Engine Running Rich vs Lean

How can you tell if your powersports vehicle engine is running rich or lean? There are several ways to determine whether an engine is getting too much fuel (rich) or not enough fuel (lean).

Sometimes a simple spark plug check can give you the answer. Our resident powersports expert John Talley goes live on our YouTube channel every Friday at 3pm Eastern, and in this edition, John answers a viewer's question on how to tell whether an engine is running lean or rich.

Engine running rich or lean spark plug

Running Rich or Running Lean?

How do you tell if you’re engine is running rich or lean?

John Talley: Well the first indicator if you’re running rich is that usually you can smell raw fuel coming through the exhaust system. But the best way to tell for sure is to go ahead and take the bike out, get it on a straight-away, put it in 3rd gear, go ahead and nail the throttle, rev it and let it run all the way out, and then do what they call a hot cut, or cold cut. Cut your ignition, roll to a stop and then pull your plug. What you want to do is to take a plug read, to see what your wide-open throttle fuel burn is. And if you pull the plug and it’s black around the ceramic and around the base of the plug, you’re running way, way rich. If it’s completely white and looks like a new plug, then you’re very lean. Keep in mind that is at wide-open throttle, and if you want to see where you are at different points, you could just stop at whatever RPM you want to. Whether it be a quarter, half, three-quarter or wide-open throttle, and that’ll give you an idea after about 5-10 seconds worth of runtime at that RPM to give you a good read on the plug as to what’s going on inside that engine.