This is a step by step process of elimination. Only runs at high speed is contrary to weak spark syndrome. If it has a carburetor (90% of my questions go unaswered) a tach signal from the distributor cap alongside the heavy red wire is NOT needed. If the tach wire shorts pandemonium erupts. But this is pointing more and more toward the HEI module inside the distributor. The one that mounts to the distribributor base not to the cap. This gets interesting because a failed coil can fail the HEI module and not the other way around.
Access google. Search TEST HEI COIL then go to it. You do have a multimeter right?
