My 1998 135 optimax fires right up at the boat ramp when I head off for a day of fishing but, when I stop and fish then try to restart the motor it cranks and cranks and sometimes back fires and is very hard to re-start. Seems to start right up when cold it is after using it that it is hard to start. I usually just leave it run all day since I am afraid it won't start back up. Any ideas why so hard to restart??