008”-.016” (.2-.4mm). • The magneto is energized by the passing of a 3c. It is possible for the transistorized portion of pair of magnets mounted in the flywheel. the module to fail, yet resistance tests will show the windings to be good. A simple • Ignition timing is set by the location of the fly- spark-gap tester is will give an adequate wheel in relation to the crankshaft. Proper timing indication of the ignition system’s condition. is maintained by a steel key.

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TROUBLESHOOTING THE IGNITION SYSTEM

3d. Simple spark-testers are readily available 3f. At pull-over speed (~ 600 RPM), voltage and inexpensive. Thexton Part # 404 is should be at least 10,000V. available form a variety of retailers, and See Figure 4.11. similar units are available form other manu- facturers. See Figure 4.9. Pull-over speed: waveform is half-way between 2nd and 3rd reticle (10,000 V.)

Instructions on back of package

Figure 4.11 Figure 4.9 NOTE: Flash-over voltage will vary with spark plug condition and gap. 3e. At operating speed, the ignition should pro- NOTE: Pull-over speed may vary from operator duce voltage approaching 12,000. to operator. See Figure 4.10. 3g. Resistance in the primary windings of the 1 reticle = 4,000 Volts ignition module, measured between the waveform is spade terminal and the laminations, was 3 reticles tall observed to be in the 550-650 Ω range. See Figure 4.12.

Figure 4.10 Probe to laminations

Probe to stop switch spade terminal Figure 4.12

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TROUBLESHOOTING THE IGNITION SYSTEM

3h. Resistance in the secondary windings of the ignition module, measured between the NOTE: An inexpensive compass or bar magnet spark plug terminal and the laminations, can be used to confirm opposite polarity of the was observed to be in the 8K-9KΩ range. flywheel magnets. See Figure 4.14. See Figure 4.13.

Probe to spark Opposite poles of bar magnets plug terminal are attracted to opposite pole flywheel magnets for the sake Probe to laminations of illustration

Figure 4.13 Figure 4.14

4. Inspect the flywheel. NOTE: There may be slight variation in specification due to production variation and other fac- The flywheel is a frequently forgotten component of the tors such as temperature. ignition system. It holds the magnets that induce a field in the module which in turn produces a spark. But it • Resistance figures that are vastly lower may also controls the timing of the ignition system by con- indicate a short in the windings being tested. trolling when the magnets are introduced to the mod- • Resistance figures that are vastly higher (or O.L) ule. may indicate a fault in the windings being tested. A sheared flywheel key will throw off the ignition timing. NOTE: Intermittent failure requires tests for volt- To inspect the flywheel and key: age and resistance to be made when the engine 4a. Remove the recoil assembly. is cold, and again when it is hot. Typical customer complaint: “It stops after I mow for 10 min- 4b. Lift the fan shroud off of the three studs that utes and I can’t get it to re-start”. locate it. See Figure 4.15. • To confirm that the problem is ignition-based, it Fan shroud is necessary to “catch it in the act”. • Resistance normally increases slightly as temperature increases. NOTE: Failure of the magnets in the flywheel is exceedingly rare. To test the magnets, simply hold an item made of ferrous metal roughly 1/4” Shoulder (.635cm) away from the magnets in the flywheel. bushings It should be drawn to the flywheel. A wrench or screwdriver is suitable for this test.

Shoulder bushings fit over studs

Figure 4.15

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TROUBLESHOOTING THE IGNITION SYSTEM

4c. To remove the flywheel, remove the flywheel 4f. On installation, confirm that the key is prop- nut using a 19mm wrench. See Figure 4.16. erly seated in the key-way, and that the tapers are fully seated. Key or keyway failure may result from improper seating. IMPORTANT: The flywheel and the tapper on the crank shaft must be clean and dry. The flywheel is held in place by the friction fit between the flywheel and the crank shaft, not the key. The key is only to guide the flywheel to the proper position until it is torqued down. 4g. Install the flywheel nut to a torque of 47-52 ft-lbs. (64-70 Nm). 4h. Use a 10mm wrench to adjust the air gap between the module and the flywheel. NOTE: The air gap should be .008”-.016” (.2- .4mm). Figure 4.16

4d. Hold the safety bail down using a spring clamp, and remove the flywheel using an appropriate puller. CAUTION: If the flywheel shows any signs of physical damage such as cracks, broken vanes, or damaged key-way, replace it. A damaged flywheel poses a threat of burst failure. Burst failures are extremely hazardous to surrounding people and property.

4e. Inspect the key, keyway, and tapered mating surfaces of the flywheel and crankshaft. See Figure 4.17.

Key

Taper

Figure 4.17

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TROUBLESHOOTING THE FUEL SYSTEM

The function of the fuel system is to store, mix the fuel 3. Get a sample of fuel from the carburetor and with air and deliver it to the combustion chamber. The pour it on the section of the paper labeled sam- fuel system consists of the following components: ple from carburetor. See Figure 5.1. • Fuel tank • Fuel lines • Fuel filter • Carburetor Inspecting the fuel: NOTE: The fuel is the maintenance item most often overlooked by consumers. A lot of fuel systems problems are caused by bad gas. When inspecting the fuel: • Look for water. • Look for dirt. • Look for discoloration. Figure 5.1 • Sniff carefully to see if it smells like varnish or kerosene. 4. Place a sample of gas/oil mix where it says gas/ • Save the fuel to show to customer. oil mix, straight gas with straight gas and straight oil with straight oil. See Figure 5.2. • Look for oil in the fuel. • Test the fuel for alcohol content if there is a reason to suspect it. NOTE: Save a sample of the fuel collected to show the customer. NOTE: Customers pouring engine oil into the fuel tank seems to be a growing problem. 1. Get a plain piece of paper. 2. Divide it into four sections and label them: • Plain gas • Gas/oil mix • Gas from carburetor • Straight oil Figure 5.2

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TROUBLESHOOTING THE FUEL SYSTEM

5. Match the sample from the carburetor to the There are several alcohol test kit available commer- other three samples. This will show if there is oil cially. See Figure 5.4. in the fuel or not. See Figure 5.3.

Figure 5.4 Figure 5.3 Generally these kits involve mixing a measured amount Test fuel for alcohol: of water and gas together and seeing were the boundary layer is. See Figure 5.5. Gasolines, currently on the market are not pure gas. Today’s fuels have alcohol and other additives in them to reduce emissions. The fuel make up can vary sea- sonally and geographically. It is hard to find a good quality fuel and unfortunately the fuel has to be pur- chased before the quality is known. Alcohol in fuel creates a lot of problems for gasoline engines. The biggest problem is that alcohol attracts and holds water. This corrodes the metal components of the fuel system, especially the carburetor. Alcohol also does not produce as much heat as gasoline when burnt. This results in less power for the engine. A 10% alcohol mix or a 15% of MTBE is acceptable for MTD engines. Anything higher than that will result in performance issues.

NOTE: E85 fuels are not to be used in any MTD Figure 5.5 engines. The test kit should come with a chart to compare the boundary layer height to alcohol percentage.

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TROUBLESHOOTING THE FUEL SYSTEM

Fuel tank vent 2. The tank should not hold any pressure nor any vacuum. The fuel tank vent preforms the important task of allowing air into the fuel tank as fuel is being used. As fuel is • Replace the cap if either pressure or vacuum being used by the engine, the fuel level in the tank builds using the hand-pump tester. drops. The dropping fuel level then creates a vacuum • A cap that maintains pressure will cause the in the tank. If the fuel tank could not suck air through engine to run rich as the fuel in the tank heats the vent, the vacuum would prevent the fuel from get- and expands, forcing it’s way past the float valve ting to the carburetor. in the carburetor. The tank is vented through the cap. See Figure 5.6. • A cap that maintains vacuum will cause the engine to run lean as the fuel is depleted and no Tank air comes in to replace it. Cap liner / gasket • The two conditions may both be present, but the symptoms vary with fuel, fuel level, and operat- Filter ing conditions. Cap • Usually presents as a “Runs and quits” scenario. Chokes All MTD engine are equipped with a manual choke to help start the engine. The choke must be closed to start the engine and opened when the engine starts. This can be a source of starting issues with customers

who are not familiar with manual chokes. Sleeve (mtg. hole) Hose barb The choke is operated by a pull knob on the handle Figure 5.6 bar. If the choke plate fails to close fully when the knob is pulled, the mower will be difficult or impossible to To test the cap vent start when cold. 1. A hand-pumped vacuum / pressure tester may NOTE: The rod connecting the choke lever to be connected to the fuel barb (after draining). the choke arm on the carburetor can be bent See Figure 5.7. slightly to facilitate adjustment. See Figure 5.8.

Hand-pump vacuum / pressure tester connected to fuel line Choke rod

Figure 5.8 Figure 5.7

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TROUBLESHOOTING THE FUEL SYSTEM

Inspect the fuel filter: Inspecting the carburetor: The fuel filter is located in the fuel tank. It can be 1. Remove the float bowl and check for dirt and/or removed and cleaned with a can of carb cleaner or varnish. replaced. See Figure 5.9. 2. Inspect the needle valve and needle valve seat for dirt and/or damage. 3. Inspect the gaskets and O-rings for damage. 4. Inspect the vents and orifices, verify that they are free of debris. 5. Check the height of the float, adjust as necessary. NOTE: If a little cleaning and new gaskets will fix the carburetor, do it. If the carburetor requires extensive cleaning it is better to replace the carburetor. IMPORTANT: Never try to mechanically clean orifices. That will damage them and ruin the carburetor. Figure 5.9

Inspect the fuel lines: • Are they cracked? • Are they clogged? • Are they brittle? NOTE: If the answer to any of the above is yes, replace the fuel lines. Carburetors Troubleshooting the carburetor is a process of elimina- tion. If everything else on the engine checks out, the carburetor is probably bad. NOTE: It is important to perform a compression test before condemning a carburetor. An engine can have a borderline compression reading and not create enough of a vacuum to draw in a sufficient fuel/air charge. NOTE: If the engine has border-line compression, a quick test to see if that is the problem is to remove the spark plug. Squirt a little bit of oil into the combustion chamber to seal the rings. Reinstall the spark plug. If the engine starts and runs ok, the that was the problem. If it does not start, move on to the carburetor.

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FAILURE ANALYSIS

A properly maintained engine will provide years of ser- 1. Abrasive particles that enter the engine through vice. Occasionally an engine will fail. An important part the intake system can be sand or dirt. of working on engines is finding out why they failed. See Figure 6.1. Was it something the customer did? Was it a manufacturing defect? Did the engine just wear out? All of these questions need to be answered when a failed engine is found. Engines can fail in a variety of ways but most failures can be classified in the following categories: • Abrasive ingestion • Insufficient lubrication • Over heating • Over speed • Mechanical breakage/ wear NOTE: There may be a combination of failures.

Finding the cause of an engine failure requires the Figure 6.1 complete disassembly of an engine and careful examination of the parts. 2. An abrasive particle can enter the engine by With a good understanding of how the engine works, bypassing an improperly installed air filter or close examination of the parts and experience, an through leaks in the intake system. Usually there understanding of why the engine failed can be will be tracking marks were the particles enter reached. the system. Use these marks to find the source of the abrasives. Abrasive Ingestion NOTE: Dirt can also work its way through a Abrasive Ingestion is when hard particles are intro- poorly maintained air filter. See Figure 6.2. duced into the engine. Particles can be introduced into the engine by leaks in the air intake system, through a dirty oil fill plug or by particles of metal that wore off of a part, especially during the break in cycle. Particles may also be introduced through worn or improperly installed seals or gaskets.

Figure 6.2

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FAILURE ANALYSIS

3. Particles that enter the intake system travel at Other sources of abrasives that get into the engine great speed and act like sand blasting media includes carbon that builds up on the top side of the inside the engine. This causes wear to the parts piston, metal shavings from the wear of engine parts or affected. dirt entering through the oil fill port. leaking gaskets and seals also have the potential of allowing debris to enter NOTE: Choke and throttle shafts are very vul- the engine. nerable to this wear. If an air filter becomes clogged, the vacuum produced by the engine will A symptom of abrasive ingestion is smoky exhaust. As try to draw air in by any means possible. This the cylinder walls wear pressure from the combustion usually happens around the throttle and choke chamber blows by the piston and pressurizes the shafts. Because the throttle shaft moves more engine sump. This pushes oil past the piston rings and than the choke, it will wear faster. into the combustion chamber where it is burnt. See Figure 6.4. 4. The particles can pass through the intake system to the valves and valve seats. 5. When particles enter the combustion chamber, the up and down motion of the piston grinds the particles into the side of the cylinder walls and damages the cylinder wall, piston and piston rings 6. This can be identified by the scoring along the vertical axis of the piston and cylinder wall or the cross hatch on the cylinder wall being worn off. NOTE: To help in the lubrication of the cylinder walls, and help with the seating of the piston rings, a diamond cross hatch is honed into the cylinder wall. Debris entering the cylinder will polish the cross hatch off of the cylinder wall. See Figure 6.3. Figure 6.4

7. Abrasive materials that enter the engine get absorbed by the oil and thickens it. Cross hatch polish off See Figure 6.5.

Figure 6.3

NOTE: Abrasives that enter the engine through the intake system will cause the upper portion of the combustion chamber to wear more than the Figure 6.5 lower portion. Measurements of the cylinder bore at the top and bottom will show this.

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FAILURE ANALYSIS

8. Because the oil absorbs the abrasive particles, Insufficient lubrication the engine components that are immersed in oil The bearing surfaces in an engine are not smooth. As will show definite signs of abrasive ingestion a result of the machining processes to make the engine especially around the connecting rod and main parts, there are little peaks and valleys that are only bearing journals. See Figure 6.6. visible on a microscopic scale. These peaks are called asperities. As the engine breaks in, the asperities break off leaving plateaus that become the bearing surface. The valleys become reservoirs for the lubricant. When the parts are at rest, they push the lubricant or oil away resting on the bearing surfaces. As the parts rotate, they climb over the oil, pulling the oil between the bearing and the part, riding on a film of oil. When there is insufficient lubrication, the parts make contact with the bearing surfaces while in motion. This creates friction which creates heat. That will lead to the asperities welding to the moving part, which leads to more friction and starts all over again. As more asperities weld to the moving part, they remove material from the bearing, which looks like gouging or galling.

Figure 6.6 Insufficient lubrication can be a result of several factors: not enough oil in the engine, contaminates in the NOTE: Abrasives that are trapped in the oil will oil, wrong oil viscosity, oil break down from age, break- cause the lower portion of the combustion cham- down of the additives in the oil or oil breakdown from ber to wearing more than the upper portion. heat. NOTE: Wear of only one bearing surface on a When examining an engine, metal to metal transfer is a new engine could be a sign of a manufacturing sure sign of insufficient lubrication. Metal to metal defect. transfer only occurs when there is a lack of lubrication. See Figure 6.8. NOTE: Abrasive particles can also be imbedded into materials that are softer than the abrasive. Discoloration This will cause the affected part to act like a piece of sand paper or a grinding wheel. See Figure 6.7.

Metal to metal Imbedded abrasives transfer

Figure 6.8

An important thing to note is that just because there are signs of insufficient lubrication, that does not mean that was the cause of the failure. It may only be a symptom Figure 6.7 of the real cause of the failure.

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FAILURE ANALYSIS

Larger size abrasive particles can render the lubricants Engine Overspeed ineffective, leading to an engine failure. An overheated The MTD engine is designed for a maximum speed of engine can cause the oil to break down leading to a 3300 rpm. When the governor is unable to control the failure. In an engine overspeed, the oil is pushed away engine rpm the engine can accelerate past the safe from the bearing surface leading to a failure. maximum speed. In all three of the above cases, the signs of insufficient When an engine runs beyond its designed speed a lubrication are symptoms not the cause. There will also couple of things happen. The first is vibrations develop. be signs of heat or discoloration around the parts All engines have vibrations and are designed to handle affected by the lack of lubrication. See Figure 6.9. those vibrations, but in overspeed the vibrations change resonance. Parts that can not handle the new resonance will crack. This may result in parts flying off of the engine which is an unsafe condition.

Discoloration The vibration can also lead to fasteners loosing up. Evidence of this could be elongated mounting holes. The area around the mounting holes may be polished due to the two surfaces rubbing against each other. See Figure 6.10.

Elongated mounting hole

Figure 6.9

Figure 6.10

The other part of an overspeed condition is the internal failures. Overspeed failure can be challenging to figure out due to the multiple failures involved. When an engine overspeeds, the oil on the bearing surfaces is flung away allowing metal to metal contact. This will show signs of inadequate lubrication. Another sign of an oversped engine is a broken connecting rod. As the piston moves up and down in the cylinder it builds momentum. The higher the rpm’s the more momentum produced by the pistons. As the momentum builds, the connecting rods will start to stretch. When the connecting rods stretch, they get weaker. Generally speaking this is at the narrowest part of the connecting rods. On most engine that would be about an inch below the wrist pin, but on the MTD engine it is at the wrist pin.

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FAILURE ANALYSIS

The force on the connecting rod is greatest when the Overheated piston transitions from the upward stroke to the down- The MTD engines are air cooled engines. Because of ward stroke. Because of this, most overspeed connect- this cleanliness of the engine is very important to the ing rod failures will occur with the piston at top dead life of the engine. Dirt, grass and sludge all form an center. insulating layer on the engine. This will trap the heat in When a connecting rod fails, the piston stops moving the engine and cause it to over heat. but the crankshaft is still moving. This will allow the bro- A sign of an overheated engine is discoloration. The ken connecting rod to get knocked around in the cylin- discoloration is usually due to material being baked into der causing more damage to it. Usually the connecting the metal. On the outside of the engine that could be rod will be in several pieces after it breaks making it dirt or grass. As the metal heats up it cooks down the hard to find where the first failure was. debris on the engine and the pores of the metal open When trying to diagnose an overspeed failure, look at up. this allows the cooked down material to soak into all the pieces. Individually the lack of lubrication, piston the metal discoloring it. position and condition of the connection rod will usually Inside the engine the metal will soak up some of the oil, indicate separate failures. Collectively they would indi- turning the metal parts a dark color. See Figure 6.11. cate an overspeed failure.

Discolored rockers

Figure 6.11

Another sign of an overheat failure is warped parts. As metal parts heat up, they expand. In an engine a cer- tain amount of expansion is expected. Engines are built so that when parts are at operating temperature, the parts will expand to be within the tolerances needed for the engine to run. A problem occurs when the parts are over heated. They expand more than they were designed to. Some parts are mounted firmly, like cylinder heads (the hottest part of the engine). As they try to expand, they fight against the head bolts. The head bolts will not move to allow the expansion so the head warps to allow the expansion.

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FAILURE ANALYSIS

This warping of the head allows the head gasket to Mechanical Breakage/ Wear leak. A leaking head gasket allows the compressed Sometimes an engine fails because a part breaks. gases in the engine to escape, lowering the compres- There are generally three causes of a broken part, out- sion in the engine and hurting engine performance. As side of the previously discussed engine failures. They the cylinder head cools, it shrinks back down to its nor- are abuse, wear, and manufacturing defects. mal size, but there will still be some warpage of the head. See Figure 6.12. A very common way to abuse an engine is a bent crank shaft. Crank shafts bend when they, or something bolted to them hits something. A prime example of this is when a mower blade hits a rock. See Figure 6.13.

Bent blade

Figure 6.12

Figure 6.13

As the engine runs, there is friction between the moving parts. This friction wears down the parts. Lubrica- tion slows the process, but wear can not be prevented. Over time the parts wear to the point they break or fail in some way. Car tires are a good example of wear. A tire will only last for so many miles before all the rubber is worn off and the tire goes flat. Bushings are another example, they are designed to wear so that the wear of other parts will be minimized. Manufacturing defects are wrongly blamed for alot of failed parts. A manufacturing defect is when a part is made wrong. It could be a porous casting, parts assembled wrong, the wrong parts used or so on. A manufacturing defect will generally show up within the first couple of hours of use.

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