Long Story

[Woodsie]

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.........................

 

[KiwiBird]

Ouch! Check your fuel flow rate, filter in the fuel tap, filler cap vent, carb jet blocked.

 

[Woodsie]

Gas flows well when its sitting there, line off, valve open.
Never thought about cap vent.
If you run outta gas, the motor wouldn't blow up. Right?

 

[Layton]

It might. Think about it; if the engine is out of gas then it has no lubrication.

Before you run the bike again put a really LARGE main jet in it. Then work your way down.

Back in the mid 80's I bought a ATK that had blown up two times. Not sure but lets say the stock main was a 270. I put a 330 in it and worked my way down until it ran good. Ended up with something like a 310 and never had another problem with it. Yes, that was far larger then stock but who cares. By the way, this bike was new and only had four races on it when I got it.

Good luck!