I have used the sullivan 2 ounce roound and square with the flex pickup and not had any problems if the motor is adjusted right. Start it up and hold it nose high, the rpms should pick up slightly and you should be able to hold it nose high without sagging or dying. Now, when level you won`t get the last rpm out of the motor and it will drop in rpm. For some reason, alot of people think that you adjust the needle until you have squeezed the last possible rpm out of a motor but then can`t understand why they always have trouble while flying.