Revving out of control

Hello all, I just bought a 2005 yz250. When I bought it everything seemed swell. Took it home, started it up the next day and it revved out of control (maybe half throttle). Only thing that changed is the temperature was cooler but not cold, around 8c or 46f. The kill switch didn't worked so I yanked the fuel line. After 15-20 seconds (total run time) of bad screaming it died but didn't seize. I did some research and I think it was my fault, I didn't turn the gas back on and others online say if I turned my choke off it probably would have died sooner.  Did simply not turning the gas on cause this? Could it be something else? Everything seems fine, should I dive into the engine to check for damage? Ill probably do it anyway. Just looking for some input. Thanks.

Edited by willerd


Edited by willerd

Yank fuel pipe off too many times = starve engine of fuel/oil...runs lean = idle speed rises & seizes up - same thing when fuel tap's shut while the engine's running - same thing when accidentally running out of fuel/oi. If needing to kill the engine then put it in 6th gear & slowly let out clutch lever while holding both front/rear brakes.

Not turning in the gas valve wouldn't cause a lean problem to this degree for this amount of time. Ensure ur jetting is right and your throttle cable has no hang ups or snags.

Only time mine has ever revved out of control has been when I got the throttle cable stuck in the wrong spot under the tank somehow after doing some work on it. Took the tank bolts out, lifted it up, and all was well. Put the tank back on and never another problem.

I think there is an air leak somewhere.  What causes an engine to do this is an extremely lean condition, what happens is the plugs get so hot they turn into glow plugs and they start igniting any unburned fuel it can scavenge up.  How to stop this is to do what someone said previously which is to put the bike in 5th gear and drop the clutch or you can hold the kill switch and hold the throttle wide open and that should stop it.  But to fix the problem you need to find where the air leak is in your intake.

When it was revving out of control I was playing with the throttle to see if it was stuck, I don't think that is the problem. I just remembered that I did indeed clean the air filter but I don't think that caused it. How can I check for a leak? Should I first check the top end and if it seems good to start it again while its running spray some starting fluid in different places?                          I want to add that I bought the bike from a friend and I honestly believe it was running good before I bought it(but who knows). It always ran good, started up with one good kick cold, he hasn't changed jetting for a few years and he always runs yamalube 50:1. I did ride the bike down a short hill with the clutch in to get away from my house before I started the beast, I don't think that matters though. In the panic I also forgot to turn the choke off and I've seen some forums where people blame it on the choke 

Edited by willerd

I've heard of people doing leak down test to check for leaks. You could look into that on google or some one else here probably knows more about leak down test. The only way I've used is by spraying starter fluid where I think it might be leaking and seeing if the bike revs up.

next time it happens pull clutch in , click it in to 5th gear and let clutch out to stall bike

Well I took it apart and the crank, cylinder and I think its called the power valve were all full of coolant. I'm stumped, I guess my head gasket wasn't sealing. Its not in bad shape but its stretched a bit. I'm not very mechanically inclined but I can't think of or see anything else. Bike running away because of coolant? Seems unlikely to me..

Edited by willerd

It probably got to hot from running away so it blew the head gasket.

Well I'm going to flush the crank, I guess the only way to drain it is to pull the bottom end out so I can tip it upside down? There was a large carbon deposit on the piston head that I cleaned off. Piston looks brand new, cylinder is scuffed but nothing deep or that I can feel with my fingers. Thank god. Ill throw it back together with fresh gaskets then start it again and hope fresh top end gaskets solve the problem.. I might have a mechanic check the head/cylinder for warping 

I would agree that having a professional measure the cylinder and head is a good idea.

I found a rather large crack in my flywheel cover, I don't know how I missed it......  :banghead:   I guess this is where the air was coming in from. But can air even go through the flywheel cover into the crank? I'm hoping someone says yes so my headache can be over

The plastic cover? No, it's not relevant.


You need to make a leak tester and find your air leak.

Quiet common for yz's rev up when started cold with the choke. Closing the choke usually gets it back down, with maybe a little turn pf the throttle.


Its amazing how few people know what to do when their bike revs out of control. It is quiet a common issue with KTM for a totally different reason, but there are quiet a few hilarious videos out there of guys trying to pull the plug lead off a hot motor or the gas line while doing the funky chicken panic dance.

That was me ^

This is definitely what they call a run away. Did you determine whether fuel was on or off when this happened? I had a runaway on a jet ski with the fuel off. No clutch or gearbox to stop it, only quit after all the fuel ran out. That was with the spark plug disconnected. Luckily was on the trailer.

fuel was off

Was it damaged?

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