Motorcycle Repair/CT90


I have (2) 73/2 CT90s that has gotten a the 80-81 CT110 engine installed. The charging systems seem to be different on the two engines. I have used the AC style coils so that the bikes can run with headlights on without the batteries with no issues. I have done this by some creative wiring under the seat. Shutting the bikes off was an issue so I wired the turn signal switch as a kill switch. "R" is for run. My problem however is keeping the battery charged I need the battery for dash lights and most importantly Tail/brake lights and horn. It works fine for a week or so and then we have to charge the battery up and it will work for another week or so. I seem to get plenty of voltage 6-8v back to the battery while the bike is idling and reving, but for some reason I can not get the bike to charge the battery. I am using the 6v battery from the 70vintage bikes. Up in the mountains where the bikes live running with the headlight on and no battery is more important than the taillights, but it would be nice to have tail lights.

Any advice would be greatly apprecaited. Oh and by the way I use a lot of your parts and literature for my 1966 Scrambler.


Mike, you certainly have done some creative work on your bikes.  First, I would have the batteries fully charged and load-tested. With only 6-7 volts to start with it doesn't take much to run them down with a full electrical load, especially when you have a bad cell in the battery. Most lead-acid batteries only last a year or two at best. If the battery is chemically sound, then look for other drains in the system. The big one to look out for is the brake light staying ON. The charging system can't keep up with that much of a drain going on all the time.
You can also use an ammeter to check the actual current output going to the battery. It is great to have 6-7 volts headed that way, but there needs to be some current delivery as well. Otherwise, it is a cell phone charger system. for some of the wiring diagrams for CT90s, but I don't see ones for CT110s.

OR go here....   more helpful.

Whatever you do, don't try to upgrade the light bulbs to higher wattage units, as the charging system is designed to maintain a normal configuration setup. Going brighter on the headlight bulb or adding other lights will throw the system out of balance.

Bill Silver  

Motorcycle Repair

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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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