Steve,

I removed the battery cable running to the starter, and using a spare
battery, and a jumper cable for the ground, I cranked the starter with
this battery, which was isolated from the airplanes electrical system.
It had good spark on both rotors .... yeah !!!

It still sounds like low battery voltage during cranking.

 

There is a minimum voltage below which some components will no longer function. A volt meter on the battery during cranking would give you the actual voltage available to run the electronics. Starters can draw over 100 amps during cranking. Quite a load for a small battery. The race car will start about 4 times on the little garden tractor battery. Then nothing. So we start up with a big group 24 paste battery with an aircraft style plug in cable made of welding wires. That jump battery is in parallel with the tractor battery as well so we charge the racer and the jump battery by leavng the tractor running.

 

Lynn E. Hanover