Check you have really good clean connections between battery and leads so that you are getting good power from the battery. Make sure terminals are done up tight.
Check if it is a Briggs & Stratton OHV engine that valve clearances are correctly set. They decompress on the valves just before top dead center and if the clearance is too large it just locks up as you describe.
Clearance 4 thou inlet 6 thou exhaust with piston 1/4" past TDC
eparkmowers
January 2009