1. You will notice that the carburetor slide has its bottom edge put away on the air intake side. This is called the slide cutaway.
1/8 to 1/4 Throttle: Slide Cutaway and
Needle Jet (Fig. 135)
2. There is a tube protruding from the bottom of the carburetor bore. This tube is called the needle jet. The tapered needle in the slide projects down into this tube, and as the slide is raised, the needle is drawn up out of the tu be, metering the f low of fuel accordingly.
3. As the slide moves from 1/8 to 1/4 throttle, air flows past the 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 bottom of the slide), the smaller the fuel flow (leaner mixture). The lower the cutaway, the greater the fuel flow (richer mixture).
5. The mixture at 1/8 to 1/4 throttle can be varied in two ways. A needle
jet with a larger hole could be fitted. However, the selection of these
needle jets is limited. It is always best to vary the mixture at 1/8 to
1/4 throttle by installing a slide with a higher or lower cutaway.