Messed up piston and cylinder, looking for opinions as to how it happened

2 possibilities come to mind for me . . .

  • Piston started to melt due to lean condition - I don't think this occurred because of the very uniform shape of the damage
  • Update - I now see this is does not have PVs - I think it is lean - check carb boots for leaks, and carb jets, air passages, floats, etc. - Also seals

The other cylinder looks rich and almost washed clean.
 
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Sled is new to me this season so I dunno what happened before but the carb on that side was dirty as hell when I cleaned it... so probably lean. The other piston has 150psi and is most likely burnt from me pouring fuel in the cylinder to get it started and running on one cylinder before I knew I had a crap piston.
 
thats a classic lean burn down. see how the exhaust side is rounded off from melting? pouring fuel in the cylinder wouldn't have done it. you have an air leak some where or a plugged jet. check your pilots and mains on that carb.
 
The other piston is black from pouring a little fuel in it, not the burned down one. Good to know tho. Carbs are all cleaned and boots are good, so I should be good to go with my new piston and cylinder. Thanks!
 
The first time I cleaned it I just sprayed and prayed, didn't check for debris. The bowl was caked in brown stuff and some floating in the fuel that was in there so the jets were probably clogged. It's been thoroughly cleaned twice more in the last week. I will check to see if the crank seal spins with the crank. I'm just trying to make it to the end of the season, full overhaul of entire sled is going to happen over the summer.

I'm sourcing a cylinder right now, what can anyone tell me about exciter cast numbers? Mine says 569cc 86M00. If I can't find one with that same cast number, are there others that are compatible? I've already ordered a new piston kit at standard bore of 73mm
 
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86m00 is 89-90 Exciter. I will tell you that is a classic Exciter burn down, most likely due to a bad mag side crank seal. I have had two dozen or more of these sleds and 99 times out of 100 the mag seal will go bad and take a piston out.
 
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