That blue collar with the pin on it is the limiter for the low speed mixture adjustment. Get rid of the pin and the collar. I have yet to work with one of these engines that is set correctly on the low speed limiter. If it on the H9 P-51 PTS, that pin has a tendency to hit the cowl when you throttle up, causing the low end mixture to be set to its full rich limit, causing idle problems again.

Brad