I inherited a snowblower from a relative. It says yard machine by mtd and has a 10 hp tecumseh. It has the carb with the fixed nut on bottom of bowl along with the spring loaded drain valve. I’ve had it for 8 years. Not sure how old it is.15 years? It’s always run well. This winter I started getting the occasional backfire and it has been getting worse. Seems to run good for 5 minutes or so then starts backfiring. When it’s under load and throwing snow it runs good. If it’s not under load then it starts backfiring. Fresh 91 gas. Always use 91 fresh gas. New plug. Cleaned the valve in carb bowl. When I look into the muffler I can see pretty steady blue flashes/flame. Not sure if it’s always been like that or not? I doubt it is supposed to.