I just bought this 91 B2200 with an auto transmission. It has a problem when accelerating. When I first step on the gas, there is a stumble, then it takes off. This only happens 80% of the time. Some days when I warm the truck up enough It does not happen at all. I did a full tune up, plugs, wires, cap, rotor, fuel filter etc. (The plugs were white with deformations on the electrodes). It still happens. There was a vacume leak that my mechanic diagnosed, and fixed, still happens. I get real bad fuel economy too. The truck starts perfect, idle's perfect, but just has that stumble. It also makes it difficult to drive at around 50 - 60 MPH. I am thinking it may be the accelerator pump. Ant help would be appreciated, and Happy Holidays!..Anthony

Yes, it could be the accelerator pump, but it could be the linkage.
When I rebuilt my carb, I had the same problem with hesitation.
It turned out that the linkage pivot shaft has a worn spot making it a little bit sloopy and delayed in pumping gas when the throttle plate was already open after stepping on the gas.
Try removing the linkage and try rotating the shaft that is attached to the carb to get a good spot and reduce the slop. There is an E-clip at the end of the shaft that holds the linkage in place,
Good luck.

if you think it is your acc pump (obviously carb)then remove the wing nut and four clips take top off and look down inside carb running grab thottle linkage and do a snap thottle can you see asquirt inside then acc pump is working..... most common problem on these is a bad fuel air sol or bad o2 rec checking both see this forum under wont pass emission on how to check or write me good luck