Wayne Cook
May 11, 2010

Tech | Carb Tuning Secrets
For nearly 100 years, a carburetor has been at the heart of almost all internal combustion engines. Only in the last 20 years or so have car makers used electronic fuel injection exclusively on gasoline engines. There have been examples of mechanical fuel injection for automotive applications in the past, and of course diesel engines use fuel injection. However, the classic Ford cars we love were all carburetor-equipped, from the 170-cubic inch six-cylinder with a one-barrel Autolite carb to an R-code 427 with eight barrels of Holley induction in the form of dual-quads. Carburetors still compete effectively with fuel injection on engines when it comes to making power and when we do an EFI conversion, horsepower production is not the single largest motivation for doing so. The fact is that when the time comes to run the quarter-mile or make a few pulls on the rollers, a carburetor gets the job done just about as well as fuel injection. Even though simple in principal, in that a carburetor functions as an atomizer, there are many subtle variables to be considered and adjustments that can be made to get your engine running its best.

Of course, once engine modifications begin then things become a little more involved. When one enthusiast decides on a mild 289 with single exhaust, while another equips his K-code with long-tube headers then carburetion requirements will differ. However, two rules of thumb apply across the board when it comes to carburetor tuning and selection. The first is sizing the carburetor appropriately to the displacement or flow capacity of the engine. For example a 750cfm carb would be too large a choice for an A-code 289 that the factory equipped with a 480cfm unit. However, adding compression, cylinder heads, and a higher-lift camshaft increases the flow capacity of the engine and then a larger carburetor might be called for. A 600cfm unit is a good bet for a healthy short-deck engine while a 650 to 750 cfm would be a good size for a stock or stroked Windsor engine. Save the 850 size for your 428 or larger displacements or otherwise radical engines. According to the experts at Pony Carburetors the following table lists approximate flow capacities of different Ford engines in their stock state. These numbers are calculated at 90 percent volumetric efficiency.

DisplacementCFM Needs
289/302ci 453 cfm
351ci 550 cfm
390ci 609 cfm
428ci 668 cfm

When we spoke to various experts in the carburetor field about the most common problems that folks encounter with a carburetor on a street performance car they cited stumble off idle, bogging at wide open throttle application, secondary engagement issues, and lean or rich operating conditions. These problems can be addressed with adjustments to the float bowl fuel level, accelerator pump, secondary opening systems, and jet settings. In this short overview let's examine these common engine performance complaints that can be addressed by simple carburetor adjustments and then look at a dyno-documented example of the benefits achieved when carburetor size and engine timing are optimized for the engine.

Matching the carburetor size to the capacity of the engine ensures good throttle response throughout the rpm range and also the best economy of operation.

The second rule of thumb that the experts we spoke to stressed over and over again is that a properly tuned carburetor will be complemented most in terms of performance by the correct engine timing. Correct initial timing for most stock V-8 engines occurs at 10 to 14 degrees before TDC. On engines with high- performance components added, the initial ignition timing should be advanced. For example, high-performance camshafts often have a longer duration for valve events and so they generate less depression or signal at the carburetor at idle. As a result air-fuel droplets are bigger, atomization is poorer, the burn rate is slower, and the combustion more incomplete. To overcome these tendencies initial timing should be increased.

Use Filtered Fuel
Before anything else, be certain that a clean and filtered supply of fuel is provided. Contaminated or dirty fuel is one of the largest causes of improper carburetion. When installing a carburetor, renew the fuel filters. Place one directly before the fuel pump and the other before the carburetor.

Float Bowl Facts
After a rebuild or before attempting any other adjustments, checking and adjusting the float level comes first. This should be the first thing you do before attempting to make any carburetor adjustments. The float functions like the float in a toilet tank and shuts off the flow of incoming fuel entering the bowl by closing a needle on the seat of the fuel inlet. The float level should put the fuel level just below the bottom of the sight plug hole if your carburetor is equipped with one. You can make the adjustment with the vehicle on a level surface and the engine idling.

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Accelerator Pump Issues
The most common cause of a stumble off-idle or bogging is an inadequate or incorrect accelerator pump shot. The first thing to do is to look at the discharge nozzles and make sure you are getting a good, strong shot of fuel when you pull the accelerator linkage back.

If there is no shot of fuel then you need to inspect the pump diaphragm for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. The accelerator pump system consists of three main components: the pump diaphragm, the pump cam, and the pump nozzle. This is the system on your carburetor that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By doing this, it acts to smooth the transition between the idle and the main operating circuits so that no stumble, hesitation, or sluggishness will be evident during this transition phase.

Once this clearance has been set, make a careful inspection of the pump linkage and work the throttle. Make sure that the accelerator pump arm is being activated the moment that the throttle begins to move. This will assure that pump response will be instantaneous to the movement of the throttle. These adjustments can be made by turning the accelerator pump adjusting screw that is located on the accelerator pump arm together with the pump override spring and lock nut. The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for a pre-determined amount of fuel to be delivered is affected by the pump nozzle size. A larger pump nozzle will allow fuel to be delivered sooner than a smaller pump nozzle. During acceleration tests, if you notice that the car hesitates at first and then picks up, it's a sign that the pump nozzle size should be increased. A backfire or lean condition upon acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size may be called for.

Throttle at wide open application often calls attention to secondary opening issues. To see if your vacuum-operated secondary is opening, its operation can be checked using a paper clip. The engine needs to be under a load before they will open. Take a normal paperclip and clip it onto the secondary diaphragm rod. Then push it up against the bottom of the secondary diaphragm housing and go out and drive the vehicle including a pass at WOT. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod, then you can tell that the secondary has opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.

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Size Your Jets
Jet size changes can be called for if the engine is determined to be running too rich or too lean. An air/fuel meter used during a dyno session concluded that our engine was running in a too lean condition at over 14:1 air/fuel ratio at wide open throttle. We knew that besides being a dangerous condition, more power could result with a jet change on the secondary side of the carburetor.

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Carb upgrade? Don't forget the timing
Once you've got your engine running right with any carburetor issues ironed out, is it possible to wring more performance out of your engine by going to a larger carb size? We wanted to find out so we took our car, which was running fine with a 600cfm Holley four-barrel on it and decided to step the carburetor size up a bit. The 351W engine has AFR aluminum cylinder heads on it, headers, and a reasonably rowdy roller cam. We felt fairly confident that the engine had enough oomph to absorb and successfully utilize some extra induction capacity. We wanted to see what differences might emerge when comparing a Holley 4160 600cfm single-feed carburetor with vacuum secondaries to a 4150 650cfm double-pumper with a mechanical secondary.

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Here are the dyno results with the new carb and the same timing. A simple carburetor swap to the slightly larger 650 got us to well over 300 horsepower with a solid 304.3 horsepower at the tire at 5,300 rpm. Although the difference in peak-to-peak horsepower is only 5.6 hp, the curve is better overall. For example, at 4,000 rpm the difference is 7.6 more horsepower with the 650cfm carb. The bigger carb made a nice improvement on torque. The peak improvement in torque output ended up being 9.1 lb-ft, or 344.2 lb-ft with the 650 carb as opposed to the 335.1 lb-ft reading we got with the 600cfm carb.

BaselineCarbTiming Change
RPMHPTQHPTQHPTQ
3,000182.0318.6175.1306.6186.4326.3
3,200196.0321.7202.0331.5210.4345.4
3,400207.3320.2219.5339.0227.6351.6
3,600226.2330.1235.9344.2240.1350.3
3,800242.5335.1247.3341.8247.9352.0
4,000250.7329.1258.3339.2253.1349.9
4,200261.1326.5266.6333.4263.1345.4
4,400270.9323.3273.5326.4273.9342.5
4,600277.1316.3277.8317.2282.3337.0
4,800284.7311.6282.1308.7287.8328.6
5,000294.4309.2293.3308.0296.9324.8
5,200298.7301.7302.4305.5309.1324.7
5,300304.3301.5314.2317.4
5,400302.7294.4320.2310.7