Correction

I just checked my 4100...the secondary actuating rod is NOT @ 90 degrees to the throttle shaft. From rear to front, it angles toward the carb body, then runs parallel for a short distance, then angles out as it approaches the lever. Make sure the rod's not binding against the carb body, although that situation would seem to force the throttle plate toward the passenger side, the opposite of your problem. I would still make up a different rod just to try different angles-of-approach. Also, if you know the cam specs, post them. For instance, the HiPo 289 has a pretty radical cam and is only expected to generate 15 inches at idle. If all else fails, you could install a light , short spring between the carb body and the throttle arm lever, similar to the bushing idea that seemed to partially work for ya.