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Motorcycle Repair/1970 honda ct90 trail bike

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Question
Im getting good spark at the points but no spark at the sparkplug last time it was started it was cutting off but it made it home and hasnt started since could it be the rectifier

Answer
Donald,  The bikes are battery powered so you must have a fully-charged, load-tested 6v battery in the system. Once the voltage drops down around 4.5-5 volts the coil can't make enough energy to fire the plug.

OTHER issues can be a bad condenser/loose wire connector at the coil, a bad spark plug cap, dirty spark plug that is arcing down the side of the insulator, insufficient point gap (set to .014" at maximum opening phase).

The rectifier only changes the AC voltage from the stator to DC voltage to charge the battery. If the bike runs okay for awhile with a charged battery but starts dying when you have the headlights on, the rectifier could be bad. After 43 years, it may be time to change it out.

Always start with a good tune-up... adjust valves (.002" on compression stroke), clean and adjust points (points OPEN at the F  mark alignment with the mark on the stator), service air filter, use a new spark plug, make sure all wiring connections AND GROUND connections are clean and tight. Check compression to be sure that you don't have worn rings/burned valves.

The spark advancers on these bike scan seize up, so I would remove the point plate and see if the weights are free to move and return under spring pressure, then adjust gap and set ignition timing.

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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