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Motorcycle Repair/Checking Spark Plug Boots 1972 honda cl175 k6


I checked the ohms on my spark plug boots and one read 8.38, the other 17.6.  Obviously, they should both read the same. Could this be a clue to my bogging down at high speeds issue?  The bike has 150psi compression on both cylinders, and Iíve only put 100 miles on it since having a tune up by western hills Honda.  Should I replace the coil based upon these readings; or just the boots?

James, the nominal resistance value for most m/c spark plug caps with built-in resistors is 5k ohms. If you have non-resistor caps, the readings should be near zero. If your readings are on the lowest scale of your meter and are in the single/low double digit range, they are probably okay. If you are reading K ohms scale, then they are out of range.

Honda lists 153 to 187 psi range for compression readings, so if you are accurately checking the compression with the throttle held wide open, the engine seems on the low end of the scale. I generally see about 175 psi on most good-running Honda twins.

You didn't state the mileage on the bike/engine or whether it has been apart previously or not. Even and low readings usually indicate either worn rings/cylinder walls or a camshaft that is one tooth out of time.

There are index marks on the points plate and when the bikes came from the factory, a point setting of .014-.016" should put the point backing plate somewhere in the middle of the adjustment slot. If you are finding the point plate off to the end of the adjustment slot in order to get the correct ignition timing to line up, then the cam is off a tooth.

Inspect the spark advancer for proper function (advance/return) to ensure that you are getting full spark advance at high speeds.

Finally, I am finding that use of today's gasoline is leaning out the engines with stock jetting, so going up a step on the main jets can help recover some top end power.

Bill Silver

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Bill Silver


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.


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.

VJMC (Vintage Japanese Motorcycle Club) of North America

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

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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