I think you're describing compression release. On most LT/GT and bigger engines, the camshaft has an arm that bumps the valve at low RPMs. This allows the crankshaft to spin faster & start/run. Cam arm slings out of the way at higher RPMs for full compression.
How do you set the coil air gap? I use a business card between the coil legs & magnets.
Can you do a visual check on the timing? Flywheel key has sheared - can be just a tiny bit. No other answers - pull FW to check well.
You have fuel/air, compression & spark - gotta think timing.