The jumper cables are your problem. John's new 261 also refused to start on jumper cables, even when we used a 12V battery. I went to a welding supply store and had them build a set of #0 battery cables with crimped-on ends, using a hammer tool to attach the terminals. We installed a 12V coil and used a 12V battery against the 6V starter to spin the engine. It took less than 10 turns to fire up. After a cam run-in we swapped back to the 6V coil, and a well-charged 6V battery. No cranking problems at all! Unless the jumper cable copper conductors are the diameter of one's thumb, there's just too much voltage drop to crank a new engine.
I had a pair of 12 foot long jumpers made from #00 welder cables that I used when I was running a Diesel shop and road service. I had 1K amp rated spring clips on the ends that were usually used as ground clamps for welding machines. They cranked big Diesels with no problems. Of course, the 12V battery in my service truck weighed 120 pounds, the same one used in a D-8 Caterpillar dozer!
Jerry
