Lean vs. rich

I'm curious about diagnosing lean vs. rich carbs. What will a bike that is running lean do or sound like, and the same for one too rich? Thanks!

The only true way to tell is by the spark plug. Whitish color=Lean, Dark black and oily= Rich. You want in between :banghead:

Lean usually gives you the common popping on decel that WR's are known for.

Lean also means it will run hotter and have a better chance of overheating in the tight stuff, so go for Rich if anything.

It's an easier process with 2-strokes, cause you have the tell tale smoke to help you out. A rich 2 stroke can make a LOT of smoke. Rich conditions will cause the bike to be slow to respond, and if the main jet is too big, it will not rev out properly, it will reach a stuttering and will refuse to rev more. Lean conditions can cause the popping and sometimes backfiring. If the bike runs better with the choke on, you can bet it is too lean. Two strokes will foul plugs often when jetted too rich, and will "load up" when not ridden at high rpms. If a bike is too lean, it will seem very "cold blooded", ie. when started cold, they will take a long time to settle into a nice idle without stalling and be willing to rev up.

These are some of the one I've experienced, I'm sure some others on here could give you a complete rundown of all of the symptoms. Looking at the plug is a good idea, but there are symptoms of being lean/rich at small and large throttle openings, as the carburetor uses different circuits, that may not be told on the plug.

Hope this helps

Cedric, thanks for the info. Your discussion about popping, revving, issues etc., is that limited to 2 strokes or is that for 4 strokes too?

The popping seems to be more of a thumper thing, while the inability to rev out with a too big main jet applies to both in my experience. Keep in mind that I'm no expert and I'm sure others would have much more experience than me in this department, just thought I would throw out what I knew.

A well jetted 2 stroke will make very little smoke and will run very clean. It will also have a bit of a different sound, rich jetting = dull exhaust sound, nice jetting = more of a tinny sound.

Excellent thread. It will be great to get some common symptoms for rich/lean conditions.

heres one, what if after a quick snap it hangs above idle a 100 or 200 drops to idle, then after a sec or two dies?

lean or rich? hmmm

Dont forget to check the Jetting D-base sticky thread at the top of this forum! Very helpful, and very thourough!!

My JD kit is ordered and on the way. My bike is stock. SO, been doing a lot of reading here and plan to hit the grey wire, airbox snorkel, throttle stop replacement to YZ, Zip Ty mixture screw, etc as soon as the kit arrives

heres one, what if after a quick snap it hangs above idle a 100 or 200 drops to idle, then after a sec or two dies?

lean or rich? hmmm

lean pilot!


These bikes are actually easy to jet. The problem comes in that for some reason, some really wierd inexplicable reason, people have this urge, this drive, this wild fascination with jetting these bikes overly rich.


Stop the madness.

One; Change only one jet at a time.

Two; If your plug looks rich, it is.

Three; If you puff black smoke, see Two.

That mysterious midrange surge in 1/2 to 3/4 throttle, where it feels like its cutting out and misfiring, you know what I'm talking about....that's a rich needle and/or main.

Thus you need to investigate the possiblity of leaning things out a bit.

Remember your AP? That makes you , NAY, requires you to jet your needle properly.

Quit screwing with your TPS systems and jet your bikes correctly.

You'll also never foul a plug again. Won't that be nice? :banghead:

Couldn't agree more!

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