I assume that is with the points open???. If not, open the points then trace the voltage from the battery to the points. Start at the terminals on the junction box on the firewall. Unscrew the wire into the distributor, if you can, and check the voltage there. When you put it back in don't screw it in so much that it shorts out. If you still get low voltage with the points open then try a new condensor and check the lower plate and the wire that leads to the upper plate in the distributor.
You should be getting zero voltage with the points closed. If not then the points are dirty and need to be dressed. I use a folded up piece of 400 grit wet and dry sand paper with the points closed and then clean the points afterwards with a clean piece of printer paper. There could be other problems too.