Dirt in the booster

My experience is that on the 2100 or 4100 nearly always there is dirt or crud somewhere inside the booster.

One way to tell is if you can get it to idle at all, then close the choke plate slowly after it warms up. If it speeds up suddenly as the plate is nearly closed, it could be either a vac leak or dirt in the booster.

I'd pull the carb and the booster assy and clean and blow it out carefully. I've actually had cases where I did that, and then it ran good, but after a few minutes of driving would pull more crud back into the holes inside the booster. Clean, clean, clean. But at least then you have found the problem.