I have a 120 AX, whch is my 8th os engine. I broke it in with a 16 8 APC prop with no problems, except for excessive vibration at an idle. I changed out to a 15 6 Evolution Prop, which at first had a very smooth idle, but after a tank of 5% Omega, same fuel used with break in, it now either cuts out when it reaches top rpm; or it sustains top rpm (10,050) but stalls out, when I cut back to an idle. I also noticed there is some in and out freeplay movement in the crank when the prop is removed. I checked the tank and all hose connections. Could the smaller prop have damaged the engine?