2006 RM250 suddenly started idiling fast and bogs

Hey All,


Need a little help here with my 06 RM250 potentially diagnosing a problem before I tear the bike apart....Went riding in the hills of SoCal on Saturday and everything was great until about 3/4 of the day. When riding my bike I noticed that the idle went up for almost no reason at all and the bike was idling sounding like it was running out of gas. I stopped at the truck and went to put gas in and noticed I still had a half tank. I started the bike on the stand and it continued to idle faster than normal and everytime I snapped the throttle the bike would bog then rev up. I also noticed a decent amount of smoke coming out of the powervalve vent hose that wasn't previously happening. Any ideas on what could be causing this before I tear it down? Id like to have a targeted idea before I go into it but Ive never experienced anything like this before ...


Bike has been the same since last season, no jetting changes, air filter has 2 short rides on it....I assume the top end needs to be replaced as it has a decent amount of hours on it but never experienced this with a worn top end. Other than that the bike seems perfectly normal, no excessive smoking or other problems this just happened mid ride and I am baffled...


Any suggestions?

Idle rising for no reason is usually an air leak.

Probably the left side crank seal.

Idle rising for no reason is usually an air leak.

Probably the left side crank seal.


Is there any way to properly determine this with testing?  

Is there any way to properly determine this with testing?  

leak down test?

A leak down test is the best way

You will have to make plates to cover off the intake and exhaust on the cylinder however...


You can change a crank seal in that motor, and get it back in the frame in about two hours.

Spray some contact cleaner at flywheel. If idle changes its sucking air through seal. Usually when you take cover off or the flywheel itslef,you may see oil at seal. This is a dead giveaway its a bad seal. Another thing to check for is the carb boots. They can crack or come loose.

I have an update......so I pulled the ignition cover off the bike and noticed there was oil BUT it was from the clutch actuator seal and very minimal. Definitely NOT 2 stroke premix oil. Yesterday I fired it up and let the bike warm up nice and good and flat out ripped it around the neighborhood and the bike ran perfectly! I restarted it and ran it a few more times with and without the ignition cover on and I BLASTED contact cleaner behind the flywheel where the seal is and it didn't affect the idle at all......very strange. 


I did let the gas in the float bowl somewhat clean out and before I started it I leaned the bike over to refill the float and she seems fine. Maybe there was a speck of dirt in the carb? Also the powervalve vent tube was not smoking at all...it was very noticeable before when this happened.


I have a fresh top end coming and cant wait to ride again!!


Any ideas on what that may have been?

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