Dune, because these small cheap engines do not have an ignition advance mechanism to get their timing more or less sensible across the rev range, they tend to be severely retarded except at idle. Since we usually don't expect them to idle often or well, it would make sense to advance their ignition timing from the standard setting, if it were not for one problem: they would end up with excessive advance when starting, and would kick back. That would not just be unpleasant - it would be dangerous.
I suggest that if you want to do some experiments, you try advancing it a bit from the mid-point position and watch for the first signs of kick-back. However remember that at any time the engine might forcefully pull the rubber starter cord handle out of your hand, and you wouldn't want it to dislocate or break any fingers when it does that. The old round knob that Victa used long ago was unlikely to hurt you that way, but the T-shaped ones used nowadays could be more of a problem.
With electric starters, you can hear the kick-back and retard the ignition, but with a hand-pull you can get bitten before you know you're on dangerous ground. If you want to experiment, be careful.
One of the built-in factors that protects you against kick-back is the flywheel: the heavier it is, the less risk of being hurt. On the other hand if you try to start a vertical-spindle mower engine without the blade-plate fitted, you may get hurt even with standard ignition timing, because those engines rely on the inertia of the blade plate to keep from kicking back.
