Section 10:

1/4 to 3/4 Throttle: Slide, Needle Position and Booster System (Fig. 136)

1. From 1/4 to 3/4 throttle, the slide regulates the flow of air. As the slide is raised further above the top of the needle jet, the cutaway becomes decreasingly effective in directing the air downward. Because of this, the air flows more squarely across the top of the needle jet. This increases the amount of vacuum formed in the needle jet, thereby drawing a greater amount of fuel from it.

2. At this throttle opening, the flow of fuel is regulated by the tapered needle. As the slide moves from 1/4 to 3/4 throttle, the needle is withdrawn proportionately from the needle jet, thereby allowing for a greater flow of fuel.

3. The mixture at 1/4 to 3/4 throttle can be adjusted by raising or lowering the needle. The needle is suspended in the slide by means of a clip fitted to one of three grooves in the needle. Putting the clip in a lower notch raises the needle, making the mixture richer. Moving the clip to a higher groove lowers the needle, making the mixture leaner.

4. Because the venturi of the carburetor is round, a small upward movement of the slide at or near 1/2 throttle position will allow for a much larger volume of air to flow through the carburetor bore than the same movement at a higher or lower throttle opening. This creates a greatly increased demand for fuel. The needle jet cannot supply this fuel alone, because only the volume of air flowing over it has increased, not the vacuum created in it. To compensate for this, a booster system is provided.

5. To boost the flow of fuel, air enters the center small hole beneath the intake mouth of the carburetor. (Fig. 137) This air is routed directly to the needle jet through several small holes drilled halfway up it. This passage of air up the needle jet boosts the flow of fuel which is necessary to meet the demand. Although the booster system is working at all throttle openings above idle, it is most effective around 1/2 throttle. There are no provisions for adjustment of the booster system on the 24 mm and 27 mm IRZ carburetors.

6. The proportion of the mixture at 1/4 to 3/4 throttle also affects the mixture at wider throttle openings.

Fig. 137

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