Motorcycle Repair: ign prob 1 cyl. poor ign., 1980 honda cm400, engine fuel systems

Question
recently purchased an 1980 Honda CM400 T with an ignition problem on the left cyl. misfiring most of the time,comp good,fuel good but a spark problem.  Where should I start, obviously the plug itself has been repl. Any ideas?

Answer
Dave, the coil is a double-ended one, feeding both sides at the same time. Possibility of a weak coil, but I would check the spark plug caps for high resistance (should be around 5k ohms), if you are SURE that it is a spark issue.

Check the air cut diaphragm on the carb body. If you have a tear/hole in it, that creates a really lean condition, ripe for misfiring.

Go here for illustrations. Most of the CB/CM400s are similar in engine/fuel systems.

Bill Silver