Actually -- you need to change the profile of the *low* speed needle (the one that you do adjust with a screwdriver) to alter the mid-range.
Changing the hi-speed needle will do nothing except alter the number of turns needed to get the right hi-speed mixture.
If you look down the throat of the carby while you move the throttle arm you'll see why that is.
Note that the carby barrel moves in and out as it opens and closes. As it moves in (when you close the throttle), the LOW SPEED needle starts to go into the hole on the end of the fuel-jet. This reduces the amount of fuel that can flow and thus controls the mid-range and idle mixture.
If you make your low-speed needle more sharply tapered then you will get a rich mid-range. If you give it a more shallow taper you will get a leaner mid-range.
Sometimes to get an engine "dead right" you end up having a slight curve to the profile of the needle so that the mixture remains ideal throughout the barrel rotation.
Generally speaking, if an engine's so bad that you have to modify the needle I don't bother using it. There are too many well-priced "good" engines to waste time redesigning a bad one
RCModelReviews.com, just the facts.