Will not start now? Ran great for 30-40 minutes

[SHINTON]

Just a question - and I am not sure what all dad / Gary are buying here, but is there any reason to NOT switch to NEGATIVE ground? (So our brains won't hurt as much when diagnosing, heh!?)

[Willyr]

The ground polarity depends soley on the charging system. Everything else could care less. 6 volt systems run on 12 volt, just cant expect 12 volt to run in a 6 volt application. By this I mean if you have a 6 volt system in place and go 12 your fine. You cant use a 12 volt system / wiring harness and expect to take it to 6 volt (wires are bigger on 6 volt).

[shinnery]

Most (NOT All but most) alternators are internally wired to work in a Negative Ground system. They will only put out a current from a positive voltage at the screw terminal of the alternator. So to use most alternators you have to wire the complete electrical system as negative ground. You connect the screw terminal of the alternator to the positive terminal of the battery and then to the ignition switch. The terminal of the coil that is the same as the ground system you are using should be connected to the wire from the distributor and the other terminal of the coil should be connected to the ignition switch, this is to reduce arc buildup (pitting) on the points.
Bryce

[Tigerhaze]

You may want to look at the starter solenoid if you switch from 6V to 12V- I have read that sometimes using a 6V starter with 12V causes the pinion to slam into the flywheel until the solenoid is replaced with a 12V.

However I am not sure on that starter if the solenoid is replaceable or not. Lavoy or others should be able to indicate if that is an issue.

[Lavoy]

It is just the voltage that is the issue. You double the voltage, you double the watts avaialbe at the same CCA, so there is a lot more energy there. That coupled with the increased starter RPM really slams the starter drive home. On the rare occasion I convert to 12 volts, I use a small battery, 400CCA give or take, and small (4 gauge) cables. This alleviates the problem somewhat. We had a 440 gas forkift once that someone had put a 1000CCA 12 volt battery in. Starting it was real scary, never knew what was going to explode when you pulled the starter lever.
Lavoy