Jump Startin + ground AC-- B

Steve at B&B is right, there's more to this jump starting bit than it used to be. Plus, it's really always been dangerous and folks have gotten burned and hurt doing it. But we so often just don't think about things or take them for granted and go on.

That said, we all know it's done, and I've done it many times myself. And I've jump started plenty of 6 volt system with 12 volts and gotten away with it. But like in a lot of things, moderation is the key. Don't connect the terminals until you're ready to start it. Connect it correctly and carefully, start it quickly then unhook it right then. You should be through in a few seconds. I've seen folks get one started then leave it connected for whatever reason and that's when the 12 volts starts to work on a 6 volt system. We watched a self propelled combine do that trick one day. The smoke started back at the engine then just worked its way right on up to the front where the controls were. Not a pretty sight.

Some think that Allis-Chalmers was weird making their system a positive ground, but where it might be somewhat unusual, it's not so much so. There were Studebakers (of course they were weird too!), Chryslers, De Sotos, Dodges, Fords, Hudsons, Henry J's, Kaisers, Lincolns, Mercurys, Nash and Plymouth to name a few. Not all of those were positive ground, but many were. The theory of an electrical system does in some measures support grounding the positive. But it didn't catch on so hot, did it? God Bless and good luck.

When jump starting a tractor with 12 volts it not necessary to hook the 12 volts to the 6 volts battery at all. As a matter of fact I strongly reccomend against it.
Instead hook the 12 volts directly to the frame of the tractor for the ground and with the transmission in neutral and out of the way of the wheels of the tractor so you don't get run over touch the other cable from the 12 volt source to the post on the starter.
By doing it this way you don't even have to worry about the polarity of the batterys or which cable you have to ground or positive to the frame or the starter as the 6 volt systme is not entered into at all with the 12 volts. Just turn on the switch and touch the hot cable to the starter post and it will go.

I would get some starting fluid and spray into the air intake before attempting to start. Turn the ignition switch on and hit the jumber wire to the starter post. If it will start on starter fluid sprayed in the air cleaner you have problems in the fuel system. If it won't try to start with starter fluid you have ignition problems and should start from the battery and follow the path of the current to the coil and then the points and condenser etc. Sparkplugs etc.