New modern jumper cables could also be part of the problem. They are not big enough for the old 6 volt systems, you can't flow enough amps. What size wire are your cables? The smaller the number the bigger and better the wire. Some of the cheap jumper sets are made with number 6 or 8 gauge wire which are border line for a 12 volt system! The better cables will be number 1, 2 or 0. I made a set out of welding cables that are 25 feet long so I can jump a car even if we aren't nose to nose.
Mike
