i am overhulling a 12 hp briggs motor and when i try to crank
it the motor back fires and shears the fly wheel key. i thlink
i did not time it right.

wesley sterling, June 2005

i had this problem and i found that i had not tighten my fylwheel nut the right torgue.

jebvtx@sbcglobal.net, June 2007

It may not be the timing. Here are some other causes. Check these first. Lowering engine speed too fast. Gasoline, which contains higher blends of alcohol. Carburetor adjustments set too lean. Muffler construction can induce backfire. Higher than normal engine temperatures. If these solutions don't help check out this site.
http://www.briggsandstratton.com/miscpdfs/RNT/crankshaft-camshaft%20synchronization.pdf

cholbach@yahoo.com, June 2005