what seal to fix air leak on CR250?

[tjohnsen]

I have been searching threads on the site trying to figure out what is wrong with my 95 CR250. It revs out of control after doing a top end and, re-sleeved jug, and seal replacement on the right side at the water pump. I think based on what I have read here that the bike has an air leak. It appears to be good at the cylinder base and intake boot so that leads me to believe that the seal on the left side is leaking causing the problem. I looked at the OEM parts finder on one of the parts websites and there are several seals on the left side. Could somebody please let me know which seal needs to be replaced?

Thanks!

 

[tjohnsen]

I little more info sorry, I don't know if this is pertinent to the problem.
This bike sat for about two years before I got it. I cleaned the carb and replaced the float. The first time I started after the fresh top end it ran fine with giving it minimal throttle. I let it cool down completely. Then the second time I started it I gave it about 1/4 throttle on and off....then it took off on me. I tried to kill switch no luck. I pulled the spark plug wire and that didn't work it was firing on straight compression. I ultimately pulled the gas line and waited for it to die!
I pulled the carb and went through it again...no apparent problems. I pulled the subframe so I could see directly into the inlet side of the carb and put my hand over it to choke it off it it took off on me again. I worked the throttle with the engine off and gas off until I had arm pump and the slide never stuck open.
I started it again and after a couple minutes of it warming up it started to rev out of control again. I choked it off and it has been sitting since.

 

[Matt90GT]

left crank seal. replace it

 

[mideastrider]

91205-KZ3-861 OIL SEAL (20X37X7)
Left side crank seal part#

 

[Pete Payne]

Best way to find the leak, remove the pipe and carb , plug the reed cage boot and exh port , adapt an air hose to pressurize the engine through the spark plug hole to 7 - 10 psi , then bubble-check the center case gskt , the reed cage area and the base gasket.

 

[tjohnsen]

thanks mideastrider!!

 

[nsxxtreme]

The reason its taking off on you is because the combustion chamber is getting so hot the fuel self ignites. Kill switch will do nothing when this happens. I find it easier to throw it in gear lock the breaks up and let the clutch out slowly. Engine will stall and die.

You have a lean condition most likely left side crank seal as others have mentioned. A leak down test is the best way to check for this.