I used to have the same sled and on a pretty cold day the same thing happened about 100 yds from the house. It was just some water in the gas line and it froze one carb up...i wasnt sure what it was at first cause the plug on one side was fine, the other was white. So we towed it home, figured i blew something, but when it thawed out in the shop it was fine...put some gasline antifreeze and she was good.
anyway just an idea...if youre not in a heated garage...
Oil injection problem maybe?

But i'm not sure what it could be...just some ideas..

 

I had the same thought (frozen carb), but after 30 hours in the garage... same symptoms.
That carb does seem to blow back into the airbox however, and the scraps of metal in the airbox are confusing. Took apart and cleaned the carb... looks fine.

Is it possible that the piston skirt blew-out without damaging the cylinder walls... maybe partially clogging the exhaust port?

Thanks for the quick reply.

 

I would say it looks like you've blown a piston skirt. Take off the carbs, boots and y pipe and inspect the pistons through the ports. If it is indeed blown, make sure you measure the cylinders and (new) pistons for proper clearance.

 

I had the same problem. The piston skirt blew and I was finding aluminum on the spark plug but somehow the cylinder wall was perfect.