A lean fuel mixture can cause an engine to have a surge or miss at idle and part throttle stumble on acceleration, leading to engine overheating and lack of power. A rich fuel mixture can cause an engine to “load up” at idle, foul the spark plugs, and also lack power or run sluggish.

If the A/F mixture that is delivered to the engine is excessively rich for too long the engine could leave leftover fuel from the combustion process, washing the oil off the cylinder walls. Without the oil to act as an anti-wear agent, the pistons and rings will make metal-to-metal contact with the cylinder walls.

Photoshop Chroma Key Plugin Free Download more. Also, if enough fuel gets past the rings and into the crankcase the oil can become diluted and lose much of its lubricating properties and accelerate engine wear. Theoretically, the ideal stoichiometric A/F mixture (the chemically ideal mixture of air and fuel that is required to provide a complete burn) for a properly tuned engine running on pure gasoline is 14.7:1; that is, 14.7 lbs. Of air to 1 lb.

However, because of operating losses in the induction system due to intake runner and cylinder wall wetting, plus the fact that fuel may not fully vaporize in the combustion chamber, a 14.7:1 A/F mixture is often too lean for actual operating needs. A more realistic light-load, cruise A/F mixture for a stock carbureted engine running on reformulated unleaded gasoline is in the 14.1:1 range. The A/F mixture always varies from cylinder to cylinder, therefore we tend to tune the average A/F mixture slightly on the rich side to avoid engine misfire in the leanest cylinder. It is possible to target an A/F mixture leaner than 14.7:1 for maximum fuel economy but this can lead to driveabilty problems if any one cylinder is leaner than the others.

The power mixture we target for maximum horsepower is in the 12.2:1 – 13.5:1 A/F range, depending on the engine package and its combustion chamber design. The original equipment carburetor(s) that came on a muscle or classic vehicle’s engine was tuned for the leaded gasoline of the day, so in most cases the engine will tend to run lean with the reformulated and/or oxygenated unleaded gasoline of today. The gasoline of today also has lower volatility than the leaded gasoline of days past, which will cause most carbureted engines to need a slightly richer A/F mixture at idle and light load part throttle driving conditions to have the same drivability as it had with the leaded gasoline of the ’60s and ’70s.