Section 2:

0 to 1/8 Throttle: The Pilot Metering

System (Fig. 124)

1. The pilot metering system feeds fuel and air from 0 to 1/8 throttle. Fuel is metered by the pilot fuel jet. Air is metered by the pilot air jet and the throttle slide. There is also a low-speed mixture screw, which meters pre-mixed fuel and air into the venturi of the carburetor.

2. When the throttle is closed and the engine is idling, the upward movement of the piston creates a vacuum in the intake port. This vacuum causes air to be sucked into the small left-hand hole beneath the mouth of the carburetor. This air is metered through the pilot air jet which is located just inside this hole. The air then flows over a passage which houses the pilot fuel jet. This causes fuel to be drawn up through the pilot jet and become mixed with the air, forming tiny droplets.

3. This pre-mixed fuel then flows past the low speed mixture screw and enters the carburetor bore through a small hole just in front of the slide. The mixture then enters the engine.

4. As the throttle is opened just slightly, the fuel mixture can also enter the engine through a small hole just insdie the front edge of the slide. On the 1971 250 Pioneer models, this hole is in a small brass tube located just inside the front edge of the slide. Due to the slide being raised slightly, larger amounts of air flow over these two holes, causing more fuel to be drawn up out of them.

5. You can vary the proportion of the mixture from 0 to 1/8 throttle by either changing the pilot fuel jet or pilot air jet (which is seldom necessary), or by adjusting the low speed mixture screw beneath the mounting flange on the right side of the carburetor. Rotate the screw clockwise to make the mixture leaner; rotate it counterclockwise to make the mixture richer.

6. Correct adjustment of the pilot metering system is as important to a competition engine as it is to a street or trail machine. Even though you do not want the Stiletto or T.T. engine to idle when the throttle is closed, you should still make certain that the pilot metering system is properly adjusted. If this is not done, the engine may hesitate badly when accelerating from a lower RPM if the mixture is too lean, or a loading up condition may be present if the mixture is too rich.

7. The pilot metering system continues to work in conjunction with other metering systems at larger throttle openings, although its influence is greatly decreased.

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