Strange electrical problem

[ZMX]

I just picked up a 1982 CB750 last week.  Bought a new battery for it, and by Friday it would not hold enough power to start the bike, and by Saturday it wouldn't hold enough to even power the ignition.

Assuming I was sold a dud battery, I got another, and it's already starting to struggle with the starter.  I tested the alternator with a voltmeter before I got the bike, and it showed 14.x volts when the engine was revved.  Also, when revving the engine, the headlight gets brighter.

I'm not sure what the issue is, so I'd appreciate some advice.  Thanks!

[Re-run]

Well I had a car that would do things like that. Faster I went, fast the tapedeck was, brighter lights, ect. A lot of time a voltage regulator is the culprit. You may need to adjust it or replace it. be sure to check the wires too. that would be a start. I know the dohc models in the early 80s had stator issues so you might want to dig up a manual and find out how to check that.

[ZMX]

Manual is in the mail.

[Rick V]

It sounds like you have a drain on the battery when the bike is sitting. You said that you got 14 volts when you revved the engine , so the bike is charging the battery. The  headlight will get brighter when you rev the engine because you are going from the battery voltage at idle to the full charge rate. You need to connect an amp meter in series with the battery to check for a drain .With the key off disconnect the ground cable and connect between the battery terminal and cable, if the meter shows any reading you have a drain causing the battery to lose power. Does the bike have any accessories like a clock that runs all the time? If you have a drain try unplugging  wires until you see the meter showing zero and that will tell what is causing it.

[woodsrider250-2]

These bikes will drain power...Mine does anyway(in about 2 weeks won't start).  Most will keep a simple float charger on them.  Charging system is not enough to charge battery unless you're going to go on a big trip....Generally just enough to keep it charged.  Next battery you get, hook to low charge for 24+ hours to get to peak charge, then see how she does...

[ZMX]

I got it worked out.  This thing has been ghetto rigged and is in a serious need of rewiring.  In one place, a fuse has been replaced with a wire... Dear Jebus Retarded.

The problem with the bike not starting is caused by the wiring to the starter being so worn down that sometimes it can't flow enough current!  Kind of scary...

Right now, the fix is to turn the ignition to "P" (Park, I assume, but I can ride it around like this), remove the key, then use it to short the connection for the starter.