Carburetor And Air/Fuel - Getting In Tune

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

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3.The easiest and most accurate method that we have found is using an infrared exhaust gas analyzer. This type of unit allows us to determine what the air/fuel mixture is by reading the exhaust gases. By using a infrared exhaust gas analyzer, the carburetor's jetting (air/fuel mixture) curve can be checked at idle, cruise, or power loads and then be tailored to what your engine needs to run at its best at all race/driving conditions. A high NOx reading taken from the exhaust gas analyzer can be used as a method to determine if the ignition timing is too advanced, creating excessive heat in a cylinders combustion chamber.

4.An optional method of checking air/fuel mixtures is by using a wide band oxygen sensor installed into the exhaust header, the sensor is read by using a digital air/fuel meter, the unit I have had the best results from is available from Innovate Motorsports. This method determines the air/fuel mixture by looking at the oxygen/unburned combustibles in the engines exhaust; the readings can be very accurate, but false readings can be created by a exhaust leak, engine misfire, or a high overlap cam at lower rpms (these false readings are caused by the oxygen sensor misreading the extra oxygen in the exhaust from the misfire, exhaust leak or high overlap cam)

Jetting With A Infrared Gas Analyzer Or Wideband Oxygen Sensor
The most accurate and easiest way to check the jetting (air/fuel mixture) of an engine is by observing the CO reading from an infrared gas analyzer and/or a wideband oxygen sensor. First place the sample probe into the tailpipe and then the unit will read the exhaust and supply the readings necessary to determine the air/fuel mixture. The infrared exhaust gas analyzer and/or the wideband oxygen sensor method allows the part throttle air/fuel mixture to be checked which otherwise is almost impossible, the readings from either method can be read in real time or recorded and later played back. It is important to note that any changes other than jet changes and other basic adjustments should be done by a proficient carburetor expert.

A starting point for air/fuel mixtures for most race engines is:
Idle: 1 to 3-percent CO or a 14.1-13.4:1 air/fuel mixtureCruise rpm: 1 to 3-percent CO or a 14.2 -14.0:1 air/fuel mixture
Power mixture and acceleration: 6.6-percent CO or a 12.0:1 air/fuel mixture for a normal engine; a high performance engine with improved combustion chamber design such as a Pro-Stock or a Winston Cup engine, in some cases, use a slightly leaner power mixture of 4-percent CO or a 13.0:1 air/fuel ratio.

Tuning the air/fuel mixture with a infrared exhaust gas analyzer
The reading from an infrared exhaust gas analyzer will indicate air/fuel ratio, engine misfire, engine combustion efficiency and excessive combustion chamber heat (detonation) by looking at the CO exhaust gases. The reading from an infrared gas analyzer is the reading that we use to determine the air to fuel ratio. (Note: CO is partially burned fuel.)

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The other readings that exhaust analyzers provide are:HC (hydrocarbons): The amount of unburned fuel or an indicator of an engine misfire, the best mixture gives you the lowest HC.

CO2: The product of complete combustion, the best mixture gives you the highest CO2 reading

O2: A high O2 reading indicates a lean mixture; an exhaust leak or the engine has a hot cam. Note: if O2 is above 2 to 3%, the CO readings may not be accurate.

NOx (oxides of nitrogen): A gas created by excessive combustion chamber heat in many cases a high reading may be related to excessive ignition timing creating detonation that can result in engine damage.

The best power and cruise air/fuel mixtures (CO) will burn all the oxygen in the cylinder and create the lowest amount of engine misfire (HC) and the ideal air/fuel mixture for each engine rpm and load condition will also cause the engine efficiency (CO2) to be at it's highest.

Tuning With A Digital Air/Fuel Meter
The digital air/fuel meter method using a extended range oxygen sensor requires you to know what air/fuel mixture your engine needs for each driving condition, this data should be available from your engine builder or you can use a infrared exhaust gas analyzer to help you determine what air/fuel mixture your engine needs to run its best. The air/fuel meter method uses a wideband oxygen sensor to determine the fuel mixture by analyzing the unburned combustibles in the exhaust gas.

A extended range oxygen sensor can read air/fuel mixtures as rich as 9 to 1 or on the lean side it can read air/fuel mixtures lf 19 to 1 or leaner (a standard oxygen sensor is only accurate at air/fuel mixtures of around 14.7 to 1). This method has the advantage of extremely fast reaction times for the readings but it can be less than accurate on an engine with a race cam or a supercharged application at light load/low rpm testing conditions because of the excessive oxygen in the exhaust created by the cam overlap or the superchargers blow thru effect at low engine speed and low load conditions.


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