Thanks for the helpful replies. It looks like I had a marginal battery. I went to
TSC and bought a full size battery. I now get 13.5v at the battery terminals
and it only varies from 13.3 - 13.5 between min to max throttle. Also, the tractor seems to run better and has a higher full speed.
In regard to the regulator, I'm still not sure. As cubs-n-bxrs pointed out, there is a unit called the Regulator/rectifier but from my electronic experience
it looks like a full wave rectifier. To me, the voltage regulator should control
the field current to the generator in order to regulate the output voltage.
I really need to get the service manual.
Here's a link to B&S alternators that I found useful
http://www4.briggsandstratton.com/mi...eplacement.pdf
Page 25 of the pdf shows a general wiring diagram which seems to apply to the
1641 and there is nothing showing how the field winding is controlled. My knowledge of regulators is probably just out of date.
Thanks again.