What are all the variables that would cause the top of the carb to catch on fire while trying to crank over the motor? My guess is timing being retarded too much. But what else?

valve adjustment or timing is my guess.

Incorrect firing order. Cam chain worn out and has slipped

for me... i put the sparkplug wires in the wrong way... instead of 18436572, i did 27563481

Timing advanced too far, incorrect valve timing, broken pushrod or rocker arm, sticking exhaust valve.

Mike

Distributor can be 180* out of sync.

But I didn't do anything to the motor except change the carb from a Demon 850 to a Demon 750. The car is in winter storage right now. I changed the carb about 2 months ago. And it started doing it when I fired it up with the new carb, to check and make sure it worked.