Fuel is quite heavy compared to air. Remember that the only thing that keeps it well vaporized at idle is the high manifold vacuum produced by the closed throttle plate. But when you open the throttle abruptly to accelerate from idle, manifold vacuum drops to zero. Also recall that the only thing that keeps fuel well vaporized at cruising speeds is the high venturi vacuum produced by the speed of the air rushing through the carburetor. But when you open the throttle quickly to accelerate from cruising speeds, air speed through the carb -and therefore venturi vacuum lags behind engine speed momentarily.
In either case, some of the bigger fuel droplets in the vaporized air/fuel mixture condense back into liquid form (an especially thorny problem on big-port and large-plenum manifolds since there’s more surface area for fuel to condense onto). A throttle
bore that’s suddenly opened needs a significant increase in fuel to replace the fuel which falls out of suspension. But the throttle plate is already too far past the idle and idle-transfer ports for them to help out. And, without a strong vacuum signal in the venturi area, the main circuit can’t flow a drop. The result? Air rushes through the carburetor, but fuel flow is momentarily halted, so the mixture sucked into the cylinders is ultra-lean. A moment later, some pressure difference returns to activate the main circuit, but by then it’s too late -the engine hesitates or stumbles on the lean mixture.
In other words, when the throttle is moved quickly, there’s atime lag between demand (throttle opening) and main system activation. Part of the reason for this phenomenon is engine load. Put the transmission in neutral, rev up the engine and
what happens? The engine accelerates smoothly. Why? Because when the throttle is opened in the absence of a load, engine speed can build smoothly and quickly. So manifold vacuum returns to its normal 14 to 17 in-Hg rather quickly. But put
the transmission in gear and do the same thing. Now the engine may stumble badly if the accelerator pump is dysfunctional or missing. Why? Because when the engine is put under a load, airflow and vacuum stay low for a lot longer than when the engine is unloaded. During this period, the carburetor flows little fuel because without airflow and vacuum, the regular circuit can’t do much .