Here is a Jim Carter tech article on how to jump a 6 volt electrical system.
Applying 12 Volts to a 6 Volt Starter Excerpt from article.
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.
