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By Larry Carley, Technical Editor [email protected] Understanding How To Tune Carburetors n engine only runs as well as with the choke adjustment (street If you are building a street it is tuned. You can build a applications only) to major performance engine, you may need killer motor using all the best modifications such as changing an emissions legal such A as this Carter AFB. parts and machine and assemble venturis, venturi boosters, air everything with the utmost care, but bleeds and emulsion tubes, restrictors for the idle circuit so if it’s a carbureted engine and the metering blocks, accelerator pumps, the carb doesn’t run too rich and carburetor isn’t setup or tuned right and discharge nozzles, and increase emissions. your killer engine may never live up power valves. Almost anybody with Vintage muscle , street to its full potential. a screwdriver can make the simple machines and hot rods don’t have A good carburetor tune can adjustments or swap jets, but it to worry about emissions in most make an engine, while a carburetor usually takes an expert to perform instances so there are no restrictions that’s mistuned can literally break major modifications with success. on what kind of carburetor, the size an engine. A good tune means the A brand new carburetor from a or even the number of carburetors engine should start and idle without major aftermarket supplier comes that can be used on an engine. Any stalling, and respond instantly prejetted and preadjusted for the carb that will bolt onto the manifold when the pedal hits the metal. The “average” engine it will be bolted may be used – and that’s a real carburetor will also deliver the onto. Street carburetors are set up problem because a lot of guys over- optimum air/fuel ratio throughout to deliver good cold drivability carb their engines. the engine’s RPM range to achieve and good fuel economy and It’s true that a carburetor with peak horsepower and torque. performance, but not necessarily larger bores will flow more A mistune, on the other hand, peak performance. By comparison, cubic feet per minute (CFM) than a can create all kinds of problems. race carburetors are purpose built carburetor with smaller bores. More A slightly rich fuel mixture is for specific applications. There’s airflow means more horsepower, necessary to make peak power, no choke to restrict airflow into but mostly at higher engine speeds. but if the A/F mixture is way too the carburetor, and the venturis, Drivability is just as important, and rich, excess fuel can wash down the boosters, metering blocks, fuel that requires a lot of air velocity walls and dilute the oil. It bowls and throttle plates may all be for instant throttle response and can also foul the spark plugs. Too heavily modified to deliver the kind good low to mid-range torque. lean is even worse. It kills power of performance a race driver Using a carburetor that is more and increases the risk of engine- expects. closely matched to the engine’s damaging detonation. Many circle track rules restrict the displacement will deliver better If an engine is hard to start, type of carburetor or modifications performance and drivability than a barely idles, coughs, bogs, sputters that can be made. For these kinds carb that’s too big. or stumbles every time the throttle of venues a “blue printed” carb that A typical smallblock Chevy will opens, gets horrible fuel economy, pushes the specs to the absolute usually run best on the street with a spews black carbon from the tailpipe limit is usually necessary to be 650 to 730 CFM carb. The only way or never seems to run very well, competitive. a street small block can handle the chances are the carburetor needs to For street cars, depending on extra airflow of a larger carb is if the be tuned. the vintage of the vehicle and state engine is built and cammed to rev Carburetor tuning runs the laws, a carburetor may have to over 7000 RPM and the drivetrain is gambit from simple idle speed be “emissions legal.” That means geared with a higher numerical final and idle mixture adjustments, vacuum hookups for the PCV and drive ratio. resizing main jets and fiddling EGR valves, a choke and possibly

26 February 2016 | EngineBuilder Carburetors

How Carburetion Differs have the same effect because molecules squirts additional fuel into the throttle From of water displace air. bores when the throttle opens to The basic difference between a Many other variables can also compensate for increased airflow. The carburetor and electronic fuel affect the air/fuel ratio such as open volume of the pump, the shape of the injection is that a carburetor is totally headers versus closed headers, and position of the linkage that dependent on vacuum for fuel changes in header length, operates the pump, and the size and metering and delivery. Intake manifold changes, gearing and tire size changes, position of the discharge nozzles all vacuum pulls fuel through metering track length, etc. Often, re-jetting the affect how much fuel is delivered when circuits while atmospheric pressure carburetor and changing power valves the throttle opens. The goal here is to provides the push. Fuel injection is not and accelerator pump rates is required. deliver enough fuel for crisp throttle dependent on intake vacuum because response without going too rich and the fuel is sprayed into the manifold, Basic Carburetor Circuits flooding the engine with excessive fuel. intake ports or directly into the Before the actual tuning process, let’s An engine that stumbles, hesitates or combustion chambers. The powertrain take a qlook at the various carburetor bogs when the throttle opens usually control module (PCM) controls the fuel circuits and what they do: needs more fuel from the accelerator mixture using inputs from the engine’s • Cold start circuit. This includes pump, or the carburetor is way too exhaust oxygen sensors, throttle the choke, fast idle cam and linkage, large for the engine. position sensor, manifold absolute and fast idle adjustment screw. These • Power enrichment circuit. A pressure sensor and airflow sensor. components are usually necessary vacuum-sensing “power valve” opens Electronic fuel injection is also self- for most street driven vehicles, when intake vacuum drops to allow tuning and is constantly readjusting especially in cold climates, but can more fuel flow through the main the A/F mixture based on feedback usually be eliminated in most racecar metering circuit. This enriches the from the oxygen sensors. Mechanical applications. Manual chokes require mixture when the engine is under load carburetors lack that ability, although no adjustment but do require a certain to maintain the A/F ratio. If the power some of the electronic feedback carbs amount of finesse when it comes to valve is too big, it may cause the A/F from the early ‘80s could vary the A/F starting a cold engine. Automatic ratio to run too rich when the engine mixture. Consequently, mechanical chokes are supposed to make cold is under load. A power valve that is carburetors require constant fiddling starting easier, but usually require too small may not increase fuel flow and readjusting for prevailing weather some trial-and-error adjustment to enough under load, causing the A/F to conditions and track conditions. find the best setting. Automatic choke go lean. problems can also cause hard starting, • Float. Inside the fuel bowl is a Optimum Air/Fuel Ratio cold stalling and idle speed problems. brass, hollow plastic or foam plastic For peak power, most engines like • Idle circuit. Adjustable tapered float that maintains the fuel level. The an air/fuel ratio of 12.8 to 13.2. If you idle screws for each barrel control how float opens and closes a needle valve can tune a carb to deliver a consistent much fuel is pulled into the engine at that allows fuel to enter the bowl. The A/F within this range, the engine will idle by intake vacuum under the closed float level is important because the usually run at its full potential from throttle plates. Proper adjustment is fuel level inside the bowl affects how off-idle to wide open throttle. critical for a good idle. Misadjustment much fuel flows through the main The air/fuel ratio will change with can cause stalling, poor idle quality metering circuit. A low fuel level can ambient temperature, barometric and hesitation when the throttle make the A/F ratio run lean, while a pressure and humidity. Hotter air initially opens. Small adjustments (only fuel level that is too high can make the is less dense than cold air, so the a quarter of a turn) can make a big carburetor run rich or even flood the carburetor main jets usually need to be difference. engine with fuel. reduced in size to compensate so the • Main circuit. The jets (and rods engine doesn’t run too rich. Likewise, in Carter/Edelbrock) carbs control Tuning Precautions colder air is more dense than hot air, so the A/F ratio from off-idle to wide One of the most important things to the main jets can be increased in size open throttle. Airflow down through always consider before you touch a so the fuel mixture doesn’t go lean. A the venturis (and boosters) creates carburetor is its history. If it’s brand 10-degree temperature change one way low pressure that pulls fuel from the new, it’s history doesn’t matter. But if or the other may require increasing or fuel bowl through the main jets and it’s a used carburetor, beware because decreasing the jets by two to four sizes. into the airstream. Increasing the jet a lot of unpleasant surprises can be Altitude and barometric pressure size enriches the A/F mixture while lurking inside: also affect air density. Higher decreasing the jet size leans the A/F • There may be dirt or corrosion altitudes and decreasing barometric mixture. The main circuit also includes inside that can plug circuits and restrict pressure thin the air, which causes emulsion tubes and air bleeds that fuel flow. the carburetor to run rich. Installing premix air with the fuel to improve • Watch out for leaking or heavy smaller jets will compensate for the fuel atomization. floats. Hollow metal and plastic floats difference. High humidity can also • Accelerator pump. The pump can leak and foam plastic can absorb

28 February 2016 | EngineBuilder Continued on page 33 Carburetors

fuel. Either condition can make the 1,100 to 1,500 RPM for its initial break- can see if the carburetor is going too float sink and over-fuel or flood the in period. Once the rings have seated, rich or too lean causing power and engine. you can make the initial idle mixture torque to drop off at certain engine • Are the fuel bowl inlet valves in and speed adjustments. The goal here speeds. This, in turn, tells you if you good condition and not sticking or is to turn the idle mixture screws in need to richen up or lean the main jets leaking? and out to achieve the fastest idle and/or change the power valve to • If the carb has screw-in fuel filters, speed, then adjust the idle speed screw correct the A/F mixture. are the filters clean and unobstructed? to set the idle speed. If you’re tuning a carburetor the • Is the accelerator pump, pump Obtaining a good idle can be a “old school” way, test driving or taking linkage and power valve in good challenge with a long duration cam a few hot laps will quickly tell you if working condition, and properly because of the reduced intake manifold the engine feels strong or not, and how matched for the application? vacuum, so the best you can do is aim well it responds to throttle changes. • Has somebody drilled out the for an idle mixture and speed that If the engine feels sluggish when the passages in the main metering blocks keeps the engine from stalling. driver steps on it, the carb may be for more fuel flow? How you proceed from here going too rich or too lean. Trial-and- • Has somebody screwed around depends on what kind of equipment error changes to the accelerator pump with the main jets, float settings, you are using. If the engine is on and/or main jets and/or power valve metering blocks, venturis, air bleeds, a dyno, your exhaust gas oxygen should eventually sort out the problem. boosters or and really messed readings can guide you as you monitor Reading the spark plugs can also things up? the A/F ratio at changing engine give you a rough idea of what’s • Is the carburetor body, top plate speeds. As we mentioned earlier, a happening to the A/F mixture. Black and throttle housing flat and free from slightly rich mixture (12.8 to 13.1) carbon deposits on the center insulator fuel or vacuum leaks? usually produces the most power and and electrodes would indicate an • Are the and seals in good torque. A balanced or neutral A/F ratio overly rich A/F mixture (or the spark condition, or are they dried out and of 14.7 to 1 (stoichiometric) is good plugs are too cold for the engine). A leaking? for fuel economy and emissions, but yellowish or blistered appearance on • If you’re building an engine that not so much for power. The one thing the electrode would tell will run on alcohol, is the carburetor you want to avoid is going too lean you the A/F mixture is much too lean properly jetted for alcohol (which (especially at high RPM) because a (or the spark plugs are too hot and the requires a much richer A/F mixture) lean fuel mixture is a prescription for engine needs colder plugs). Heavy and additional corrosion protection)? detonation, preignition and burned black soot in the exhaust or tailpipe The bottom line is you need to make . would also indicate an overly rich A/F sure the carburetor you’re going to If the engine is on a test stand (not mixture. use is in good working condition and a dyno) or in a vehicle, you can use a When changing jet sizes up or hasn’t been screwed up, otherwise you handheld A/F meter with a wideband down, go two number sizes larger may never get it to work well. O2 sensor to measure the A/F ratio or smaller rather than one to see if it in the exhaust. Keep in mind that the helps or hurts performance. If it helps, Carburetor Tuning meter readings are an average for all of increase the jet sizes again to see if it Step-By-Step the cylinders that are dumping exhaust helps even more. If larger jets hurt Okay, now comes the fun part. A into that pipe. So check both pipes with performance, go the other way and few preliminary adjustments can be a dual exhaust. Also keep in mind that decrease the size of the jets until the made before installing the carburetor differences in fuel distribution in the engine feels stronger. on the engine. One is to preadjust intake manifold may cause the end Keep in mind that the jet sizes, the idle mixture screws to about one cylinders to run leaner than the center power valve and accelerator pump full turn out. Do not overtighten the cylinders. adjustments that provide the best idle mixture screws as doing so can Another check to make before you performance today may not produce damage the needle tips. If the carb has start playing around with the jets, the same results tomorrow. As soon as a choke, preset it to the “0” or midpoint accelerator pump and power valve the weather changes, track condition setting. Leave the locking screws for is the fuel level inside the fuel bowls changes, or other changes are made the choke housing cap slightly loose so with the engine idling. On a Holley (like valvetrain lash, spark timing, etc.), the choke can be easily readjusted as 4BBL, the level should be even with the carburetor will be out of tune and needed. Preadjust the idle speed screw the inspection hole when the plug is in need of further adjustments. on the throttle linkage so the throttle removed. With carbureted engines, it’s an plates are closed, then tighten the Once you have the carburetor ongoing process of tweaking and screw one full turn for the initial idle baselined, you can start to look for adjustment to find the settings that setting. power improvements. With a dyno, deliver the best performance. So unless As soon as the engine starts you can see what’s happening with the you’re ready to swap out the carb for a (assuming this is a newly built engine), A/F mixture, horsepower and torque bolt-on fuel injection system, keep that the engine needs to run at fast idle curves as the engine makes a pull. You screwdriver handy. You’ll need it! n

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