This information has been posted on the VFR List in various forms this last year. Some Viffers have reported voltage regulator problems with their VFR750F's from model years 1990 to 1993 especially after 18,000 miles or so. Generally, the symptoms seem to be something like a bad battery, weak lights and other electrical problems. Some riders have said it occurred all at once and for others the problem comes and goes. Sometimes the battery is allowed to fully charge and other times the battery is weak.
Henrik posted this very detailed message to the VFR List which was especially helpful.The rectifier is a little size unit (pack of cigarettes) that takes the AC voltage from your alternator and converts it to DC. It's located under the seat and mounts directly to the frame (and gets pretty hot since the frame is a heat sink). Anyway, these units are known to fail and cause other things to break. As the rectifier begins to fail, it may put out too much voltage on the electrical systems of the VFR. Usually, this will literally cook the battery on the bike..."Hon, what's that sulfur smell coming from your bike?" Some sloppy repair shops will just replace the battery, only to have it fry again. Eventually, the electronic dash and alternator can also be damaged by a bad rectifier. I've seen this happen on a friend's '87 VFR.
To check the rectifier you will need a voltmeter (a cheap Radio Shack model will even do). With the engine turned off, check the battery to see if it puts out 11.5 to 12.5 volts. Then turn the key on, but don't start the bike. The voltage should be about the same. If your voltage is low or drops more than 15% then the battery is probably shot (one or more of the battery's cells are ruptured).
Now start the bike. Check the voltage at idle and at 4000 rpm or so, and the voltage should be around 14.5 volts (this is the charge voltage level). If your voltage varies a lot with rpm (like going as high as 30 volts!) then the rectifier is broken and needs to be replaced with a new OEM or a substitute since a used unit may not last long. Rectifiers do age and wear out even though there's no moving parts. It's the temperature and usage that causes them to age. Honda bikes are known to have weak rectifiers, at least compared to other components on the bike. The replacement rectifiers are reported to be redesigned and are stronger than the OEM part.
Good luck and safe rides
-Henrik '95 VFR750FS, Palo Alto, CA
