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Motorcycle Repair/Weak spark on 2 and 3

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Question
Hello, I am restoring a 1976 honda cb750, yesterday was just adjusting carburators, went for a little test ride, came back and bike shut off suddenly. Checked with spark tester and 1 and 4 are strong, but 2 and 3 are weak, almost no spark. checked coils and both give from 1.1 to 0.9 ohms. Motorcycle was running strong before it died, what can be the problem?

Answer
Judas, a 1976 Honda CB750 came with a points and condenser ignition system. The coils for that type of ignition require 4.5 ohm coils. The coils you describe are for electronic ignition systems, which use a much lower coil impedance rating value. Points and condenser ignitions draw a lot of current through the coils, so low resistance versions will usually overheat and die.

http://www.hondachopper.com/garage/troubleshooting_charging_system/troubleshooti

http://blacksquaremotorcycle.com/ignition-coils-test/  scroll to the bottom of the page for the primary coil test readings. All the SOHC Fours have similar coil types. DO check the resistance of the spark plug caps as they can fail over time. Should be 5k ohms from one side of the cap through the other side.

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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