I am in the restoration of a 1975 Honda CB500T. I got it running yesterday, but noticed I only have one cylinder firing. I tested the points and they were in spec and ran a Ohmmeter on the coils independently and one was pooped out the other was weak. I also noticed they had broken spots on the spark cables and since they were “no good” I cut the rubber on the cable and sure enough the copper wire was broke and was to blame for the coil failure. I retested from the break point and got a better reading. My question is since these are no longer available from Honda OEM what are my options? A used one from ebay sounds risky seeing they are old as well. Can the wires be repaired if the coils are fine? Would an after market 5 ohm coil work?
Thanks for any responses,
On most of the old Honda CB’s the coils and spark plug leads are molded together so the wires are not replaceable. If this is the case you do have a couple options.
I have read reports more than once from people who have converted their coils to use replacement wires. You can cut back the wire so there is just a little stub then solder/jb-weld on a cap so replacement wires can be used. This can be done with both motorcycle or auto parts. I haven’t personally done this, but see no reason why it wouldn’t work, and would certainly be the budget conscious choice.
The other option is of course to buy new coils. Classic Motorcycle carries generic/universal coils for old Hondas and I’m sure you could dig them up other places as well. 5 Ohms is correct as long as you are using a ballast resistor. I can’t recall for sure if the CB500T had one factory or not, either way it can be easily setup if not.
Are you sure your bike doesn’t already have replaceable plug wires? They appeared at different times on different models, but here is a coil from a CB500T with replaceable wires. It also has a ballast resistor wired in.
New spark plug caps and wires for the CB500T can be found many places, Parts n More is one such place.
Thanks for the help!
That looks just like the set I have, but I don’t think mine are removable as far as the plug wires go. I have heard about the ballast resistors before. What exactly are those in the picture? I am not too familiar with how those work or where they are. Would my set have one wired in already then? I can upload a picture, but it looks just like the photo you posted.
Pardon my haste – the photos above show a condenser (bronze tubular piece), not a ballast resistor.
A ballast resistor is a simple device used to add resistance to a line. In the case of moving from 3ohm rated coils to 5ohm rated coils you need to use a ballast resistor to insure the coils don’t overheat. A ballast resistor rated at 1.5ohms would be suitable for this application because they tend to over perform once they’ve heated up. It is simply wired directly inline with the leads to the coils.
Now, we are talking about all of this, but I’m not sure for certain what the factory coils were rated at. Most points ignitions have factory coils at 4 or 5 ohms. It is typically electronic ignitions which run at 3. You should verify for certain what factory coils are rated at.
Here is a Honda CB500T wiring diagram to help:
Thanks for the posting! It is nice to see the wire colors as well that really helps keep things straight. I have 4ohm coils on the way. Since I need 5ohm resistance for the honda CB500T set up here to keep them from burning out early I will need a 1 ohm resistor wired in. The correct placement for the resistor is in line on each coil on the blue and yellow leads after the connection with the condensers correct? I would have liked to find a resistor at .75 ohms or something just less than one to compensate for the over performance, but i can’t find anything rated at least 20 watts or higher at that low of resistance.
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