Feb 7, 2008
i was wondering why my 96 yz 250 keeps fouling plugs. i just put a new top end in and still does it. im running btw 32:1 38:1. would it matter on what kind of oil im mixing? when i run a b7es it last a little longer than b8es-b9es. thanks
Dec 8, 2007
Make sure your airscrew is set as reccomended in your service manual. If thats ok then replace your pilot jet with the next smaller size, if it keeps fouling do that over and over until it doesent foul or you've reached the smallest pilot jet size. You should also check your carb float level but it requires a special tool.
Last edited:


Mar 18, 2008

only two reasons i can think of, either you dont know how to ride a 2 stroke, which i highly doubt. or have you switched oils recently? i know my friend had his rm 85 up here, and he ran out of gas and used some oil we had here which was different then what he had used and the bike did not like the switch and he fouled something like 8 plugs in 4 hours, while i on my 250 have never fouled a plug with the oil i have here. not sure what type it is i allways forget.


Aug 8, 2007
Fouling plugs usually comes from rich jetting. This means that your carb is adding too much gas to the gas:air ratio. There are tons of posts on here about jetting, I recommend you read some. I'm not the best at jetting but try turning your air screw counter-clockwise a little (like 1/8 to 1/4 a turn). That should lean out your bike a little bit but the air screw only goes so far in terms of jetting. If you notice a lot of bogging after you do this, then turn it back clockwise until that bogging stops.

Hope that helps.


Mar 8, 2008

Flux3D said:
Fouling plugs usually comes from rich jetting.

That could be, but the pre owner of my bike a RM125 year of 1995 did have to change the plugs once per ride. He did run it if 30:1 to 32:1, while I run it with 45 to 50:1. He had the problem that the oil left over the pipe. I did clean the pipe up and ride it in hills, but with around 5000 RPM or even more as with less I couldn't get up there. :yikes:

Now, I never had to change the plug and the oil which leave the pipe is down to 2 drops per 4 hours how riding. I think that 2 strokes, like the RM125 or all others, like the higher RPM when riding, around 5000 to 8000 and in the MX even higher I guess. :nod:

just my 2 cents


Oct 4, 2003
the amount of oil in the gas has nothing to do w/ fouling plugs, its the amount of gas/oil the carb is letting into the engine. also if it is jetted right then dont worry about reving it up to prevent fouling the plug. your bike is jetted rich, reduce the jet sizes till it stops fouling plugs.
Top Bottom