Mount a degree wheel and set a reference pointer at zero when the engine is in top dead center.
The spark plug should fire when at 28 degrees before top dead center when the engine is turned over slowly.
When turned over faster, like in flip starting, the ignition will retard to 9 degrees before top dead center.
When rpm reach 2000 - 4000, the ignition will slowly stop all retarding and operate at 28° BTDC again @4000 rpm.

Installed like that should stop all rough running, if the carb is adjusted well and the engine is mechanically ok.