Here are a couple of thoughts:
1. in the 2100S the only vacuum tube runs up to the bottom side of the well below the diaphragm (and there also may be a tiny vacuum bleed hole running off to the inside of the carburetor bore). It is very important to make certain the bonnet which holds the diaphragm down is completely flat. Somewhere here I have shown how to use a couple of pieces of sandpaper and a flat piece of wood with a 1 1/4" hole drilled into it to correct the warping you will find in some of these. If this isn't done and if you have a leak at the point where the diaphragm meets the float bowl, raw gas will be sucked into the manifold while the engine is running and raw gas will actually leak past the diaphragm and will dribble down out of the well when the engine is cut off. Plain old gravity will take care of this. You will see wetness from gas at the seam (gasket) where the float bowl meets the throttle body and also you will see it on the throttle valve and the shaft of that valve. It is critically important you never get gasoline into that well as nothing good can follow if that happens.
2. we have no idea where you got your rebuilding kit. It may have been one with an incorrect needle and seat. If so, you probably are allowing too much fuel into the float bowl. Carter engineers spent lots of time/effort designing these and they had instruments we (and the people selling the imitation kits) will never have. The "generic" kits out there may have a seat with a very large opening. It should not cause fuel to run into the engine after you turn it off, but if you have a problem there
3. the float adjustment is also very important...but it assumes you have the proper needle and seat.
4. make certain you have a fuel pump that is only providing at the absolute most about 4 psi. Preferably 3 psi. These engines and these carburetors do not need more than that. If your fuel pump is providing something like 10 psi, that's likely to force fuel past your needle/seat and into your float bowl.
Hopefully some of that is helpful.
