I agree with piroflip2 about the YS110 jackhammer. I am a convert back to 2-strokes, at least for light 3d airframes.

My 120ax runs good with a conventional Perry setup, terrible with the perry "bypass" arrangement so often discussed.

I have been working with commercial pumping systems for 25 years now and the bypass makes no sense to me. If anyone says their bypassed pump works great, then I'll bet it runs just as good with no pump at all. With the bypass, mine is very pitch sensitive. Without the bypass its perfect. The only way it runs good though is to really back out the pump pressure screw. Mine is screwed in only 3 or 4 tuns from the "oops I lost the spring" position.

My only complaint is the fuel metering curve is off a tad. It's too lean through midrange, which is indicated by the detonation "rattle" everyone hears. If you simply open up the idle needle during this midrange knock, it goes away completely and smooths out. However, this makes it too rich at idle.

The only (60 year old) hot rod trick that I know of to fix this is to open a small hole (or in our case a small notch) in the throttle barrel so that the throttle barrel must be rotated further closed than before to get the same idle speed. This lets the idle needle seat even deeper and leans the flow, but does not affect other throttle positions appreciably. As the throttle opens, the little notch becomes a smaller percentage of effect on airflow, and does not greatly affect midrange or full throttle mixtures. This is how you can reshape the fuel curve for richer midrange with lean idle. Interestingly, YS did this very process on the latest 110 throttle.