Small Engines (Lawn Mowers, etc.): ariens st524 5hp snow thrower, snow thrower, last winter

Question
Bought this blower used last winter. It fires right up on first pull every time and runs great for first few minutes. Then about every min or so it backfires. After about 10 min of use the muffler glows red. The rpm's go up and down as well. Where should I start?? Thank you for ur advice.  

Answer
With the muffler glowing, I would guess it's a little out of time, probably the flywheel key is partially sheared. The rpm problem is probably in the carburetor, but first I would fix the other problem then address the other problem if it still persists.