When the throttle is quickly opened, the airflow increases almost immediately, but the heavier fuel takes longer to catch up, leading to a brief lean condition. The accelerator pump compensates for this by supplying the necessary fuel for smooth acceleration. Since the secondary side throttle valves of the carburetor remain fully closed during part-throttle operation, only one accelerator pump is required, which is positioned on the primary side of the carburetor.
A double spring pump plunger is employed, with the top and bottom springs calibrated to ensure a consistent and smooth fuel supply during acceleration. The accelerator pump is linked to the throttle lever via the pump shaft, lever assembly, and pump rod. During the pump intake or upstroke of the plunger, fuel from the float bowl passes through a filter screen, dislodges an aluminum inlet check ball, and fills the pump well. When accelerating or during the downstroke of the pump plunger, the force of the fuel seats the inlet ball, pushing the fuel through the discharge channel. This unseats the pump discharge ball, allowing fuel to flow through the pump jets into the air stream. After discharge, a calibrated spring returns the discharge ball to its seat, preventing air from being drawn back into the fuel channel during the intake stroke.
The pump plunger head is vented to reduce the effects of fuel percolation in the pump well. This is achieved through the design of the check ball and seat in the plunger head, allowing any fuel vapors in the pump well to rise and vent into the float bowl. This design ensures that a solid charge of fuel remains beneath the plunger head for rapid acceleration. Without this feature, vapor pressure buildup could deplete the fuel charge in the pump system, leading to poor initial acceleration and difficulty in hot starting.
Some models lack inlet check balls; in these versions, fuel enters through a slot in the pump well. During the upstroke of the plunger, fuel flows through the vapor vent ball passage and between the plunger and the pump well wall, filling the area below the plunger with fresh fuel.
Additionally, the carburetor features a pump plunger boot that prevents dirt and foreign particles from entering the fuel bowl through the shaft opening at the top of the air horn and ensures the proper seal is maintained to keep the correct air pressure within the fuel bowl.
To keep the 4-jet accelerator pump well in optimal condition, it’s recommended to polish it with crocus cloth. Since gasoline no longer has the same lubricating properties as it once did, a smooth well surface is essential to prevent the accelerator pump from failing due to a rough surface. The 4 Jet is notorious for eating up the accelerator pump cup.