OK, here's how I've been teaching fuel/air mixture for almost 40 years:
Air is about 20% oxygen, which is what combines with fuel to burn it and release the thermal energy to power an engine. Since virtually all fuels are hydrocarbons, the oxygen combines with the hydrogen to form water vapor, and the carbon to form C02. If there's a shortage of oxygen, the incomplete combustion also allows carbon Monoxide to form (CO). The whole point to computer-controlled fuel injection is to constantly monitor the oxygen content in the exhaust stream, and instantly adjust the fuel/air mixture for the most efficient combustion efficiency. Since that adjustment can happen several times a second with a properly-programmed computer, electronic fuel injection is far superior to a carburetor for wringing the best efficiency and performance from an internal combustion engine.
Anything that drastically alters the mixture away from a 14.7:1 air/fuel ratio (by weight) affects performance, and can do major engine damage. Gravity makes our atmosphere weigh 14.7 pounds per square inch, which is just a coincidence, I think. Look at it this way- - - -the atmosphere extends to about 100,000 feet above sea level, and the force of gravity acting on the air makes a 1 square inch column of air 100,000 feet tall weigh 14.7 pounds. Combine that column of air with 1 pound of gasoline, and we've got the perfect air/fuel mixture. Since gas weighs about 6 1/2 pounds per gallon, it takes a column of air about 3 1/2 inches square, and 100,000 feet tall to burn a single gallon of gas.
If there's too much air for the available gasoline, all the fuel gets burned early in the power stroke, and the remaining hot oxygen has to burn something- - - - -like spark plugs, valves, pistons, etc. A lean mixture gets extremely hot, which heats up any exposed metal or foreign material in the cylinder- - - -spark plug electrodes, carbon deposits, and/or exhaust valves, for instance. Once those things get hot enough, they become the source of ignition for the fuel instead of the spark plug, and the fuel begins burning early in the compression stroke.(Pre-ignition) Cylinder pressure rises abruptly, and the piston material, which has been softened by the high cylinder temperature, gerts blown away by the pressure spike. (Detonation) Gasoline detonation is the second-most powerful destructive force, pound for pound, known to man, exceeded only by a nuclear explosion. It's more powerful than Dynamite, TNT, C-4, etc.
Bottom line- - - - -don't let an engine run lean for any reason. A vacuum leak can be terribly destructive to your engine!
Jerry
EDIT: I use an unlit acetyline torch to check for vacuum leaks, but Propane would work just as well. Unscrew the burner head from your Propane torch, add a couple of feet of rubber fuel line, Open the gas valve a little, and explore any suspected leaks around manifold gaskets, vacuum lines, carb base, etc. When you find a leak, the RPM will rise, and/or a rough idle will smooth out.
Jerry
Last edited by Hotrod Lincoln; 12/21/2008 6:53 PM.
