2004 yz250, has me stumped!

So I am at a loss. Bought an 04 yz250 2s last April in like new condition. Everything was great. Started doing coal hills, and dropping the bike frequently. During the first season, I noticed a slight stumble in 5th gear while at wot. Over the winter, I tore the bike down, rebuilt the top end with all yamaha parts, put in an n3ew needle set on 2nd position from bottom, and changed air screw setting to 2 turns. Adjusted idle, and bike ran like a raped ape! Couldn't have asked for anything more, and the top end stumble was gone. 2nd ride this year, I flipped the bike on a hill and the forks took a wicked shot on a huge boulder. I immediately noticed the top end lack of power and stumble had returned, only worse than before. Now I noticed it in 4th slightly and 5th bad, and got worse the longer it was at wot. Low end was still great so I continued to ride. The bike sat for 3 months until I fixed the forks, started up fine as well. But when riding it has the same issue, only way worse. I notice it in almost every gear now, and would have what felt like a Rev limiter at half throttle as powerband hit, and will not accelerate. If I roll onto the throttle easy in first and second it will Rev out a little farther but not much. Thinking maybe the race gas went bad from sitting and is making the problem worse? The only thing I can imagine caused this was the impact on the forks felt through the entire bike, or the fact it was upside down for about 10 seconds and running. I have checked tps resistance and is ok. Fuel shutoff sol. LOOKS ok but was not tested. Raised needle clip 1 spot to the middle, turned air screw in to 1 or 1.5. Helped a tad, but not much. Seems if I let it idle for a minute it will Rev out better, almost like a lack of fuel somewhere. Good flow frI'm tank, cleaned carb, air filter clean, checked all elec. Connections. Runs the same with both plugs from carb disconnected. Did I possible damage the powervalve and the bad fuel is making it worse? Seems to foul plugs almost imediately. And I shut off the petcock and bike revs fine in neutral but didn't not ride it like this. That tells me it is rich? At wot, it feels as though it is leaning out and not firing every stroke. Can somebody please help me make sense of this jumble of info? If the pv was stuck closed, at wot it would feed the engine more fuel than its capable of burning while having the flow restricted, I think? Even in 2nd and sometimes 1st, if I go wot to fast it sputters and won't Rev out almost imediately. I'm going to start with fresh fuel and plug, but I know the real issue will still remain. Please help!

Also, the bike has an fmf sst pipe installed at time of topend. After I flipped it, the pipe has no major damage, but seems to be tweaked a little (mounts are a little crooked) the pipe has a moose aluminum gaurd on it and could have sent a jolt from the impact to the jug. Performance wise, everything else is stock but the pipe and jetting, and the jetting was spot on perfect before I flipped it.it did have a low end rich condition and would drop idle and try to stall now and again. I didn't mind seeing as I only trail ride the bike, but is ridden at wot ALOT. Have only run vp112 with benol at 32/1 since rebuild/pipe/jetting changes. Last weekend when ithe ran so bad I couldn't ride it, I mixed my gas and my dad's kx500 gas and ran that. 32/1 and 50/1, roughly same amounts leaves me around 41/1. I planned on going to 40/1 like I have always ran once the topend broke In and I had to get more racefuel

Not jetting. You have mechanical issues, time for a leak down test, check of the power valve and crank seal.

Leak down as suggested, before you do that.... When was the last time you re packed the silencer? When was the last time you checked float height?

Not jetting. You have mechanical issues, time for a leak down test, check of the power valve and crank seal.

Do Yourself A favor and perform A leak down test. Sounds like crank seals

Never noticed mention of plug reading either.

Plug fouls rather quickly. I left the n3ew needle in the middle clip position, adjusted air screw, and put in fresh fuel, seems to rev out ok in neutral, will know more one day this week when I ride it. It only started the issue after I flipped it. The vp fuel I used last week when it ran so bad I couldn't ride it had been premixed about 3 months ago. Last night I tore it down and checked everything I could; timing, reeds, carb, float, wiring, piston, fuel tank and petcock, and air cleaner. Visually checked for cracks in intake boot or any signs of a leak on the mating surfaces. Did not test compression, but it only has 2 rides on its very first top end replacement. I used all new yamaha parts, including the d size piston. It doesn't seem to have lost any compression, and feels super tight. The pipe was ever so slightly tweaked, but wasn't leaking at the jug at all. Loosened the mounts and it fits perfect again. I moved the float hight up a tad, maybe that will help. It's never had the packing replaced, but I did make my own silence tube for the center, and drill my own holes through it. May have to pick up a shorty, just love the sound! Will update as to how it's runner after next test ride

i also think its the crank seal on the gearbox side . 


Have you been monitoring the oil level in the gearbox ?

They say pump gas starts going bad in 30 days, not sure on race gas. Premix kills it even faster. Only other thing I can say is to quit wreckin...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now