Took my Murray 42" select out of mothballs from winter storage. Replace the battery, checked and cleaned fuel line, checked float valve and needle, cleaned the fuel bowl, filled tank with new gas.
Started first crank, but after a few seconds it began to backfire (shot flames) and continued until I shut it off.
Can someone help me with this issue on my Briggs 16.5 engine vert. mounted engine?

Either a timing issue, stuck valve, or carb problem.

Check the engine oil sump for any gas that might have leaked down during the winter

Where are we on this?