Colin,

I would recomend lengthneing the pipe to cure that problem. I changed pipes one time and the midrange was so rich it would barely run. Lenghtened the pipe about 1 1/4 inches and it finally came around to running in the midrange.
You didnt say what pipe or plug you where running and what percent of nitro. Must run 15% and any more is rarely if ever needed.

Run at least a 17x12 prop(18x10PN Apc or 15.75x10 or 11 repitched 3blade by Dave Lockhart will work too), 15% nitro, OS or YS 4 stroke plug. Set the top end first till it is just one click to the rich side and then set the bottom end so it will idle for at least 20-30 seconds with out dying. If you run it up to mid range and it starts to burble then lengthen the pipe. Reset the top end and low end and try again till it settles into a smooth 2 cycle. I have not touched the pump stteing on four OS 1.40's that I have set up and run.

Good Luck,

Wayne Galligan