YZ250 spark plug fouled after every half tank

[YZ250 kid]

i have a 1998 YZ250. The plug keeps fouling after about a half tank of fuel. i have tried 3 different plugs...NGK stock, Racing and Iridium. nothing seems to help. i also tried different premixes of 50:1 and 40:1 possibly if i try 35:1 i dont know.
please help!!!!!!

[Satch0922]

Sounds like you need to reject the carb. Go to the Knowledge Base Archive or do a search for YZ250 jetting. I would offer some advice but I don't know where you live. Altitude has alot to do with it.

[yz250guy]

It's not your plug or your pre-mix ratio. It's possible it's your jetting, even likely. What changed just before it started fouling plugs? How's compression? Just trying to get this thread started.

BTW: Welcome to ATM!

[YZ250 kid]

lets see i live in wisconsin if that helps with the altitude. my compression is just fine. before it started fouling plugs it was burning alot of smoke...before i started noticing it i thought it was something else and tried pulling it behind a truck tp pop start it well it only cracked once in a while. were finally smart enough to try a new plug. so yea probably is the jetting. please help!

[cr2504life]

If "it was buring alot of smoke" it may be possible that it is a crank seal. Did the smoke look white and have a funny smell, if so it probaly is a seal.

[Crogg]

I learned from doing dyno runs that when you use an iridium plug your jetting has to be almost spot on or they foul. The electrode is way smaller and easier to foul. Just for fun change to the stock plug and see what happens....or just rejet....good luck!

[Xidium]

My jetting is never spot on and I dont foul Iridium plugs anymore. And if you are fouling plugs try a different premix oil all togather. And remember, 50:1 uses less oil, so if you went to 35:1 you would have more oil causeing to to foul more. I would sudgest running it at 32:1.

[Crazy]

They will foul unless your on it hard all the time. If your putting around in the woods use a ngk b7es plug its the next hotter one and helps prevent fouling alot.

[Bad Dos]

I would check the crank seals like cr2504life said. How you do that, would be to remove your cylinder and look down into your bottom end. If there is excessive oil in there, your seals are probably toast.

Are you running low on motor oil in the crankcase?

[KTM250rm393]

A quick test for crank seals is to use either Bel-Ray Gearsaver 80 wt or ATF fluid Type F. If you shoot out white smoke out of the exhaust, you need to replace your crank seals.

If you don't want to do the above, drain your tranny and fill it to capacity by measuring it. Ride it until it fouls the plug, then drain the tranny and re-measure what you drained out. If its alot lower, its your crank seals.