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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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.

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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.


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