The main circuit utilizes the venturi to create a low pressure system, -essentually a vaucuum, to draw fuel into the engine. It works very well at high velocity due to the nature of the venturi. In fact, it works so well that at really high velocity (relatively speaking) the signal to the fuel circuit is actually too high, and it needs to be "bled off" by the air jets in the tops of the emulsion tubes.

That defines the idle fuel rate. The idle mixture is regulated by adjusting the amount of air entering the carb via the idle speed screw. Idle speed is controlled by a screw located on the lower right hand side of the cast iron throttle body. The bottom end of the idle speed screw serves as a physical stop for the primary throttle shaft. As it's moved in, the "closed" position of the primary valves is opened up, and more air flows in. Consequently, the idle speed is increased, but the mixture changes dramatically with even a minute adjustment of the idle speed screw, which is why it is necessary to adjust them alternately until the desired result is achieved.

There is a throttle stop screw located on the secondary shaft as well, but this serves only to keep the valves from being wedged closed inside the throttle body. It is not meant to be user-adjusted, and if it's been tampered with, or if the secondary valves have been removed and replaced without being properly seated, the result will be a permanently raised lowest idle speed setting. Whether the raised lowest setting actually compromises the idle depends on how much air is getting through the secondaries. It is highly suggested that the secondary throttle shaft and these associated components never be adjusted.

There are eight very important jets associated with the idle circuit on the Nikki carburetor. Four are all referred to as "slow air jet #2"; two for the primary main, and two for the secondary main. These jets serve as air bleeds for the idle mixture, and are located on the outer edges of the top of the main body. Set closer to the center of the carburetor are four "dual purpose" jets. These are all referred to as "slow air bleed # 1", and are specific to the primary and secondary main circuits just as the other four are. These jets play a much smaller role in the idle circuit, and play more of a role in circuit transition. They serve to ease the transition from idle operation to primary main circuit operation, and then to secondary main circuit operation.

Ideal idle air / fuel mixture for the 12a with the complex stock emissions control system in place was specified to be a rather rich 10:1, as it required a rich mixture to help fully burn up the idle exhaust. With a header exhaust (or even just the rat's nest removed), this is slightly too rich and can result in fouled plugs. I recommend an idle mixture of just under 11:1.

Circuit Transition

Circuit transition in the Nikki is typical of most four barrel carburetors. As the primary throttle is opened, a pump called the accelerator pump shoots a steady, small stream of fuel directly into each primary venturi. If it were not for the accelerator pump, opening the primary throttle would cause a lean condition that would result in a stumble, even if it were opened relatively slowly. But with the aid of the accelerator pump, the mixture is made very rich, which is ideal for the load associated with acceleration.

On the stock Nikki, the secondary main circuit is vacuum operated, and only opens under significant engine load. As such, there is no bog associated with the opening of a vacuum operated secondary throttle shaft.

But on a modified Nikki with mechanized secondaries, the same transition problem that occurs from running on the idle circuit to running on the primary main circuit also happens while transitioning from the primary main circuit to both the primary main and the secondary main circuits. The accelerator pump can be modified to accommodate this transition the same way it does the prior one. This is done by both modifying the linkage to keep it pumping throughout the opening of the secondaries, as well as modifying the inside of the pump to increase the volume of fuel that it can hold.

Circuit transition is tunable on the nikki via the accelerator pump. The accelerator pump can be adjusted to give a longer or shorter shot of fuel, and if it's too much either way, the result will be difficult to discern. In both cases, it will cause a bog. Optimised accelerator pump tuning is subjective to an individual's driving style; -[to a point. Remember, nothing will make the rotary accelerate as fast as you can slam the pedal.

For information on tuning the accelerator pump, visit the Tuning section.

There are four jets associated with the transition from the circuit idle to the main circuit. These are basically dual purpose jets as they serve to bleed signal as well as emulsify the fuel coming into the idle circuit. Fuel emulsified in a carburetor is fuel that is mixed physically (in stead of molecularly) with air. The small air bubbles trapped within the fuel make it atomize easier when it hits the incoming flow of air that carries it into the manifold.

These four jets are removeable, but not replaceable in the Nikki. In other words, there is no variety from which to choose. But they should be inspected upon a rebuild to ensure that they are not plugged with debris or solidified fuel residue.

Finally, there is one last adjustment that can be made with regards to circuit transition on the stock Nikki with vacuum secondaries, and that is the vacuum diaphragm box spring rate. Changing this spring rate can make the secondaries come on with either less load associated engine vacuum prompting with a lighter rate spring, or more load prompting with a higher rate spring. generally speaking, for performance gains, it would be desireable to lessen the rate of the spring. Often people will simply cut the spring, but in order for it to push fully against the diaphragm, the spring must then be stretched slightly to make it long enough. This can actually make the spring have a higher tension than it originally did.

I have found that the best way to lessen spring tension rate is to apply localized heat to a section of it and let it air cool.