New key on flywheel. Has spark. New sparkplug. Carberator cleaned thoroughly. Set fuel mix needle at 1 1/2 turns out. Installed new drive and starter drive. Adjusted valves. Cleaned valves and piston face. Still backfires out filter housing and exhaust. What are some solutions?

Hey everyone I've been working on moweres along time hope to help hear. Check lower end of push rods

Umm, could be the flywheel key, even though you just installed a new one, check it anyway.