Small Engines (Lawn Mowers, etc.): backfire 16.5 b&s, briggs stratton engine, craftsman lawn mower

Question
I have a craftsman lawn mower with a 16.5 Briggs & Stratton engine. It has started Backfiring through the carb and will not stay running. I checked the key way it seems fine.
My son used it last and it poured rain at the end of his cutting. Now I go to use it and it won't start or runs as if exhausting though the carb no higher than an idle.

Answer
Backfiring through the carburetor is usually caused when valve timing or bad valve clearance is off.  It might be while unit was not being used the carburetor overflowed and filled your crankcase.  I would check the oil level, and if it is too high drain the oil and refill to proper level with fresh oil.  If this is the case, you should check the carburetor inlet needle and seat to be sure that there isn't anything blocking, the needle from seating or that the float as not failed.

If there is gas in the crankcase and the carburetor seat is okay.  You have a crankcase air leak.  It can be the crankcase gasket, crankcase seal, crankcase breather or breather gasket.  Check the dipstick to see if it is loose, where it goes into the crankcase.  I hope some of this information helps.