Motorcycle Repair: CT 90 Stalling and wont Re-Start, honda ct, headlight circuit

Question
i recently inherited a 1977 Honda CT 90, odometer read 335 miles on it, for atleast the last 20 years its been stored in a barn. so i performed a complete tune up. the bike startes on the first kick. after about 30 miles of riding it started to misfire in first gear on wide open throttle.
did the same in second, but third and forth seemed to be ok. but eventually started to misfire as well. the bike would stay running as long as i rolled off the throlle slowly. once throttle was completely closed the bike stalled and would not re fire. i had asked around for information and was told the stator more than like was the cause. replaced it with a known good one and after about 30 miles same thing happened. my low beam is burnt out so i drive with the high beam on. i found that when the head lamp is turned off the bike did not misfire. any help would be appreciated. thanks

Answer
Brandon, with only 336 miles, the stator should not have been the issue. The rectifier probably is the cause if you are having problems keeping the battery charged. I hope that you did install a new battery before you went driving.

The ignition is battery powered, so if the charging system is not functioning, then the ignition quits when the voltage gets down below 5 volts. The fact that the low beam is burned out indicates that someone ran the engine with a dead battery in it and the charging system went to full output and fried the light bulb and perhaps the rectifier.

Normally, when the headlight circuit is engaged, there is an additional coil from the stator that helps offset the extra current draw. I think that turning the headlight off took the extra voltage out, but also the extra electrical draw.

http://oldmanhonda.com/MC/WiringDiagrams/MCwiring.php for wiring diagram.

Be sure that the gas cap is in vent position, if that is an option and the passageway is open. Plugged gas caps cause vapor lock which adds an additional troubleshooting issue. If you were able to go 30 miles, it is probably okay, though.

Recharge your battery or buy a new one, check and/or replace the rectifier and you should be okay again.

http://www.bikebandit.com/houseofmotorcycles/honda-motorcycle-ct90-1977/o/m9471
for illustrations.

Bill Silver