To best explain, I will start with a mean. The mean is say 10:1 compression ratio and a 90 octane fuel rating number. For this combination ,with an air cooled engine , you would expect the engine to achieve its best fuel expansion point by starting the ignition at 35 degrees BTDC. If you were to increase the compression ratio to 11:1 you would have to do one of two things, increase the octane fuel rating number to, say 100, this would slow the burn rate by 5 degree of crankshaft rotation allowing the expansion point of the fuel burn to be equivalent to 35 degrees BTDC , or retard the ignition to 30degree advance BTDC. The afore ,is the best by far, as you are taking advantage of the H.P. gain ,by increasing the compression ratio. The expansion of the fuel taking place at the best crankshaft position but is not giving up fuel burn expansion to a larger volume but to a smaller volume , this gives more effective push of expansion on the piston crown. Keep in mind that the jetting at half throttle, running at say 3000 rpm, the fuel /air ratio is 14 to 1 which is good.. Take that same jetting to 6000 rpm and the fuel/ air should be much richer say 11.5 to 1 , so when tuning with a plug chop, make sure the needle is in its bottom grove, this will insure the needle is out of the picture, when tuning top end fuel/ air ratio. Then tune the needle position for mid range.