2003 yz250f problem

I recently had my sons 250f totally rebuilt. We have put 3 races on it and now it is smoking out of the big tube that comes off of the top of the valve cover and goes to the bottom of the engine (I think it is called a crankcase vent). There is also smoke coming out of the exhaust and inside of the pipe is black. Rings, Valve seals, burnt valve? What might be the problem?

are you losing /burning oil? what does the dipstick reading say, or better yet, when you drain it is there a diff between what you put in and what you got out?

It sounds like you've got a bunch of blow-by gases and it's venting out the crankcase vent hose. Some is normal, alot of gases is not. Rings not seating properly would cause this.

It has just started doing this. I have not noticed any change in oil level at this time. The bike also seems to have lost compression and will not start without rolling it off.

I recently had my sons 250f totally rebuilt. We have put 3 races on it and now it is smoking

take it back to the rebuilder. something is not right -- could be valve seals, rings, scored cylinder -- the fact that you've lost compression means that you are going to have to take it apart. and the guy who rebuilt it should do it right this time, or give you your money back.

jim aka the wrooster

'01 wr250f

As a matter of fact I was just now able to get ahold of the rebuilder and he wants it back. He is going to run a bleeddown test on it and says that it is most likely a ring that wore improperly for some reason ar a bad exhaust valve seal.

ok, that's a little good news that he's being upstanding about it -- let us know how you make out. always nice to get some reference info for when the next time something like this happens. good luck with your fixes and your son's race series.


my money would be on valve seals or compression ring end gap.

jim aka the wrooster

'01 wr250f

As it turns out the aftermarket piston installed in the bike was cracked (some off brand piston that starts with an A). The mechanic changed it to a Weisco (no charge for labor) and it is fixed.


The other piston that he put in only had 1 compression ring while the Weisco has 2.

Seems like to me that 2 compression rings are better than 1.

I know which one your talking about. They use 1 ring to reduce friction and subsequent loss of hp. They are strictly race pistons and will wear the bore much sooner as the rings are also used to reduce piston slap.

Yea, thats all well and good but I prefer longivity and dependability. It is a YZ250F after all. How much more power could you possibly need?


It already has a big bore kit, a set of hot cams and a power now in it.


I used a wiseco piston myself. But there are plenty of money racers that want the race piston, enough so that they actualy made one.

Bike ran good this weekend (and held together). He got 12th overall out of a 35 gate in 125C and 10th overall out of a 21 gate in 250C on his 450.

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