Carburetor And Air/Fuel - Getting In Tune

Nothing Makes An Engine Sing (And Make Good Power) Better Than The Correct Air/Fuel Mix

Sucp_0407_13_z Carburetor_air_fuel Stoichiometric_chart 1/8

The Innovate Motorsports Digital Air/Fuel Meter can allow you to sample and record the air/fuel mixture data at a rate of 12 samples per second over a period of up to 44 minutes, this data can allow you to tune the fuel mixture to the ideal air/fuel mixture curve that the infrared exhaust gas analyzer can help you establish. The use of an infrared exhaust gas analyzer, while slower in reaction time, has the advantage that it not only reads the oxygen/unburned combustibles content of the exhaust, but also allows you to determine the air/fuel mixture by observing the CO reading; the engine's rate of misfire can be determined by observing the HC reading; the engine's efficiency can be determined by observing the CO2 reading, and detonation caused by overly advanced ignition timing can be seen by observing the NOx reading.

On-Car Testing
After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, as well as checking to be sure there are no vacuum leaks, the next step is to determine what the air/fuel mixture is at idle through 3,000 rpm. If the cruise mixture is off, first change the jets in order to get the air/fuel mixture correct at the 2,500-3,000-cruise rpm range. Then check and set the idle mixture. If the air/fuel mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle jet.

If the mixture is still lean at 1,000-1,800 rpm, the idle channel restriction on a carburetor such as a Quadrajet or Edelbrock Performer or Thunder series may have to be slightly enlarged to allow more fuel to be delivered at part throttle. This lean condition at part throttle condition will cause the engine to miss or stumble, this is due to the lean air/fuel mixture, this problem is very common on many of the performance carburetors sold today. If the air/fuel mixture is too rich at idle and part throttle, the idle jet/restriction may be too big and may need to be replaced with a smaller one.

The next step is a road test using a portable infrared exhaust gas analyzer and/or a wideband oxygen sensor to check the cruise speed air/fuel mixture-main jetting, followed by a check of the power air/fuel mixture under load. During a road test you are able to read and then correct the air/fuel mixture so you can have them correct at idle, cruise/light throttle and full power.

Tuning The Carburetor
A carburetor has an accelerator pump, idle, main jets, and in most cases a power system that is designed to supply the correct air/fuel mixture for the engine's demands. An idle system will have an idle jet/restriction that must be changed to supply the desired fuel mixture for idle and off idle engine demands. For a carburetor that uses a power valve, the main jet size is what determines what air/fuel mixture is delivered to the engine at light load/cruise speeds (1,500 rpm and up). The power valve restriction (under the power valve) is the determining factor in what the air/fuel mixture the carburetor will supply when the power valve is open; a 6.5-inch power valve will be open and supplying the richer air/fuel mixture needed under high power demands anytime the vacuum is below its 6.5 opening point.

A carburetor that uses metering rods in the primary jets such as a Quadrajet or Edelbrock Performer/Thunder Series will use the metering rods to change the air/fuel ratio for both the power and cruise mixture demands of the engine; the larger the metering rod diameter the leaner the air/fuel mixture will be. The accelerator pump system adds fuel as the throttle valves are opened, tuning the accelerator pump squirter volume and duration it's tuning is mainly trial and error.

For a Demon/Holley style carburetor the combination that is used most is a 0.031-inch squirter along with a pink pump cam, we often modify the accelerator pump duration spring on the Demon/Holley style carburetors and the Edelbrock Performer/AFB carburetors to make the pump more active and help avoid hesitation problems on acceleration. The above chart shows the gases in the exhaust that an infrared exhaust gas analyzer reads and how the gases change as the air/fuel mixture changes.

If you are buying a engine package that has been dyno tuned or developed and run on an engine dynamometer, it would be a good idea to have the engine builder supply you with the initial timing and the ignition timing curve they recommend for your engine and also find out what air/fuel mixture they recommend for the engine for both maximum power and cruise rpm loads and then be sure they are the same with the engine in the vehicle. If possible when the engine builder runs the engine on the dyno have them use a air/fuel meter such as the Innovate Motorsports unit and then you can use the recorded data to tune the fuel curve to supply the engine with the same air/fuel mixture that the engine builder used on the dyno.

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