Rich / Lean... What does it mean?

4 replies to this topic
  • SilverBurr

Posted September 11, 2006 - 04:41 AM


What does it mean when a bike is running rich or lean? How do you determine and fix this problem. Is this for two strokes only or does it happen to four strokes?

  • tpunzy

Posted September 11, 2006 - 04:57 AM


Lean and rich can be applied for almost all internal combustion engines. Gasholine engine generally like around 14.5 parts air to 1 part gas. Basically when you have more air you are lean, when you have more gas you are rich.

What confusesess people with 2 strokes is the ratio of engine oil added to the fuel. With the same carb settings a pinger running 28:1 will be leaner (a/f ratio) then running 40:1.....................the added oil displaces the amount of gas passing through the carb while the amount of air remains the same.

  • MrSmooth

Posted September 11, 2006 - 05:01 AM


It applys to both 2 and 4 strokes. It also can apply to two things. The first is the air/fuel mixture in the carb and that applies to both 2 and 4 stroke.

An over rich carburetor adjustment is when the proportion of fuel in the combustible air/fuel mixture is so high that the fuel does not burn well

A lean carburetor adjustment is when the proportion of fuel in the combustible air/fuel mixture is so low that there is not enough fuel to burn

It is also used to discribe the mixture of oil/fuel in a 2 stroke. Lean meaning more fuel and less oil ,rich meaning more oil and less fuel.

  • milje

Posted September 11, 2006 - 05:12 AM


Those two guys already got what rich/lean is. To determine if you're running rich or lean, easiest way is to pull the spark plug. If it's white, you're lean, black and sooty you're rich, and a light tan color you're good to go. To correct a lean/rich condition is a matter of jetting your carb correctly.

  • sidekick

Posted September 11, 2006 - 05:21 AM


Rich means you have too much fuel and not enough air. Lean means too much air and not enough fuel. Learning how to read the color of your spark plug.
Dry and white means you are running lean. Black and wet too rich. Plugs that are a tan to brownish, cocoa, is the color you are looking for. Re jetting your carb is the usual way to correcting richness or leanness. There are a few other things that can effect fuel flow through the carb, how clean is the air filter, obstruction to intake and or exhaust, position of the jet needle and air screw or fuel screw setting.
This pertains to both, two strokes and four strokes. With two strokes you have one more variable to deal with, oil ratio.

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