** YZ 125 Engine Problems

[mattyp]

hey guys i'm new to these forums, looks like a nice solid community. Anyways, i have a 2000 YZ 125 that toasted the top end a matter of probably 5 hours of riding time after it had been rebuilt, i bought it in this condition for a really cheap price, and the guy has receipts for the rebuild at a local reputable shop (so it was more than likely done correctly).

When i went to tear down the top end i instantly noticed that the piston had some scoring on the exhaust side, and the ring is melted to the piston on that side, i cant get it off the piston AT ALL :yikes: . The other issue that i ran into is the cylinder is scored on the exhaust side and also has a little chunk missing on the intake side.

i was wondering what would have caused this problem as i dont want to have the cylinder re-plated and rebuild the top end and have the same problem happen.

I checked out the reeds and they look OK.

I was thinking it could be a dirty carb maybe or even improper fuel/oil mixture.

I am really at a loss here as to what could have caused the problem, so id appreciate your input greatly, thanks! :nod:

 

[todd36]

It sounds like it was running way to lean . I would clean the carb,verify the jetting,check the timing and give it a leakdown test before starting it after the rebuild. Go with a 32:1 gas mixture . You probably are going to have to split the cases to clean any foreign material out of the bottom end.

 

[Coop]

I agree, sounds way too lean. What does the plug look like?

 

[mattyp]

the plug looks really really dry and really clean, almost like they stuck a new plug in it after the piston went because they didn't know what happened.

thanks guys i was hoping it wasn't anything too major, kinda verified my suspicions.

how do you go about cleaning out the bottom end, just a visual check or what?

 

[CarnageZ28]

Well, you have to split the cases to clean out the bottom end. If you never did this before, get a tech manual for the bike. Its really not that bad once youve done a few of them though. Alot of manuals will say to use a special holder to hold the inner clutch hub as you break the nut loose, if you have a compressor, usually you can just use an impact gun on this, no need for the hub holder. But yea, once you split it, remove the crank, wash it in a parts washer, thoroughly. And be really careful checking the crank bearings. They are what just toasted a brand new top end on my RM ...

Once you clean the crank, the cases are nothing special, spray with break clean, wipe with a fresh, clean, lint free rag. Get all the pieces out.

Lightly coat all the new parts with oil. And dont cheap out on it, buy new gaskets. Or at the very least, use "The Right Stuff" as it seems to be the best silicon on the market right now ...

 

[mattyp]

right on, thanks.

ill get on that here in a week or two since we still have snow on the ground :pissed:

if i run into any problems ill probably be poppin up here again