hot 500
No mystery on seizures, piston crown gets hot and swells up the piston diameter until its bigger than the hole in the cylinder and everything grinds to a halt.
Obvious question why did piston crown heat up . From picture there is only a hint of some past piston wash, so for sure fuel mixture was lean just before seizure. I would check with other snow bikers and get a handle on main jet range. Appearance of piston would lead me to believe that there is a definite lean area in the carb fuel delivery. Almost always lean mid range. So needle position very critical.......too far down/clip postion too high means lean running and bad for sustained load partial throttle opening.
If this motor seized being ridden as a snow bike and survived as a dirt bike with the same jetting, very believable, mortals cannot load a KX500 motor on the mx track or trail riding......maybe in the dunes, in the snow with cool air and sustained throttle position at moderate load, quick feeling lean mx jetting has potential to ruin a piston crown running in the snow.
Also keep in mind the age of your bike means it was never designed to be ridden with cheeeesy 91 octane 2014 poor quality gas..........all of which leads to detonation at marginal jetting partial throttle opening sustained modest load........for sure you need richer jetting across the board to compensate for todays chittttty gas. KX500 out of the box 15 years ago had deto issues anyway, bad gas/kx500head/sustained load/lean jetting/cool good air........all conspiring against you on an old lean jetted engine.
On the karts I tuned for guys with kx500 motors, we found that motor can swallow huge amounts of main jet gas WOT............same on any big piston diameter motor and thats what you have. So when in doubt make sure you have fat top end which makes hp, and tune low and mid range carefully.
Millions of pages of good big piston two stoke tuning tips available at dyno tech and try dale cutlers web site.
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