2 stroke degrease at higher RPM and 4 stroke increase.
BUT.....!
If you want some more power in low rpm, you have to set the timing to BTDC + (x) degrees.
If the normal timing is 34 degrees, for a little more power into the lower band you have to set the ignition to 34 +(x) degrees.
If you want more high rpm (no power !!) you have to set the ignition to 34 - (x) degrees.
Becourse you can't do this with normal breakers or magneto-ignition, you need a programmable ignition like a CDI or TCI.
Petrol need some time to ignite, at low rpm there is time enough but at high rpm this time will be very short.
Thats why you have to degrease the ignitiontime.

This is the way I test mopped-engines.....12-13hp / 50ccm