Carb needles!!
I read carefully that there was no mention of attempting to open up the carb H and L adjustments. Try an eighth to quarter turn open on the H needle. It is important to understand, the H affects the L and vice versa. If the L needle is in too far the engine is typically difficult to start and will quit in the air. If the L needle is too rich the engine will be "blubbery" and run only on three or four cylinders in the lower to midrange throttle positions.