I discussed this problem with Randy at the time. The workmanship wasn't the issue, turbulent flow around the butterfly/throttle plate was the problem. I had similar problem 20 years ago with manual throttle body for Rabbit/Golf by Weber. at small throttle openings, it would pull air, and would get a "cavitation" for lack of precise term behind throttle plate. It would vibrate, and the idle would hunt all over the place, from 800-1800. Once it was shut, it was fine, but small openings confused the airflow meter and it would hunt. I think the dual servos on the electronic TB would start a see-saw like response when the flow would eddy like this, thus recreating the same idle probs I had with the Weber TB.

 

>>I discussed this problem with Randy at the time. The workmanship wasn't the issue, turbulent flow around the butterfly/throttle plate was the problem. I had similar problem 20 years ago with manual throttle body for Rabbit/Golf by Weber. at small throttle openings, it would pull air, and would get a "cavitation" for lack of precise term behind throttle plate. It would vibrate, and the idle would hunt all over the place, from 800-1800. Once it was shut, it was fine, but small openings confused the airflow meter and it would hunt. I think the dual servos on the electronic TB would start a see-saw like response when the flow would eddy like this, thus recreating the same idle probs I had with the Weber TB.
>>
I think you're onto something! When I talked to Randy he told me that he had to work out, using trig (which I've forgotten, or maybe I've forgotten whether I ever learned it) the exact angles at which to machine the butterfly so it would close properly and not cause this swirl. At 63mm, the angles were not optimal.

 

i held to the stock angle for the closed butterfly at 61 mm; it seems to me if you change the closed angle, the servo might get confused. i will machine one at 63mm to see if it gets hinky using the same geometry as stock and 61mm. By the way, no trig was needed; a much harder problem was extracting the pin retaining the throttle shaft.

Workmanship is very important if you want the bfly to close tight and still open freely.