1 . You will notice that the carburetor slide has its bottom edge cut away on the air intake side. This is called the slide cutaway.
1/8 to 1/4 Throttle: Slide Cutaway and
Mid-Range Needle (Fig. 125)
2. There are two tubes protruding from the bottom of the carburetor venturi bore. The shortest one, or the one closest to the engine, is the mid-range needle jet. The long one is the high speed needle jet. The tapered needles in the slide project down into these tubes and, as the slide is raised, the needles are drawn up out of them, metering the flow of fuel accordingly.
3. As the slide moves from 1/8 to 1/4 throttle, air flows past the mid-range needle jet. If the air could flow squarely across the top of the needle jet, it would create considerable vacuum in it, therefore sucking too much fuel from it. This is the reason for the slide cutaway, so as to direct air downward against the mouth of the needle jet. This decreases the amount of vacuum formed, thereby decreasing the amount of fuel flow.
4. The higher the cutaway (stamped in millimeters on the top of the slide), the smaller the fuel flow (leaner mixture). The smaller the cutaway, the greater the fuel flow (richer mixture).
5. The mixture at 1/8 to 1/4 throttle can be adjusted in two ways. A
slide with a higher or lower cutaway can be fitted to make the mixture
leaner or richer at that point. It must be remembered, however, that changing
slide will also affect the mixture at 1/2 to 3/4 throttle as you will see in Section 5.
6. It is always best to vary the mixture at 1/8 to 1/4 throttle by raising or lowering the needles. Each needle is suspended from the slide by means of a clip fitted to one of three grooves in the needle. Putting the clip in a lower groove raises the needle, thereby enrichening the mixture. Moving the clip to a higher groove lowers the needle, making the mixture leaner.