I would get Bennet's points eliminator kit to eliminate one source of problems. Seem pretty reasonbly priced to me. Install prop 90 degrees to the magnet (dead sticks leave prop horizontal). Mark the magnet on the OD of the flywheel or prop spinner, someplace easily seen while in starting position. Turn the motor until you see your mark is just past the coil, then wind the spring back 1/2-3/4 turn and let it go. If my 42 is primed properly, it starts first time every time....

If your's is only running 3 seconds, it's likely just running on the prime, not getting any/enough fuel?