Jetting & gearing Related?

Has anyone experinced this? Switching to taller gearing making the bike run richer & fouling plugs.

My friends 99 WR is almost stock. Stock pipe with core removed, airbox lid removed, trottle stock cut. Stock jetting. He rode the bike all last year this way.

He installs a 15 tooth counter sprocket & fouls a couple plugs in the last few weeks. Plugs come out rather black. He says he changed nothing else except the sprocket.

Would the taller gearing put more load on the bike? The bike would be spending more time at lower revs? He might be lugging the bike more with the taller gearing & shifting less?

Any ideas?


I think the lugging is it. But, I would guess it his JETTING in that range that is the real culprit.


99 WR, all YZ mods, de-octopused, OEM YZ tank and seat, jetting by Clark.

I aggree, tell him to move the needle 1 clip leaner and turn the pilot (fuel) screw in 1/4 turn.

Better yet, use a DVP or EVP carb needle. DVP should be used in the same clip he has been using, and EVP used 2 clips richer. These will both run slightly leaner when lugging the engine than the stock DTM.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now