Small Engines (Lawn Mowers, etc.): 12hp ohv tecumseh 143.376012, compression stroke, airbox

Question
A friend just helped me rebuild (new valves, rings, and seals).  This Motor did run before but, smoked like crazy.  The motor will not start now, it just intermittently pops when turnig over, the air box is soaked with fuel, and every now and then the motor shoots backwards after turning over.  It has spark, the ignition coil is gapped at .010, the valves were gapped at .010 intake and .020 exhaust when piston was at the top of compression stroke.  we had all the gear marks lined up when reassembling the motor.  I'm an industrial maintenance tech and this is my first time rebuilding a combustion motor (a christmas gift for my son) i'm pritty disappointed in my self!  local shops are not being very cooperative.

Any help you can give me would be greatly appriciated
                                   thanks
                                    Mike  

Answer
Hello Mike,
 Sounds like you have a sheared or missing flywheel key.  The key holds the flywheel and flywheel magnets in the correct position so that the ignition will fire at the correct time.  You could also have a bad float or needle and seat in the carburetor causing the fuel in the airbox.