2002 YZ125 Engine Problems HELP!!!!

[calistang1966]

my brother has a 2002 yz125 that has been having some problems lately. the engine went down once because a circlip came loose and ruined the cylinder and piston. we had the cylinder replated and put a new piston in. we put it all back together and it ran fine for 5 rides except for a knocking noise in the engine at low rmps but would leave at high rpms. after 5 rides the bike just died again and so we tore it apart and the ring was stuck. so after replacing everything we got it running again for about 10 min and it went down again!!! the knocking was still there and we cant figure out why and now it wont start.

we tore it all down again and there was nothing wrong. so we took it to a shop and showed the guys and they thought it could be the lower rod or bearing was out but after checking that with them it was fine. we also checked the bearings behind the side case and they were fine. the guys at the shop were dumbfounded and could not figure out what the problem is.

is there anyone that has had a problem like this or has any suggestions on what is wrong? what could be causing the knocking sound? why cant we get it started?

PLEASE HELP ANYTHING WILL HELP. thanks

 

[Zeuski]

what kind of octane do you use for it?, i have a 2001 yz 125 and i remember i just wanted to start it and i mixed some regular gas that i had for a weed whacker with some premix and it was knocking and pinging. Once i put in high octane premium gas it was gone. I talked to the guys at the trails that i ride on, if that has ever happened to them and they said no cause they always use high octane. Those engines thrive on high octane, one of the guys even adds octane booster to it.

 

[TimberPig]

The knocking noise, plus the stuck ring after the first rebuild =failed lower rod bearing. The bearing is loose, and the ring was likely stuck by a piece of one of the rollers that came out of the bearing. Unless they have completely torn it down and inspected the crank, a quick check with the cases still assembled may make it appear fine, when it is actually gone.