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Carburetor And Air/Fuel - Getting In Tune

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

By Henry P. Olson

Tuning a carburetor to supply an engine with the correct jetting-air/fuel mixture has always been a job that almost impossible for most hot rod owners and tuners. In the past, most performance engine tuners looked at the spark plug, the exhaust port and the first 6 inches of the header for proper color and then make a guess at what jet size change is needed. One of the disadvantages of this method is that the header and spark plug can only indicate what the mixture was at the exact rpm and load condition the plug check was done at, so you were mainly just tuning by trial and error.

Now a new, more scientific-modern method of checking the air/fuel mixture is the use of an infrared exhaust gas analyzer and/or an extended range oxygen sensor in the exhaust system; now the fuel mixture can be read at any rpm and load condition you wish to see. The content of the engine's exhaust can be read to indicate what the air/fuel mixture is at any rpm or load and how efficiently the engine is burning the fuel.

The proper tuning of any engine can make the difference between a great running engine and an engine that always sounds and runs like it needs a tune-up. For most hot rodders, one of the biggest mysteries is how do you jet the engine in order to obtain the correct air/fuel ratio necessary for your performance engine, to not only supply drivable horsepower when you want to go fast, but also supply the engine with the correct air/fuel mixture for when you are driving in heavy traffic or while cruising down the highway.

If the air/fuel mixture is too rich for the engine while you are running at cruise speeds, the engine may tend to load up and foul the spark plugs, while if the air/fuel mixture is too lean the engine may misfire at idle and at light loads or tend to run hot or overheat. Having a air/fuel mixture correct for all driving conditions will allow you to get all the horsepower out of the engine while getting as many miles as possible from a tank of fuel without overheating or causing any engine damage from having too lean of an air/fuel mixture.

The new advances in exhaust gas analysis technology and extended range oxygen sensor technology has made it possible to read and/or record what the air/fuel mixture actually is under almost any driving condition. In the past exhaust gas analyzers have tended to be large and expensive, but the new units on the market are not only compact and portable, but also affordable.

The performance and replacement carburetors sold today have a generic tune-up or jetting unless the carburetor is built for a specific engine package and fuel. A carburetor not built and tuned for a specific engine, exhaust system, and fuel should supply an air/fuel mixture rich enough for a variety of engines (but this is not always the case). If the carburetor is supplying too lean of an air/fuel mixture, the engine will run sluggish, overheat or the lean mixture could cause engine damage. If the carburetor is supplying an air/fuel mixture that is too rich, the engine may tend to load up, foul the spark plugs, run sluggish and lack power.

Correct carburetor selection can make the job of fine tuning the air/fuel mixture a easier job, my favorite performance replacement carburetors are: for a mild engine a Quadrajet, Edelbrock Thunder or Performer 650 cfm or smaller, on a high performance engine I prefer the Mighty Demon carburetors from Barry Grant Inc. or the Holley 4150 HP, on a supercharged engine the blower carburetors available from Barry Grant have supplied us with outstanding results.

The fuel you use (pump or race), the air density (i.e. altitude, barometric pressure, air temperature, humidity), compression ratio, camshaft, exhaust system, ignition timing curve, engine condition, fuel pressure, air flow thru the air cleaner etc will all effect the carburetor tune-up needed to get the correct fuel mixture for your engine.

The first order of business is to get the correct ignition advance curve for the engine and fuel being used, then the fuel pressure must be checked to be sure it has the proper system pressure at all engine load conditions. If the fuel pressure drops below the proper pressure, the carburetors air/fuel mixture will go lean and engine damage may follow. Once the ignition advance curve has been confirmed to be correct, many of the problems that we see can be traced to the fuel mixture being incorrect for the engine's needs.

Ignition Timing And The Advance CurveBefore checking the air/fuel mixture, the ignition timing and advance curve must first be correct. No mater how what ignition system you use, if the ignition spark timing is not correct for the engine needs, the engine will not produce all the potential power built into it. Any distributor, performance replacement or original equipment, must have the mechanical and vacuum advance curves checked and then tailored to the engine and the fuel being used. (Note: MSD distributors come with a very conservative mechanical advance curve and included in the box are the bushings and springs to get the desired curve).

Barry Grant Inc. has a very good reference for the recommended initial timing using the camshaft duration at 0.050 valve lift that I find very useful, just go to the Barry Grant website and click on the demon selection guide. The advance curve we see used most on a mild cam 9:1 compression Chevrolet V-8 engine is 12 degrees initial timing plus another 24 degrees of mechanical advance in by 3,600 rpm, if a vacuum advance is used it should supply a MAXIMUM OF 10 degrees OF ADDITIONAL advance at a engine vacuum above 12 inches! An engine equipped with a hot cam or an air-gap/race designed intake manifold may respond well to 18 degrees of initial timing combined with a shorter mechanical advance curve of 18 degrees at 3,200 to 3,400 rpm.

If the engine does not have enough ignition spark timing advance it may lack power, have poor throttle response, use too much fuel and cause the engine to run hot, while if the engine has too much ignition spark advance the engine may lack power, ping, use too much fuel or also cause the engine to run hot.

The correct ignition timing will create maximum cylinder pressure at about 12 degrees after the piston goes past top dead center, only then can you get all the energy out of the fuel, creating maximum power and engine efficiency. There are two methods we use to check the distributors advance curve, the best method involves the use of a distributor test stand to check and set both the mechanical and vacuum advance curves, and the second choice is the use of a dial-back advance timing light to check the advance curves while running the engine at various engine rpm and vacuum conditions.

Reading The Air/Fuel Mixture
A lean fuel mixture (too little fuel for the amount of air in the cylinder) can cause an engine to have a surge or miss at idle and part throttle, stumble on acceleration, engine overheating, cause a lack of power, and create possible engine failure from the lean air/fuel mixture. A rich fuel mixture (too much fuel for the amount of air in the cylinder) can cause an engine to load up at idle, foul the spark plugs, and also lack power or run sluggish. There are several different methods to determine if the air/fuel mixture is correct, among them are the following:

1.Reading the spark plugs using an illuminated magnifying glass. This method involves looking at the base of the spark plug insulator (white part of the plug) for a slight coloring on the insulator just above where the insulator comes through the steel case. If the mixture is too lean, it will leave no color, while a rich mixture will cause the fuel ring to become more prominent. Over-rich mixtures will give the plug a sooty appearance.

Pulling the header off and looking at the color of the exhaust port in the cylinder head and at the first 6 inches of the exhaust header is also used as a way to determine what the air/fuel mixture is, but the header and spark plug color can only show what the air/fuel mixture was at the load condition you did the check at.

In the days of leaded fuel and point ignition, this method worked well, but today the use of unleaded fuels and high-energy ignition systems has made this method much harder because very little color is seen on the spark plug and thus a job for an expert. Looking at the spark plug insulator for signs of detonation, which is seen as specks of aluminum, can be an effective way to determine if the ignition timing is too far advanced for the octane rating of the fuel being used.

2.The second method is by using timed acceleration runs or top speed for the power system, this involves using trial and error jetting changes to obtain the best results. Obtaining the correct cruise mixture (which is the air/fuel mixture the engine operates at while driving under light load conditions such as pace laps and yellow flag conditions) is not as easy since this involves jetting the carburetor to get the highest vacuum, then trial and error to get the best engine drivability.

When setting the power and cruise mixtures, it's always advisable to stay a little rich in order to avoid engine damage. The idle mixture is set using a tachometer to get the max speed from each idle screw and then go leaner to get a 20-rpm drop in speed; this is known as the lean drop method.

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.)

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.

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.

Many of the engine packages we have checked the ignition spark timing and air/fuel mixture curves on have had the correct ignition spark timing and air/fuel mixtures for high rpm/wide open throttle operation but need a lot of tuning work at low rpm/part throttle/normal driving conditions. In most cases when an engine is run on a engine dynamometer, they only check for maximum power while using race style headers with open exhaust and they supply the engine with outside air to a carburetor with no air cleaner in place.

The air/fuel mixture and ignition timing curves should be corrected for the real world operating conditions of your cars engine compartment with the hot air coming from the radiator and exhaust system along the changes in exhaust backpressure created by the headers and mufflers you are using which may cause the engine to not perform with the same power that was seen on the dyno.

A properly tuned fuel and ignition system will allow your performance engine to perform up to its potential and will supply you with a better running, more reliable, hot rod! The use of an infrared exhaust gas analyzer and/or an extended range oxygen sensor and then reading the spark plugs (for signs of detonation) is the best way to know if the air/fuel mixture is correct for your engine. Taking the time to properly tune your performance engine's fuel system will not only allow you to unlock all its power, but it may help avoid destroying a expensive performance engine due to a incorrect fuel system tune up. For a more detailed explanation of the use of a five-gas infrared exhaust gas analyzer in fuel system diagnosis on the Web go to: www.automotiveu.com.

SOURCES
Barry Grant Inc.
1450 McDonald Road,
Dept. MMFF
Dahlonega
GA  30533
Innovate Motorsports
5 Jenner, Ste. 100
Irvine
CA  92618
949-502-8400
www.innovatemotorsports.com
Bridge Analyzers OTC/SPX Corporation
www.otctools.com
Hot Rod Tuning-John Bishop
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By Henry P. Olson
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