Ok thank you makes perfect sense now and i heard 270 is good for 20 over?The correct way to see if an engine is jetted correctly is to do a WOT plug chop.
If you WOT a fresh build you can shorten the life of the engine by excessive ring wear.
It is not wise to do a WOT plug chop on a fresh engine.
The main jet comes on at above 3/4 throttle.
If you have chosen the incorrect main jet, then by keeping at no more than 3/4 throttle, you should not fear a too lean a mixture.