serious carb issues

I think JoeDust has something and I agree. It will never be right. Have you tried spraying water around the shaft ends? I bet the idle tightens up. Just ask the guy what the shaft to bore clearance was. I bet you get a big fat "uhhhhhhhh". Not that there's a set number really, but it will at least get him thinking.

On the flip side, if your guy was right and it is indeed dirt in the idle circuit, either way he should have caught it. Time to buy the book and do it yourself.