Why do flames shoot out of snowblower muffler?

Answer 1

The air/fuel ratio for the carburetor may not be adjusted properly, the exhaust valve may not be seating properly or the flywheel key may be partially sheared which affects engine timing. A cheap muffler can also cause flames and backfire when going from high to low RPM because it does not provide proper back pressure for the combustion chamber.

Answer 2

Possible valve problem. Most likely the exhaust valve is worn or burnt. Try lapping the valves & seats. Better yet buy new valves, and new valve springs if available, particularly the exhaust. Be sure when you take engine apart to keep all the parts for the intake and exhaust separate from each other, as not only have they got accustomed to each other due to wear, SOME engines the exhaust spring is stronger, and most engines the exhaust valve itself is different to withstand heat.

Inspect valve seat for damage as well. On some engines seats are replaceable, and can come loose as well. Also, check & adjust as necessary the valve to tappet clearance on a flat head engine, usually that means grinding off the valve stem, and on an OHV engine, you'll need to loosen a keeper nut, and adjust with screw or other nut. Study up online, or some old books on small engines at your local library & good luck. And yes, you'll need some special tools to do this or take to a shop.