I ran into the exact same thing recently and wondered if you ever figured it out? I am using essentially the stock return spring (repop) and my primaries close fine when the engine is off. When its running it won't close completely and when I pull to a light it will idle 1500-1800rpm (4spd). I rap the throttle and it drops down to 800-900ish. I checked it when I got home and it simply doesn't return to the stop when the engine is running? It doesn't appear to bind anywhere at all. Cliff Ruggles recently rebuilt the carb. I am sure a heavier spring would do it, but I too am now curious what the root cause is... I'll also take some of the suggestions off here and check them tomorrow.