Let me jump in here, if I may. The idle "speed" is determined by the idle stop screw. The low speed needle valve sets the "low speed mixture". If you reset the needle, all you do is make the idle rich or lean, neither of which is what you want to do.

I usually remove the idle stop screw entirely and use the throttle servo to set the speed. This way, I can reduce the throttle to absolute zero and stop the engine with the transmitter.

You may also want to look at the throttle linkage geometry. If you are only moving the throttle servo a limited number of degrees, each step on the throttle stick is a large dimensional amount. This means each "click" is a big increase or decrease in throttle position, i. e. engine speed.

Here's how I set up the throttle linkage: On the carburetor, there's a strange shaped throttle arm. I add a piece of brass stock to it using a small nut and bolt and a bit of JB Weld. This will be your new throttle arm. Attach it so that the arm can move equal distances from the center to both idle and full speed. Make the arm about 3/4" long and drill a hole in the outer end for the clevis on the linkage already attached to the engine.

Using this geometry, you can have your throttle servo move 120 degrees from off to full and the idle speed resolution will increase dramatically.

There I go rambling again, but I hope this will help you to get your US41 running the way it should. BTW, My US41 is on a brand new 90" Gee Bee Model Y, ready to test fly in a week or so, right after Top Gun.