Did you actually send it into a highside or did it do it by itself?
I was racing at Mosport with an RZ350 a few months ago, and we needed more top end speed, so we talked to a guy who had built a 150mph 350, and he said to take out the intake baffle. We were already running a bit rich, so taking out the baffle and leaning it out should've been perfect.
Flatout down the backstraight at 120mph, pop out of the bubble for the right hander at the end of the straight and the motor seizes...
Managed to keep it from sliding to far out, (dirtbiking xp lol) and pulled in the clutch, but I didn't know what had happened so I let the clutch back out and started sliding again.
Finally got'er stopped in one piece though.
Turns out that leaning it out was actually fine, the right plug was the perfect colour, but the rings on the left were worn so much that the pulse from the 2-stroke exhaust was getting back up through the rings, heating up the front of the piston.
When the piston was removed, to top part of the dome was gone, and the rings were melted into the piston.
The offending piston and cylinder.
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(Thing is, the bike had picked up a few hundred rpm after we did the change, I guess it's true that they're always fastest right before they blow.
)