By Larry Carley, Technical Editor [email protected] Carburetors 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 carburetor 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 cams and discharge nozzles, and increase emissions. your killer engine may never live up power valves. Almost anybody with Vintage muscle cars, 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 throttle 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 cylinder 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 car 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 Fuel Injection 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, cam and position of the linkage that dependent on intake vacuum for fuel changes in header length, camshaft 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.