I also own a 1981 Honda 750, I had the same problem when I got mine. I did very extensive trouble shooting to find out that is the ROTOR, which is the rotating part of the alternator on these bikes. The good news was that when I bought a remanufactured rotor it worked great, the bad news was it was like 300 dollars. When the charging system on these bikes quit, it does not take them long to drain the battery, since it has to run the electronic ignition, headlight, tail light, and the starter. I am not sure if I am in time to help or not but thought I would mention what I have found. I have about 60,000 miles on my 750 and it has been through every type of weather. I think they are one of the best bikes ever made.
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