Small Engines (Lawn Mowers, etc.): 12 HP Briggs Backfiring, briggs and stratton, hp briggs

Question
OK, Mr.Ben Thanks for taking time to help everyone out on here.  I have done everything I can think of and still no go.  I have a 12Hp Synchro-Balanced Industial /Commercial Engine.  Cast Iron Bore Briggs and Stratton.  Model 281707 Type 0411 01 Code 89072611.  This thing was sitting for years, but was an awesome mower.  I have changed out the spark plug, magneto, generator, kill switch and all wiring from Briggs dealer.  I have rebuilt the carb ( I can rebuild a quadrajet carb with my eyes closed so this one was easy.)  My true problem is that it does nothing but backfire everytime I try to start it.  Sometime it will backfire through the carb and sometimes out the muffler while trying to start.  I think it may be the timing, but I havent changed out the crank or anything.  If this could be a solution, what is the proper way to gap the magneto from the fly wheel?  The old shade tree way of using a match book has proved useless.  Also, I have GREAT compression out of the muffler, good spark (it still tingles..LOL) Please help.

Very Respectfully,
Richard

Answer
Richard

You need to take a look at the fly wheel key and your valve timing and lash.
The gap is usually set at 003-008,Use a feeler gauge not a match book how cave man is that?
A lot of times the flywheel keys get sheared and causing the timing to be off.

P.S. get a spark plug tester like maybe your kids or person you don't like LOL