Topic:

Subsection:

Other:

The purpose of the accelerator pump (AP) is to introduce a rich mixture into the engine at very low engine RPM, where the air flow through the carburetor is too low to effectively meter out the necessary mixture for acceleration. It mainly serves as a transitional component from idle to part throttle.

The accelerator pump on the Nikki has an adjustment that is simply a nut at the end of the piece of linkage that runs through the pump lever. The lever is hinged via the pin in the pump cover and actuated by the primary throttle shaft. There is a spring on the shaft between the lever and the throttle shaft link. When the primary throttle shaft is opened, the spring pushes the lever that actuates the diaphragm within the housing. The fuel is pumped up through a circuit in the center of the main housing, and a small stream of fuel is forced into each primary bore.

The accelerator pump lever finishes it's travel at about the midway of the primary throttle shaft being fully opened, and the stream stops. The accelerator pump linkage shaft is spring loaded in order for the primary throttle shaft to be able to continue to wide open throttle even though the pump lever has gone as far as it can go. The secondary throttle shaft is locked shut by linkage that's under high spring tension. The action of the accelerator pump stops just short of the point where the primary throttle linkage begins to unlock the secondary throttle shaft.

The stock Nikki has vacuum operated secondaries that only open under significant engine load. If the accelerator pump were to continue to squirt fuel into the carb while they were trying to open, there would be a pronounced stumble, loss of power, and most likely some backfire, all due to the rich condition.

However, when the Nikki is modified to have mechanical secondaries, the lack of a steady stream from the accelerator pump while opening the secondaries causes a stumble due to a lean condition. this can be corrected by modifying the accelerator pump so that the housing holds more fuel, and the accelerator pump lever has a longer span of travel. This is a modification I make to the Sterling Nikki.

Adjusting the accelerator pump is purely subjective to driving style. The basic setting is with the diaphragm partially pushed in. At this point, the portion of the accelerator pump lever that is pinned in to the slot of the accelerator pump cover is parallel with the edges of the slot. It's a good starting point from which you can increase or decrease the pump shot.

The adjusting nut is located on the threaded rod that goes through the bottom of the AP lever. Turning the nut clockwise pushes the lever out from the diaphragm, increasing the volume of fuel that goes into the AP circuit. At the end of the circuit, where the nozzles are located, is a brass banjo bolt holding the nozzle head in place. Inside that banjo bolt is a pressed-in jet that controls the flow rate of the fuel stream. On the stock Nikki, this is rather small, and adjusting the nut for higher volume within the pump translates into the AP stream lasting for a longer duration at the same rate because the action is ultimately spring loaded via the external linkage spring (the diaphragm spring simply opens the diaphragm to draw in more fuel to the pump when the linkage is returned).

Turning the adjusting nut counter-clockwise pushes the diaphragm further in at the throttle starting point (idle), limiting the amount of fuel the diaphragm is allowed to draw in. As the throttle is engaged, the pump still pushes fuel through the nozzles at the same rate of flow that the jet in the banjo bolt dictates, but the stream will run out sooner.

On the Sterling Nikki, the jet has been drilled out to increase the fuel stream slightly, the pump has been hollowed out a bit to increase the maximum amount of fuel it can hold, and the linkage has been modified to continue the stream through the mechanical secondary throttle operation to aid in acceleration even at high engine load and RPM.

In either case, the AP should be adjusted by alternating ¼ turn increments of the adjustment screw with test runs that focus on acceleration to see what happens.

 

The accelerator pump lever with an extension on a Sterling Nikki. Adjustments are made the same way as stock. The nut on the rod going through the bottom of the AP shaft controls the volume of fuel the AP can take in (and thus pump out). Turning it clockwise increases the volume of fuel.

Noteworthy:

The accelerator pump begins introducing the same rate of fuel as soon as the primary throttle is opened, no matter how it's set.

On a stock Nikki, the only thing you're really changing is how the primaries respond.

In some cases, the accelerator pump will need to be adjusted after tuning the idle.

On Sterling Nikkis with the "flip-O-matic" secondary linkage, keep in mind that the AP will have to be adjusted when going from mechanical secondaries to vacuum secondaries, and vice versa.