On my 1949 3600 (12V conversion, 235 engine, SM420) I get the same thing after it's sat a bit in the winter (wet and cold). My generator is original with the 235 (from some year in the '50's and never rebuilt) and my mechanical regulator is a two year old questionable reproduction that wishes it was a Delco-Remy.
Anyway, I get the same symptoms. A stiff whack on the cover of the voltage regulator gets it charging again, so that tells me it's cranky from sitting and oxidizing. Thereafter, cleaning all three contacts with very fine grit makes it work all summer just like it's supposed to.
I don't understand electrical magic like Jerry does, so I just clean up all three contacts. One of them is responsible for knowing the voltage to the battery and when it's time to turn on and off.
Always give it a whack first. If that doesn't work, call Jerry. ;-)
