Also, modern batteries are good for about 5 years and that's it. If you don't know how old the battery is, you may just want to replace it. I've been driving KF products for 40+ years and never converted one to 12 volts. They work fine on the 6 volt systems they were designed with. Another problem is, if the battery cables were ever replaced, and many were because they do corrode away, they may have been replaced with modern 2 gauge cables which are made for 12 volt systems. If you get the correct 1 or 0 gauge cables, which you can get at any farm equipment shop, they start just fine. I've driven my cars from Minneapolis to Auburn, IN, Sioux City, IA, Missourri Valley, IA and Sublette, IL all with the original 6 volt system with no problems at all. It is important to have a known good battery, and a good working charging system. If you have that you should be good to go.