The off idle stumble problem you discribe could be caused by a lean air/fuel mixture.We have found that most of the aftermarket carter AFB & AVS clones are both lean off idle and have a accelerator pump squirt that is not as strong as it needs to be. there are some pics of what we do to cure the problems in a article that can be found at
http://www.webrodder.com/article.php...SID=60&CID you may need to enlargen the ICR (idle channel restrictor) and use a accelerator pump with a stronger duration spring to solve the problem that is being covered up by overadvancing the ignition spark advance.
I hope this helps Henry @ olescarb