Why is my 14 hp Briggs Stratton engine backfiring through the carburetor?

I have the same problem and I am going home to fix it this afternoon. The intake valve is not sitting perfectly tight against valve seat (block opening). It is slightly open when the plug fires it will create a small back fire through the carburetor I can only guess that the engine will run rough sorta when this happen. it is getting a little explosion in the cylinder, enough to stroke the piston, but will let some of the explosion be through the carburetor Take the head off and clean the area where the valve stem sits against the head. once clean, apply some toothpaste in this seat area and put the valve stem back in place and rotate back and forth to "polish" this seat area. do this for about 30 seconds. clean off the toothpaste and reinstall valve, spring, keeper, and head. make sure head bolts are tight. p.s. make sure that the valve is fully seated when the spring and keeper are installed, to make sure the valve is closed tightly to prevent any explosion bypass. God bless.