Crank slippage!
This engine has about 10 minutes run time since rebuild with rebuilt crank, new pistons, etc. The crank has slipped out of phase. It still ran on the mag side, that is why there is a difference in piston top coloration. Engine ran great when first started, then got to running kinda crappy like a fouled plug. Ran worse and worse until it only ran on one. Had good compression on both cylinders. Kinda tough to diagnose at first. Then I just sat there and played with it, rotating it by the clutch and feeling for compression when I noticed that compression was occurring a bit too close in the rotation of the crank. Anyway, gotta tear it down and start over.