You guys always have the right answers, here's a good question for you.
Can I use a 12 volt battery to jump start (via jumper cables) a 6 volt car. Or, am I looking to burn something up. I used to do it on an old (1959) volkswagen I drove in high school, and always wondered what would break. It may take some work to get the 1940 model 51 to run after 20 years of storage. I'll be giving it the greatest chance to start on the first try (new plugs and ignition parts, rebuilt carb, rebuilt fuel pump, clean fuel lines and tank) but that 6 volt will only crank so long without recharging. And, I'll want to crank it for a while without starting to get the new oil worked thru-out the bearings and verify some kind of oil pressure.