RE: Macchi 202
Y ou can go higher on you exhaust 180 easily. the 1/4 turn to rich lean is a bit sensitive which means the carb can be opened up more. if you had to put a head shim in to run 15% i dont think will get much more by another shim and 25%. Need to optimise the rest of the engine first. What is you exhaust port shape and width? What is the measurement from the top of the sleeve to the top of exhaust port, transfer and boost ports. Measure these first then we can see if can go higher or not on the exhaust. Did you fill the crank in the front to at least a 45 ?