There are three things that a 6 volt starting system MUST have- - - - a fully charged battery, BIG battery cables, and clean, tight connections. There is simply no room for error with only 6 volts to push the current through all those starter windings, particularly when trying to crank a very cold, or very hot engine. Here's a few tests to help you do some diagnosis.
First, test the battery. Your starter is very good testing device. With the ignition switch off, crank the engine for 15 seconds while a helper monitors the battery post voltage with a DC multimeter. After 15 seconds, WITH THE ENGINE STILL TURNING the battery terminal voltage should not drop below 4.5 volts. If the voltage drops below that number or if the cranking speed slows dramatically, recharge or replace the battery. Never, ever lug a starter down to the point it can't turn the engine- - - -as the cranking speed slows, the current draw increases until the internal windings get extremely hot and the starter begins to short itself out. After one 15-20 second episode of continuous cranking, the starter needs at least 20 minutes of cool-down time before another lengthy cranking session.
Battery cables- - - -6V systems need a minimum of a #1 wire gauge conductor on both the hot and ground cables. #0 is better. I build my cables from #1 arc welding cable, with tightly crimped terminal ends. Most welding shops can custom build those cables in any length you want.
ALL connections from the battery to the starter and back need to be scrubbed thoroughly with a wire brush, and tightened firmly. I like to clean and tighten all the connections about once a year- - - -more often if the roads in your area get salted in the winter. I cut my teeth on 6V electrical systems on a 47 Mercury convertible- - - -with an 11:1 compression ratio flathead V8! It was VERY unforgiving of even one loose or dirty connection!
Jerry
