How good is the spark at cranking speed? If you're not getting enough juice to the ignition system when you crank it over,it might start when everything the plugs are at their best,butif the float level is a little high or if the float sticks once you shut down it might pull enough unburned fuel into the engine to foul some plugs,and with weak spark,when you try to re-start there's not enough spark to clear the plugs.
Speed
