Technical Q and A for classic motorcycle maintenance and repair.
Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject.
1970 Honda CB350 Custom Wiring Problems
Q: I’m building a 1970 Honda CB350 custom with bobbed fenders, Superbike bars, painted wheels, etc. I’ve put in new rings, rebuilt the carburetors and so on. I want to ensure the bike is dependable, so I have tried to update or replace with new all the items I can. My coils and wires were questionable, so I wanted to replace them without breaking the bank. I was able to get some Wat Yong aftermarket coils from Dime City Cycles. The original coils on my 350 each had a black/white wire and the second wire on each was yellow or blue, respectively. The black/white wires both went to the harness and I assume were the source of power for the coils. The yellow and blue wires went to the points (right and left). The condenser was wired across the yellow and blue wires. Of course the plug wire on each goes to the plug. The aftermarket coils have a black wire and a yellow wire. I wired the blacks to the black and whites in the harness for power. The yellows I put to the points, left and right with the condenser wired between them. Am I in the ballpark? Does it really matter which way the wires on the coils are wired as long as the high voltage goes to the plug through the plug wire? - Lynn Metzger/Lawrence, Kan.
A: I think you’ve got it wired correctly. I don’t see how you can go wrong, as the primary side of the coil doesn’t care which wire is hot and which is grounded. At worst you would have the coils polarized backward. An easy way to be sure is the old car tuner’s method of holding a pencil between the spark plug lead and the plug. The polarity of the coil will be shown by a shower of sparks from the pencil lead. A spark going toward the plug is usually desired. In the past, this was done to take advantage of the thermal characteristics of the spark plug. The center electrode runs hotter than the strap. Electrons are easier to push off of a hot surface. That means it takes less of the high voltage output of the coil to produce the spark. If you have the coil polarized correctly, the spark jumps from the center to the strap. Polarized the other way, it’s just the opposite. Modern engines, both motorcycle and car, sometimes share coils among cylinders, firing one on compression and the other on exhaust. On those dual output coils, one output is positive polarization and the other negative. I guess they decided the cost savings outweighed the efficiency. Lynn, I’m glad to know your Honda CB350 custom started easily and is running strong after installing the new coils. MC