sign in sign up
<<
Home , Head gasket, Manifold vacuum, MAP sensor, Valvetrain

WRENCHIN’ @ RANDOM YOUR GAUGE IS YOUR FRIEND Two Essential Diagnostic Tools No Hot Rodder Should Be Without, and How to Use Them Marlan Davis hI’ve been answering read- ers’ Pit Stop tech questions for decades, explaining how to improve performance, troubleshoot pesky problems, or recommend a better combina- tion. Yet rarely do any of these problem- solving requests include information on the problem combo’s vacuum reading. That’s unfor- tunate, as [Above: Two essential diagnostic tools no hot rodder should be with- vacuum out, from left: a Mityvac handheld can tell vacuum pump for testing vacuum you a heck of a lot about an consumers (some models will even engine’s condition, without the aid in bleeding), and a large, easy-to-read vacuum gauge like need to invest in a bunch of this one by OTC (this model also high-tech diagnostic tools. includes a pressure gauge for even So what’s the deal on more test possibilities). vacuum? Consider an internal- [Left: Knowing how to use a combustion engine as basically vacuum gauge is the key to a giant air pump that operates diagnosing many performance under the principles of pres- problems. It aids in tuning your sure differential. The difference motor to the tip of the pyramid. It even helps diagnose problems not between normal atmospheric seemingly engine-related, such as pressure (14.7 psi at sea level a weak power-brake system. Add at standard temperature and one to your toolbox today. pressure) and how hard this “pump” sucks under various engine-management system). that needs vacuum that—at that vacuum comes from the operating conditions and states If present, the least on the surface—seemingly engine. If an engine is down on of tune creates vacuum (usually vacuum advance system also doesn’t tie directly into engine vacuum, these devices may not expressed in inches of mer- depends on vacuum. Many performance. These include function properly; conversely, cury, or in-hg). The amount of emission-control devices— vacuum consumers like the if the vacuum consumers have vacuum in the induction tract at including the EGR valve, evapo- power-brake vacuum booster, a problem, it creates a vacuum various operating points is used rative control system (ECS) automatic- vacuum leak, degrading proper engine to properly meter fuel (through canister purge, and PCV valve— modulator, and classic-era operation. old analog devices like the - likewise rely on consistent and HVAC (heating, ventilation, and Erratic or low-vacuum gauge buretor, or via a MAP sensor, correct engine vacuum. air conditioning) controls. All readings indicate there’s a real as part of a modern electronic Wait: There’s even more stuff need vacuum to operate, and problem with the engine or the “Vacuum is everything from a tuner’s standpoint. It’s the key to virtually every component.” — Norm Rollings

74 HOTROD.COM/2019/AUGUST/ WRENCHIN’ @ RANDOM THE NUMBERS ADJACENT TO SOME OF THE BOXES BELOW CORRESPOND TO THE PHOTOS ON PAGE 78. Steady needle at idle. Find the I. ABNORMAL BUT STEADY Reads steady vacuum READING AT IDLE low or steady high leak. compared to nominal. various interconnected vacuum your tune-up, check vacuum- devices. On an old-school powered external device opera- 6 Steady but low Steady but high motor, you can also gauge the tion, or begin homing in on a efficiency of the tune-up by the possible internal engine issue, Spray carb cleaner around the -manifold–to–- engine’s vacuum output. When the Rollings tell us you can head mounting surfaces and the –to–intake-manifold 1 it comes to high performance start with just two inexpensive mounting surface. Adjust carburetor idle fuel Reading of over 18 in-hg, There is a restriction in the air specifically, understanding your devices: a vacuum gauge and mixture screws to obtain especially on a hot rod, is cleaner. Replace air cleaner or highest vacuum reading. generally way too high. install larger air cleaner. engine’s vacuum characteristics a handheld vacuum pump. If aids in diagnosing mysterious 2 Idle mixture problems—like a power-brake screws Idle mixture screws do not respond to adjustment; idle mixture quality (vacuum and setup that somehow can’t stop respond to rpm) bad. Place hand or shop rag over the carburetor air horn. the car. adjustment To get the skinny on vacuum, we visited Rollings Automotive, Idle rpm momentarily speeds up. Replace the a family-owned, full-service faulty . Some carb mounting flanges Idle rpm lack material for getting a good seal. A metal does not automotive shop that works on momentarily 3 anything with wheels. Whether plate-type might be the solution here. Quadrajet carbs are known to sometimes increase. Vacuum Some improvement, Car stalls Car you need to get a handle on leak through a loose or worn primary increases to but vacuum is still out. Spray- continues shaft. Shaft bushing kits are available (requires the normal lower than desired. out air running. complete carburetor disassembly). range for Check . bleeds with You have your : Add more initial lead carburetor a vacuum Stop, you’ve to achieve highest cleaner. leak. 5 7 Engines with found the vacuum, then back off Vacuum is now a separate problem. about ½ in-hg. Check idle circuit. On standard carbs Engines without a separate valley cover. within acceptable valley cover: There can still be range. Stop, the with the usual two idle mixture Idle screws screws, a big may result in a leak out the bottom (blind problem is solved, Idle screws side) of the intake into the but you may also now respond don’t unresponsive mixture screws and/ respond, or require turning in the idle speed engine valley. Pull the PCV need to refine the to adjustment, valve out and cap the breather centrifugal advance vacuum improves. but car does screw too far. Adjust the secondary not smell stop screw to increase rear blade side of the system. Spray carb Vacuum is curve amount Stop, you’ve found cleaner through the hole into and rate if you’ve the problem. rich. angle, then back off the primary- still low but side idle screw on the throttle shaft. the . steady. added in a bunch more initial timing (see “How Much Idle screws don’t respond to adjustment. Car also Motor speeds up Motor does not speed up Advance Do You 8 Need?” sidebar on smells rich, sounds loaded up, and/or belches black page 84). exhaust smoke at idle. Disconnect all The intake gasket vacuum hoses and Idle mixture Idle mixture screws remain is leaking out the plug all vacuum screws can now nonresponsive. The carb in its bottom. Replace. ports, or clamp Jumped or be adjusted. Stop, present state won’t work with all vacuum hoses Valve lifters incorrectly you’ve found the the rest of the combo. You shut. adjusted too tight. installed timing problem. have several choices. chain. Fix and repair as needed. Remove and inspect power valve (PV) located in the primary metering block on a Holley-style carburetor. Adjustable Net-lash Motor does not Motor speeds up. valvetrain speed up. There Reconnect each Check the Wrong PV Blown (ruptured) PV 3 3 may be a serious vacuum hose one at a last-connected Drill a ⁄32- to ⁄16- Have a specialty Get the proper internal engine time until motor slows vacuum hose for inch hole in both carburetor shop high-end carb for back down. primary throttle trick-out the the application. At issue. cracks and holes. Your Holley-style blades. existing carb. a minimum, a high- carb’s power 4 end carb (such as a Holley HP or Ultra) Properly Determine and obtain the valve (PV) should open at half the If PV is correct for should have a four- adjust lifters. proper-length pushrod. the application, corner idle system engine’s normal Bad hose found. idle vacuum check to see if and replaceable air it both holds bleeds. Replace with number. For new hose. example, if a vacuum and opens big-cam engine when vacuum idles at 13 inches, drops to the value use a No. 6.5 PV. stamped on the valve. If it doesn’t, Otherwise, the Original hose checked None of the above fully resolves the issue. You may have a PV may open replace the PV. Vacuum not system-wide internal engine issue. Hold engine speed steady fully improved: good and/or vacuum prematurely and still doesn’t improve at 2,500 rpm for 15 seconds, then rapidly release the throttle. flood the motor. Continue testing Vacuum should momentarily increase a couple of inches. If it remaining vacuum after replacing hose. doesn’t increase or pulls down to zero, you’re looking at worn circuits as neces- Test vacuum consumer rings, cylinders, or valve seats; confirm with a cylinder sary one by one the hose connects to cranking compression test. Also see diagnostic tree II until issue is found using handheld vacuum on pages 80–81 and tree III on page 82. and resolved. pump. Replace with Still not right? new unit if device won’t Check the ignition. hold vacuum.

76 HOTROD.COM/2019/AUGUST/ HOTROD.COM/2019/AUGUST/ 77 WRENCHIN’ @ RANDOM

THE BOX NUMBERS ABOVE THE PHOTOS CORRESPOND TO THE DIAGNOSTIC TREE I BOXES ON PAGES 76–77. See box 1 See box 2 See box 3 Norm Brandes

A B B A

[If the idle mixture screws don’t [Sometimes just spraying the air bleeds with carb cleaner helps free up respond, place a shop towel or idle system response. High-end Holley-style carbs like this Quick Fuel unit your hand over the carb’s air horn. have replaceable air bleeds (idle, A; high-speed, B) plus a “four-corner” Normally, this action should cause idle system that facilitates tuning for big in radical engines [Adjust the idle mixture screws the engine to immediately stall compared to old-school “two-corner”-idle systems with fixed bleeds. to obtain the highest possible out. If it keeps running, there’s a vacuum reading. Turn the curb idle vacuum leak somewhere. speed screw slightly clockwise only as necessary to prevent the car See box 5 See box 5 from stalling out while doing this. A 15 in-hg gauge reading here isn’t too bad for a hot rod, but be sure the power-brake vacuum booster can get the job done at that level. (Hint: don’t rely on a minibooster.)

See box 4 A B

A [At idle, a Holley primary-side idle transfer slot (A) should be exposed just 0.020 to 0.030 inch. If a big cam requires turning the idle screw on the throttle linkage too far “in” to achieve stable idle, too much of [Cure for excessive Holley carb primary idle-transfer slot exposure: Turn the slot is exposed, deactivating in the secondary idle stop screw located on the bottom-right side of the fuel flow through the idle discharge baseplate about 0.015 inch in from its lightly bottomed position. This hole (B) and prematurely activat- cracks open the secondary blades slightly farther at idle, which permits ing the main circuit. Result: An backing off the primary curb-idle screw to reduce slot exposure and restore over-rich, low-vacuum condition. idle mixture screw authority.

See box 6 See box 7 See box 8 B Amazon.com

[Most Holley carbs made after 1992 have built-in power valve (PV) blowout protection, but those on older Holley models sometimes rupture from an engine [To check for vacuum leaks, Roll- [There can be a hard-to-find [Not leaking through the gaskets? backfire. This results in a super- ings sprays the intake- leak out the bottom of Plug or pinch off all vacuum ports rich, blubbery engine at idle and off and carb-gasket surfaces with the intake into the valley area. and hoses; rpm should speed up. idle—as well as low vacuum. Test carb cleaner (he says it’s slightly Rollings’ solution: Pull out the Reconnect the hoses one at a time with the handheld vacuum pump. safer and less volatile than brake PCV valve and cap off the system’s until the engine slows back down— If you’re careful, its cup adapter cleaner). If rpm momentarily breather side. Spray carb cleaner you’ve found the leak. If the motor sort of works (A); better is the increases, you’ve found the down the open PCV hole. If rpm didn’t speed up, there may be a pump connected to Moroso’s vacuum-leak source. momentarily increases, you’ve serious internal engine issue. PV tester (PN 62295, B), about found the leak. $45 through Amazon.com. you really get into it, you might REALISTIC Rollings says. “It’s the first basic ing your vacuum gauge to a want to invest a little more on a step for us when a car comes in full-time vacuum port on the cylinder-compression tester and VACUUM FOR with an issue. It tells us a story. If intake manifold. Traditionally, maybe an electrical multimeter, HOT RODS vacuum is low, we need to find with the car warmed up, at idle but we’ll leave those tools for “People need to use a vacuum out why and fix the problem.” you want to see vacuum read- another time. gauge on just about anything,” Begin diagnostics by connect- ings of 18 to 22 in-hg from sea

78 HOTROD.COM/2019/AUGUST/ WRENCHIN’ @ RANDOM Irregular or Steady wavering pattern: high/low needle indicates a problem swing. Regular repeating affecting pattern: indicates a more than one II. FLUCTUATING POINTER problem affecting only cylinder or area. AT IDLE one cylinder, two adjacent cylinders, and/or is concentrated in one area.

Electrical system Engine problem. Engine problem. problem. Electrical system problem.

Faulty distributor Spark timing Cracked centrifugal too advanced Sticky, burnt, or Blown head (s) More than one Multiple Multiple Spark plugs or carbon- advance or retarded (see leaky intake valve, gasket, blown in more intake valve fouled spark bad plug gapped too tracked mechanism “How Much Fouled spark Bad spark-plug Cracked or damaged , likely on than one seat burnt or plugs. wires. wide; gap to distributor and/or excessive Timing Do You plug. wire. carbon-tracked or poorly seated adjacent location. not seating. spec. cap. distributor shaft Need?” Sidebar distributor cap. intake valve. cylinders. wear. on page 84).

None of Try pouring a little the above Try pouring a little auto-trans fluid auto-trans fluid None of the solves the above solves the down carburetor Look for water Check for water down carburetor problem. and see if there’s problem. You have You may in the oil (or in oil (and vice and see if there’s any improvement. any improvement. an internal engine have an vice versa). versa). problem. engine Carburetor problem. out of adjustment. Perform cylinder compression test to identify the problem cylinder(s). Pull head for further Adjust idle mixture diagnostics. screws to highest vacuum and Float level stabilize idle speed too high; level to 1,000 feet. But that’s wants to see at least 16 in-hg at “normal” readings will decrease your hot rod requires an aux- screw to best adjust float with a stocker and a mild cam. sea level, or he says, “We’ll have about 1 in-hg for every 1,000 iliary vacuum-assist device to quality idle (see also level. Diagnostic Tree I, Realistically, hot rods with a to compensate in some way.” feet of elevation gain above ensure reliable and repeatable pages 76–77). performance cam will be lower, Other sources maintain power 1,000 feet. For example, at 5,000 power-brake operation. sometimes as low as 10 to 11 work OK with as little as feet, normal vacuum for a given in-hg (not great if you want to 14 in-hg, but you better know combo will be down about ABNORMAL drive your car daily or go on how to tune the entire combo to 4 in-hg. But that still doesn’t Power Tour®). the peak of the pyramid. change that threshold for proper VACUUM AT IDLE For a street-driven hot rod Engine vacuum is established vacuum device operation. Liv- Under an initial hot-idle with power brakes and other relative to atmospheric pres- ing in mile-high Denver, for vacuum test, the vacuum-gauge vacuum consumers, Rollings sure, so as altitude increases, instance, makes it more likely needle should hold steady None of the above solves the A B C problem. Check for an electrical PORTED problem.

PCV

USUALLY USUALLY MANIFOLD PORTED MANIFOLD MANIFOLD PORTED

[Connect your vacuum gauge to a full-time manifold vacuum port on the intake or at the carburetor. A carb’s multiple vacuum ports may reference either [If you’ve checked everything else and the engine still doesn’t respond manifold (full-time) or ported vacuum. Ported will have zero or near-zero vacuum at idle, which you don’t want for a basic vacuum test; confirm if in properly, it’s finally time to get out the big gun: perform a cylinder doubt. These photos show common vacuum port locations on a typical Holley (A), Edelbrock (B), or Quadrajet (C). Be careful on the Q-jet; GM switched compression check. ported/manifold vacuum functionality, depending on model or year.

80 HOTROD.COM/2019/AUGUST/ HOTROD.COM/2019/AUGUST/ 81 WRENCHIN’ @ RANDOM

III. IRREGULAR WAVERING Progressively POINTER AT SPEED increase amount of throttle opening and observe results.

Pointer becomes more stable as Pointer range of Pointer range of Vacuum initially is engine speed travel increases travel decreases but normal but drops increases. along with engine wavers more rapidly drastically as engine speed. The amount as engine speed speed increases. Check of swing can be any- increases. Check for for damaged exhaust or where from 10 to 22 intake-manifold leak. exhaust restriction. in-hg. Check for weak or broken valve­ springs, depending on engine speed. Check Check ignition Possible worn carburetor and coil output. valveguides. The settings and faster the needle internal vibration, the calibration. greater the number of valveguides that are worn.

B

A

[One often overlooked vacuum consumer is the auto-trans vacuum [Vacuum secondary Holley four-barrels, as well as end carbs on its 3x2 modulator. When a good modulator goes bad, trans fluid gets sucked setups, require vacuum to work. A malfunction won’t affect idle, but will into the motor, displaying symptoms like bad rings or valves. Late, firm come into play in the midrange and top end. Before messing with differ- shifts are one sign it’s gone bad, but the surest clue is trans fluid in the ent-tension secondary springs, see if the diaphragm holds vacuum (isn’t modulator hose. Test the modulator with the vacuum pump. leaking). On Holley 3x2 end carbs, connect the pump to the diaphragm’s external vacuum reference port (A). Hold your thumb over the small D- shaped kill bleed near the casting seam above the passenger side (inset, circle, B). You should see steady vacuum when the test pump is engaged. within the preceding param- same as your buddy’s. • Low but steady: a condi- irregular or wavering eters. A fluctuating needle, When vacuum is not what tion affecting all cylinders. pattern: Conditions exist in abnormally high, or abnormally it should be, pay particular • High but steady (over more than one cylinder. low readings indicate a potential attention to not just the raw 20 in-hg): although rare, problem that calls for further vacuum numbers but also could indicate a restricted investigation and diagnosis. the range of gauge-needle . ABNORMAL That’s one reason why it pays fluctuation (and the pattern). • Fluctuating with a steady to establish a decent baseline Broadly speaking, abnormal rhythm in a regular pattern: VACUUM “normal” vacuum reading for at-idle vacuum readings can The trouble is concentrated AT SPEED your combo, as what’s normal be broken down into several in one area or cylinder. Another valuable diagnostic tool for your hot rod may not be the categories: • Fluctuating with an is closely studying the vacuum-

82 HOTROD.COM/2019/AUGUST/ WRENCHIN’ @ RANDOM

HOW MUCH TIMING DO YOU NEED? You can’t go too far wrong on timing around 7 to 8 degrees BTDC plus mechanical, as read on the a new build without a track on 91-octane or better street gas. balancer) at 3,200 to 3,600 rpm. record by starting out with a Engines that under 150 Efficient late-model engines or conservative baseline that psi are lazy and need more time even old-school engines with the takes into consideration the to complete a full mixture burn; 10 latest aftermarket quick-burn engine’s cranking compression, to 15 degrees initial at a 700-rpm aluminum heads may require which provides a substantially idle speed is a good starting point. less total timing. Either way, if reliable indicator of an engine’s On the other hand, if an engine a given combo likes more initial pump-gas compatibility and cranks over 200 psi, tuning expertise timing to develop best idle baseline timing requirement. becomes critical if you have any vacuum and response (including, For engines that crank over chance (and that chance is slim) of based on trial and error, going 150 psi, a good rule of thumb reliably running on pump gas. beyond the 1 degree/1,000 rpm is to establish an initial base For total ignition advance, rule at idle), total timing must timing of 1 degree for every 100 [Adding more initial timing can continue following the “1 degree be correspondingly reduced (the improve engine vacuum and idle rpm. For example, if the engine quality, but if the engine likes a of timing/per 1,000 rpm” rule until “curve” slowed or limited) on likes to idle at 700 rpm with the lot of initial lead, you may have to reaching maximum advance. For the top end to remain within the carburetor properly adjusted, back down the advance curve to typical traditional engines, that’s preceding total ignition advance you want to initially set the base prevent top-end detonation. around 34 to 36 degrees total (base parameters.

[Above: Does your distributor vacuum advance diaphragm actually hold vacuum? Pump it up and see with your vacuum pump. Also pop the cap to look inside the unit and see if the rod coming out of the diaphragm is actually pulling on the advance plate.

[Left: Some carbs use a vacuum-actuated choke pull-off. The blade over the air horn is normally closed with the engine off. When the motor starts making vacuum under crank, a rod connected to the diaphragm pulls the blade open a set amount, allowing air to flow into the carb and the car to start. Exhaust heat or an electrical coil then opens the choke completely as the engine warms up. A bad hose or diaphragm here creates a hidden vacuum leak (and cold-start issues).

gauge needle’s sweep under If needle sweep is irregular syncracies of a particular engine vacuum gauge. It will help you conditions of increased throttle and/or wavering as engine speed combo. In other words, get to understand how to really help opening. On a healthy, prop- increases under normal cruising know, honor, and cherish your your engine suck. erly tuned engine, the normal conditions, pay attention to how response to suddenly opening and when the amount of needle Contacts and closing the throttle should sweep and waver changes as AMAZON.COM INC.; Seattle, WA; Amazon.com be a wide, smooth sweep of the engine speed changes. This, too, MITYVAC, SKF LUBRICATION SYSTEMS USA INC.; St. Louis, MO; 314.679.4200 gauge needle. If this doesn’t hap- can help pinpoint where you (customer service) or 844.772.1341 (tech service); Mityvac.com pen, at a minimum, you have a need to investigate further. MOROSO PERFORMANCE PRODUCTS; Guilford, CT; 203.453.6571 (sales/customer vacuum leak somewhere in the We’ve included vacuum- support); 203.458.0542 or 203.458.0546 (tech); Moroso.com induction system. Otherwise, gauge diagnostic “trees” to help OTC, BOSCH AUTOMOTIVE SERVICE SOLUTIONS; you could be looking at bad you methodically sort these Warren, MI; 800.533.6127; OTCtools.com valve seats or poorly sealing issues out. It’ll get easier as you ROLLINGS AUTOMOTIVE INC.; Mira Loma, CA; 951.361.3001; rings. gain experience with the idio- Email: [email protected]

84 HOTROD.COM/2019/AUGUST/