2000 YZ 125 Bought it used. Road it 3 months. Changed nothing. Lost compression on full throttle pass. Piston melted on wrist pin sides. Top end rebuild. First full throttle pass after break-in.............
MELTED! Exact same piston damage. Electrode melted off plug. Put motor back together Checked for air leaks. Sprayed starting fluid on running motor. Carb boot, flywheel side, etc. No rpm rise. Did leakdown test. Tight. Lost 1 psi in 3 minutes. Changed crank seals anyway. Checked reeds. Had carb checked out and cleaned. Stock jets. Checked timing. Dead on. Power valve movement and linkage looks good. Fresh pre-mix. Good water circulation. B8ES plug. Rode bike for 20 minutes. Slowly. Checked for leaks, etc. First full throttle pass. MELTED. Has to be exremely lean condition at full throttle, to melt that steel on the plug!
.......................SOMEBODY HELP ME.........................