The following information applies to opposed Twin and V-Twin engines.
1.Spark is going to one cylinder only. This problem can only happen on V-Twin engines. The problem could either be a bad magneto or a bad diode. There is one diode for each cylinder.
2.Fire is going to both cylinders, but one is still dead. To find out which cylinder is "dead" start it up and remove the spark plug wire from the cylinders one at a time. If you remove one spark plug wire and it does not phase the engine any, then that is the bad cylinder. The causes of a dead cylinder are the valve seat coming out, valve stuck open, low compression, or even a bad spark plug!. Pull the head off and see what happened. If a valve seat came out, then you must take the engine to an authorized service center and have them repair it.
You will not usually notice the power loss until you put a load on the machine that requires both cylinders to be putting out power. A twin cylinder engine will run with no problem on one cylinder, just as long as the load is not greater than what the good cylinder can put out.