1988 yamaha moto-4 battery issues

[Louis Angel]

Anyone have a clue as to why my old 1988 Yamaha Moto-4 250
keeps frying the batteries?
4 years ago: Installed new battery. 1 week later, fried. THOUGHT battery was
bad, under warranty swapped for another battery. About a week later, FRIED!
Too embarrassed to try to get a third battery, ordered a third battery from O'Reilly's STILL thinking the other battery's from Interstate were just JUNK.
NOPE! 3rd battery fried like all the rest.
I am sure the carb is gummy BAD by now. :-/
Knee jerk reaction now is voltage regulator is bad. Have to go drag it out of the barn and get it up to my house and see what the actual voltage is when it is running. Will be a few days before I can get that done as when not flipping houses or farming I am also a small engine mechanic ( certified through Kohler ) and I paint houses too. So , , , , yeah, , , I can be quite busy now that I have retired. hahahahaha! Love to you all --- ride safe!!!

[FastDoc]

I'd imagine it's the voltage regulator. I don't see what else it could be.

Start it up. If you rev it and voltage goes over 13.8 that's probably it.

[cheesy]

Originally Posted by FastDoc

I'd imagine it's the voltage regulator. I don't see what else it could be.

Start it up. If you rev it and voltage goes over 13.8 that's probably it.

Electrical issues are such fun to track down. A lot easier to trace with a DVOM, though.

You could go as high 14.5 without lights and be okay. With lights on, I don't think you'd want to be any lower than 13.5 at an idle. And the output should be pretty stable throughout the rpm range, plus or minus about half a volt. If you have got decent output at idle and the output jumps to 15, or higher, when revved, it's bye bye reg. Also, if your voltage doesn't go up past 12.5, it could be the reg or the alternator.

You could also have a phantom draw or a short draining the battery. A quick check on a bike without an ECU is to make sure the ignition switch is off and disconnect the ground cable from the battery. Tap the ground cable against the negative battery post. Nothing should happen. If you get a spark, you've got a draw somewhere in the system.

If it turns out that the reg is the culprit,(most likely) I'd suggest replacing it with a Shindengen mosfet unit. http://www.roadstercycle.com/ It'll give you a very stable output, in the neighborhood of 0.1v. But they are spendy.