Here is how I have resolved mid-range problems, and I now do this on all my gas engines.

I set the low needle so that when you start a cold engine it will not transition as you move the throttle. In other words if you start the engine and immediately give it full throttle fairly fast, (with out warming it up) the engine dies or almost dies. Then after 1 minute of running at about 1/4 throttle you hit the throttle and it should transition fine.

I have my timer set on the transmitter to start at about 7% throttle. When I start the engine cold, I then slowly advance the throttle to about 1/4, watch the timer for 1 minute and then run the engine up.

Now you may already have done this as it is a common way of adjusting the low end. In that case you may have to do the intake and vent tube, but I have never had to do either.

Hope you get it figured out.