yz250 Short top end life + siezing up

The last two pistons on my bike have had a very short life (maybe 15-20 hrs tops) and they end with the engine siezing up. I'm assuming something is wrong with my jetting causing it to run lean. Also having some trouble with my idle which might be related. Once I have the engine warmed up and running for a bit it has a really high idle. When I initially start the bike and warm it up before i get on the track it idles fine. While riding the bike soon before it siezes it sounds like what I would sort of describe as a "tearing" sound coming from the engine. Running 40:1 mixture. Any ideas what could be wrong? I just ordered a new top end so I'll be replacing it this week but I need to figure out the cause of the problem. Thanks!

Check for possible air leaks.

How do you run the bike? Hard? I went from 40:1 to 30:1 and now I have less problems. Also the jetting went from 48/NP2/172 to 48/NP3/185

Do you have a play on the fly wheel or the other side of the crankshaft?


I ride/race mx so it does get ridden fairly hard. Previous owner raced harescrambles and ran 50:1 which I thought was crazy so I chose to run 40:1 but haven't changed the jetting. Will check for airleaks and crank play when I get home from work and I'll throw up some pics of the piston.

50:1 has less oil but more gas which cools the motor. Your jetting could be horrendously lean and/or you have an air leak.

Set jetting to stock if not already there. Stock jetting at sea level with 30:1 should not seize assuming all else is good.

+1 on what Shrub said.

What year bike are we talking about?

It really doesn't matter what ratio the previous owner ran his premix. Unless you are using the exact same premix and riding the same level as them, it is comparing an apple to an orange.

Lots of guys mistakenly believe that premix @ 30:1 is richer than 50:1. In fact the exact opposite is true from an oil/gas stand point. 30:1 has more oil and less gas than 50:1. The gas actually helps to cool the engine while the oil is lubricating the bearings and cylinder. But if you don't have enough gas, you will increase your engine temps and the oil will burn off before it can fully lubricate everything.

It sounds as if you definitely have a jetting issue or a possible air leak.

Stock jetting is a good starting point for 99% of riders with stock engines. Likewise, premium pump gas and a quality premix at 32:1 is good for most riders.

I run my '06 YZ250 with stock jetting, 92 octane pump gas mixed with Castrol TTS premix. Starts first kick cold or hot, idles smooth cold or hot all day, and has gobs of power. Now, my bike is bone stock (except for cosmetics) so if your bike has a modded engine or exhaust, you may or may not have similar results.

But, stock jetting, a fresh topend, and verifying that you don't have an air leak, should be pretty close for most riders.

Pictures of the damaged pistons would help as well. It is often possible to tell why a piston failed by the damage on it (i.e. 4-corner score mark means cold seize/improper warm up, burnt hole in crown means too lean, small pitting around outside of crown means detonation, etc).

The bike is an 07. What you guys are saying makes sense...I made the mistake of assuming 40:1 would be richer thus running my engine in a "safer" area. I guess I will plan on putting the new top end in, returning to stock jetting, and running 32:1. I'm also running a pro circuit pipe, what kind of jetting adjustments should i expect to make from stock because of this?

Show some pics of the pistons please.

I'm also running a pro circuit pipe, what kind of jetting adjustments should i expect to make from stock because of this?

Once you get the engine back together and the jetting back to stock, put in a brand new plug and follow the jetting FAQ. It will get you dialed in pretty quickly if you follow it properly.

Jetting is something that is affected by too many external factor (temps, humidity, elevation, riding style, etc) to have a "standards" chart for a given bike and rider.

Show some pics of the pistons please.

Will do when I get home, forgot to yesterday :thumbsup:





Piston definitely has some slight pitting

looks like no wash ie lean and deto is your enemy. you need a bit more fuel octane or a bit higher cc on the head. backing off the timing may help too.

Was the cylinder new with that last piston? It looks to have a lot of wear and blowby for being "low hours". Looking at the top of the piston edge on the side,that piston was rocking a lot and caught the exhaust port a little. Measure the cylinder at the small bridges at the exhaust side ,then compare to 'un-worn' areas. I bet there is at least 3 thou wear.

I see this alot....

Yes the cylinder was replated by powerseal. The piston I pulled out previously had similar blowby, pitting etc, but was on the stock cylinder I believe. My new piston should be in tomorrow, maybe I should send that along with my cylinder to powerseal so they can "custom fit the bore and piston clearance."?

What's your jetting currently at, out of curiousity? I didn't see it mentioned anywhere.

That looks like a ton of blow-by for such a short life on that piston. I am assuming by the look of that piston that the cylinder is toast?

Not sure, all I know is the previous owner was running a 50:1 mix and I've been running a 40:1 mix. Yes the cylinder needs replated, however it was running with the piston in the condition you see it in believe it or not. I just decided to take the pipe off the other day and check everything out and noticed that one bad area on the piston on the exhaust side.

While going from 50:1 to 40:1 IS leaner... it a tiny amount... out of a five gallon can of mix that's all of 3.2oz less fuel... I suck at math, but I think that would mean it's .5% leaner (yeah, a half of one percent). There has to be something else going on here, and it actually kinda sounds like an air leak, something like a blown ignition side crank seal (extremely lean jetting could also be to blame, but I think it would have to be really, really lean to burn out top ends like that).

Didnt get a chance to check the chamfering before I sent it away. How much do shops typically charge to do a leakdown test? Here's a few pics of the top of the piston and the spark plug.




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