If you have a weak cylinder, the chances of being the Magneto are very slim. When the Magneto is running it creates an electrical pressure (similar to a fuel pump). The full pressure is delivered by the coil to the rotor & hence to the cap-wires-plugs & finally the cylinder. If there are any carbon tracks in the cap or on the rotor, they should be replaced. If there are any holes/cuts or burn marks on the wires, they should be replaced. If you still have problems, leak down the cylinder with a proper leak down meter. As you leak down each cylinder listen by ear or open stethoscope to the intake-exhaust and oil fill hole for an air leak. If you still have problems and are running synthetic fuel check the injectors first, the nozzles for those elusive white rocks that are a byproduct of fuel being left in the system. After a race be sure to run the motor on gas to flush out the synthetic fuel from the system. Hook up a small tank or jug to the fuel pump inlet. Adjust the fuel available to the injection by partially closing 3 way shutoff valve. All it takes is a pint or two of gas to flush the system. 5 minutes of time after the race can save hours of frustration later.