Just found your post, hopefully you solved the mystery, but here's my 2 cents. I had exactly the same problem with my 1953, 3600 with stock 6volt 216. cranked well when cold, then got weaker after engine had run a little while. I figured it must be the starter, so after a new starter, same problem. I thought maybe the engine was getting tight when warm/Hot. I even had a local shop tell me I needed to boost the system voltage to 8 volts to overcome the trouble. Know what it was... The ground cable was too small, that's all. These 6 volt systems pull a huge amount of amps when cranking, a too small electrical conduit will prevent the electrons from making their trip and the battery cables will get very warm or hot. I upgraded the ground battery cable to a 1 gauge (huge cable), problem solved. Haven't ever experienced that problem again. My other suggestion is make sure the positive and engine ground to frame cables are large enough, and that all connections are clean and tight. Hope this helps.
Last edited by mannymoejack; 06/13/2012 4:04 AM.
