Small Engines (Lawn Mowers, etc.): snowblower, craftsman snowblower, valve clearance

Question
I have a craftsman snowblower w/ a snotek B%26S engine last year started backfiring and actually lit on fire @ one point replaced all external parts including hoses that were damaged, carb choke etc etc.verything was torqued to spec according to serv manual ever since this happened it dies 10 min after running then will restart then die again. do you think the ex valve is screwed or could it be something altogether something simple that I missed?

Answer
When it quits, check for spark by removing the spark plug.  If no spark replace the coil

Backfiring sounds like a timing problem...There could be a partially sheared flywheel key, a bad camshaft or possibly improper valve clearance.  These are the things i would check.

I have never seen a sheared flywheel key in this application, but its possible.

Do you have fresh fuel?  No water in the tank or carb bowl?

Hope these things help.