What would happen if you tried to start a 6 volt model t with one of them 12 volt jump starters. With the 6 volt battery still hooked up.
I don't know whether that's OK, but if your battery is down too far to run the starter it may have enough juice left to buzz the coils. Then you should be able to hand start easily.
It has been done, however I would worry more about the starter and generator and the lights when using such a device.
Best to jump start directly to the starter, make sure hand brake is set and wheels are scotched. Engine will start to turn over as soon as voltage is applied to the starter, best to put positive to starter then touch negative to ground. There is always a chance of blowing up a six volt battery so I always disconnect it. KGB
I was told decades ago to Never disconnect a battery from a running engine as it would destroy the generator.
I understand that a non running engine is being jump started, but how long would it take to ruin the generator before the battery could be reconnected?
No need to disconnect a 6v battery when jump starting with a 12v - just don't turn the light on or press the brake if you have a brake light while the 12v is connected. Usually you only need to connect it for a few seconds while the electric starter is running. Don't connect the last jumper cable close to any of the batteries, especially the 6v one - you may blow up a battery that's being charged if not careful when connecting cables.
If the battery has a little juice you should be able to crank start the engine with the coils on battery and let the generator charge the battery up.