You can't run a 12V battery in a 6V system and expect it to work properly. The electronics are not meant to take it. It will fry coils, points, condensors, generators, starters, switches, etc. Sure, it cranks over way fast, but it will play havoc. Think of it like plugging your table saw into 220v. It may work, but it can't take twice the voltage for long. Get yourself a 6V battery and probably some new points. They usually weld together first, then take out the coils.
