ringed engines have to get hot in order for the ring to expand and wear to the proper fit to the liner. you need to peak them out for 30 seconds (let it warm up) and then richen them to a 4 cycle and let the excessive fuel/oil cool the ring/piston and repeat. after a few tanks, with the engine OFF, the engine will 'bounce' freely when the prop is given a quick flip toward compression (it'll ping/pong back and forth). at that point you can fly it on a rich 2 cycle setting till the exhaust residue is completely clear.

kc

EDIT: running it at a continuous 4 cycle setting (as previously suggested) before the ring is worn/fitted you'll overheat the piston. the ring is what transfers the heat from the piston to the sleeve with the continuous 4 cycle running the heat is ring doesn't wear to match the liner and the heat is NOT transfered efficiently.