If you're pulling 20" of vacuum at idle then the timing is plenty enough advanced, that can be ruled out. In fact, that might be a little too far advanced. Be careful with setting the idle mixture screws. They have a big effect on how the off idle acceleration works. What's called the transition slot and idle circuit is in play for a lot of driving so it has to be just right. Too rich can cause an off idle stumble as well as too lean.

Carburetors are very sensitive to float level. Before making any other adjustments make sure this is correct. What you're actually doing by adjusting the float, is setting the fuel level height in the carb bowl. The whole carb is calibrated by the manufacturer on this being correct at all times under all conditions, and stay there regardless of engine demand. The initial float level adjustment is done on the bench during rebuild, but the fuel level needs checked after installation on the engine. Maybe you've done this already, but it's important to mention. The easiest way to check is let it idle for a few minutes on level ground. Engine should be warmed up. Shut the engine off. Then remove the carburetor horn (4 screws) and move it to the side.

The fuel level in the carb bowl is called "wet height" in the manual, if you can't find this information for your truck try the width of a US quarter or 29/32", from the surface of the fuel to the machined flat surface of the carb. Adjust the tang on the float up or down as necessary to achieve this. It's probably the single most important adjustment on any carburetor.

The accelerator pump rod holes - the top hole (#4) is for temperatures below 40° F. second from top (#3) is for temps 40° to 80°, and third from top (#2) is temps above 80°. 2100s also have inboard and outboard hole positions on the pump lever, the inboard one is a bit more juice. This is where the Shop Manual comes in handy, because while the 2100 carburetor was used on everything, they came in many different Venturi sizes and had several different settings, jet sizes, power valves, etc depending on the specific application.

Yeah Holley really hit one out of the park with the ability to adjust float height on the fly like that. What's nice about the 2100 design is there's no gaskets for the bowl to leak, and if one is careful, the float height/fuel height can be observed while the engine idles. Another important thing to beware of is the fuel pump pressure, a lot of the parts store replacements sold these days are way off the beam (too high).

The 2100 needs only 4 or 5 psi. The canister-filter type fuel pump I bought as a fresh replacement was running close to 10 consequently the float needed to be hung way down in the bowl to get the fuel height right. Excess pressure wastes gas and trashes the viton-tip needle and will probably flood sooner rather than later. So it's important to get all that stuff baselined and straight before thinking about power valve #s and jet size or pumprod holes & idle mixture settings, because the carb is calibrated by the factory on the assumption that adequate fuel is supplied in the bowl at all times under all conditions, not too high, not too low.

2100 is a really fun carb to mess with and the engine should run really sweet when dialed in. The shop manual carburetor specs are invaluable for setting them up because after several years lots of parts get inadvertantly or advertantly swapped, sometimes without much thought to the overall effect. Venturi booster clusters, missing check ball weights etc. Trucks and maybe other applications also included a primary float spring. The rebuild kits don't supply these for some reason. If Ford thought a .02c part was necessary and didn't leave it out I probably shouldn't either, is my thinking.

Sure, it could be jet size, but carburetor tuning must be done in a certain order and, fuel height in the bowl is numero uno, because the entire carburetor is calibrated around this. Ever wonder why the manual lists wet height being a fairly precise # like 29/32"? Why not just say 1 inch and call it good?

It's because 1/16" one way or another is equal to maybe one or two jet sizes difference, something like that. So don't worry about jet size until you're certain everything else is straight. Once the mechanic starts compensating for an error in one circuit by an additional tuning error the end result is it won't run as well as it should. Just take one thing at a time, in order is what I was trying to get across.