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Motorcycle Repair/CD175 6V coil primary resistance

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Question
I just cannot find a factory spec for primary resistance for a 1968 CD175 coil.  It's a single coil that fires both plugs, 360 degree engine, 6V with points of course.

While quite possibly irrelevant, I did find a factory spec, in the Honda dealer service manual that I have for a 1981 Passport C70,  It is also a points ignition, 6V....and the spec for primary resistance is 1.35 -1 .65 ohms which surprised me.

I measured some other 6V honda coils of unknown provenance and got in the 1.4-1.6 ohms range....

Thanks one million percent!  I bought your 250/305 guide maybe 20 years ago and still refer to it often!

Eli

Answer
Eli,  the coil resistance figures are usually pretty consistent for any type of points type coil. The 250-305 12v coils are usually about 4.5 ohms. Theoretically, a 6v coil would have about half that resistance value, making them read out around 2 to 2.2 ohms, but I can see how they could dip down into the 1.6 range on a single cylinder model, as you have discovered. If it makes sparks, doesn't overheat and doesn't melt down the points, then the coil must be about right for your bike.

Coils are still subject to ohm's law and you don't want the current across the ignition points to be more than a couple of amps.

I'm glad you have made good use of my 250-305 guide!

Bill Silver

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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