Got a 64 122 with the B-18D. Front carb hangs slightly returning to idle. A slight nudge with the thumb and it rolls right on down. Springs are good. The bellcrank seems slightly loose with 2 or 3 degrees of free play. Flats on the throttle shaft look very slightly rounded. I tried tightening the fastening nut but if I torque it too much the throttle binds even more. There seems to be a fine line between loose and too tight. Bushing wear seems not to be a problem... I tried the old Berryman trick to sniff out air leaks and all seems to be tight.

Would ya say the next step is to take the carb apart and physically check for shaft wear??