There comes a time with some 6 volt vehicles that an emergency jump start is necessary. Maybe the battery has been drained due to a light or ignition switch being left on. Or maybe it was stored over a winter without the battery trickle charger being attached. Whatever, the reason, you feel helpless without another readily available 6 volt battery to use as a jumper (and you need to move the vehicle that day!)
Buying a replacement 6 volt battery at a local auto parts store seems a last resort, particularity at the near $100.00 price. The option is to use your readily available 12 volt battery and jumper cables to get it started.
This can be an acceptable idea in an emergency but with some important limitations. If your vehicle was running when parked and the 6 volt battery did not freeze over the winter, it should start quickly with a 12 volt battery. Connect positive to positive and negative to negative when adding the 12 volt cables from the 6 volt in the vehicle. No need to unhook the depleted 6 volt battery.
If the engine still remains free (will turn over) a 12 volt jolt will get it spinning at twice the RPM as did the 6 volt original starter. It if drove and was parked under its own power, then it should start quickly! If at the higher RPM and it will not start, you probably have another problem.
Pouring a “small” amount of gasoline in your down-draft carb. Using the 12 volt cables requires less time to have it connected before starting. Less time pumping gasoline into a dry carb requires less time connected to a 12 volt system.
DANGER: Jumping 6 volt vehicle with a 12 volt battery can be done at a “maximum” of 15 Seconds! After that the solder in the commutator begins to melt, some wiring insulation is turning black, and the starter switch may be turning blue. Connect the 12 volt cables “ONLY” when you are ready to start the engine. Have a person ready to immediately to remove one of the 12 volt cables the second it becomes running. Remember: Not over 15 seconds or parts of your 6 volt system are cooked.