Lawnmower Problems?

Have a briggs&stratton 4.5 hp that will not start most of the time. When i pull the cord it locks and lifts the mower of the ground. very ocasionalley it lets me do a full pull of the cord and runs perfect.Have checked its not grounding,New plug,Fresh oil.Cleaned fuel tank,New diaphram,Checked valves, Piston ok,Spark good, Checked gap between flywheel and magnets, Checked flywheel key(thanks for that suggestion Smudge) Any ideas, Thanks Bob.