If the ratio is too rich or too lean, the engine will not burn optimally burn the air-fuel mixture which can cause performance issues or use up too much fuel. The ideal air-fuel ratio that burns all fuel without excess air is 14.7:1. This is referred to as the “stoichiometric” mixture.

If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency. Normally aspirated engines typically run anywhere between 80% and 100% VE.

IDLE / LIGHT CRUISE: 13.5 AFR With 14.6 being the industry standard stoich AFR value, we typically like to see these conditions run slightly richer.

14.7:1

As far as lean goes, higher than normal operating temps, plugs look like they’re burnt to a crisp. For figuring out which way to go on the top end, I get in high gear, pin it and when it starts to lay down pull the choke out slightly and see if it recovers. If it does, you’re lean and if it worsens, they you’re rich.

Lean Air/Fuel Mixture Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn’t have enough gasoline can cause a backfire, too. When a lean mixture combusts, it burns more slowly, meaning there will still be some air and fuel that isn’t used up when the exhaust valves open — leading to a backfire.

If one considers the tipping point between lean and rich as whatever air/fuel ratio yields the best power, then again, slightly rich is most likely the answer. You’ll loose some power vs. your best power ratio but probably not as much as if you went to the lean side.

Running lean, on the other hand, refers to when a car’s engine receives too much air and too little fuel. When this occurs, you may notice symptoms like sluggish acceleration as well as jerking. Some of the most common causes of a lean-running engine include clogged fuel injections, bad fuel pump and a vacuum leak.

10.5-12.5:1