Something else I didn't see in the previous posts (may have overlooked it), but a mag goes to FULL advance as soon as the engine starts. 30, 35, 40 degrees advance on an engine at idle. Kinda makes one wonder how they would even run.
You mentioned a rebuilt C113 with step head pistons. Well here is my take on it from my experience. Rebuilt a 47 SA, originally had flattop pistons and a mag 35 or 40 deg impulse coupling. Overhauled it with fire craters. Ran OK at full throttle but at idle and mid speeds always had a slight miss.
Finally convinced Dad to spring for the IH dist conversion kit. Put it in with the proper advance plate (22 or 25 deg I think) for the fire craters (came with the IH OH kit) and it purred like a kitten.
So if you have step head pistons and a gas head, get a dist with a 20, 22, or 25 deg advance plate and it should solve your problems.