I picked up a 02 YZ125 for my son, I know the folks personally, they had only put a couple of hours on the bike since the rebuild. The bike came with receipts for a complete rebuild of top and bottom end with a renickeled jug, from a reputable shop. They set the ring gap at 18 thous.
The jetting is all stock with clip on middle notch on needle, low speed set at 1/5 turns. We take the bike out after purchase, warm it up and ride for 5-6 minutes fairly hard. Stopped the bike for a couple of minutes, start it up and idle away a few feet and it stops. Not compression. I pulled it apart and found a broken ring on the exhaust side. Piston was still mint so I installed a new ring and reassembled. The bike ran well for maybe five tanks of fuel, then, after a hard ride, the bike is stopped and left for about five minutes then no compression. Pull it apart and find the exhaust side of the piston is burnt slightly. I then install a new piston and ring and reassemble with ring gap at 20thous. I figure to lean so I lower the clip on the needle and set the low speed so I get a good idle. Take the bike out and break it in. Take it to the sand pit and run it around a bit (10m tops) and come to a stop for 10m or so and again, no compression. I haven't had it apart yet but I'm thinking burst piston again. I haven't checked the plug yet either. The last failure the plug looked a nice brown.
I don't know what to do? The bike has an FMF pipe and silencer on it. The rads have a bit of a bend, but one can still see straight through them. The piston appears to be melting after it stops. The motor doesn't seem to be running any hotter than other bikes.
Anyone have this problem?
The jetting is all stock with clip on middle notch on needle, low speed set at 1/5 turns. We take the bike out after purchase, warm it up and ride for 5-6 minutes fairly hard. Stopped the bike for a couple of minutes, start it up and idle away a few feet and it stops. Not compression. I pulled it apart and found a broken ring on the exhaust side. Piston was still mint so I installed a new ring and reassembled. The bike ran well for maybe five tanks of fuel, then, after a hard ride, the bike is stopped and left for about five minutes then no compression. Pull it apart and find the exhaust side of the piston is burnt slightly. I then install a new piston and ring and reassemble with ring gap at 20thous. I figure to lean so I lower the clip on the needle and set the low speed so I get a good idle. Take the bike out and break it in. Take it to the sand pit and run it around a bit (10m tops) and come to a stop for 10m or so and again, no compression. I haven't had it apart yet but I'm thinking burst piston again. I haven't checked the plug yet either. The last failure the plug looked a nice brown.
I don't know what to do? The bike has an FMF pipe and silencer on it. The rads have a bit of a bend, but one can still see straight through them. The piston appears to be melting after it stops. The motor doesn't seem to be running any hotter than other bikes.
Anyone have this problem?