Section 8:

0 to 1/8 Throttle: The Pilot Metering System

(Fig. 134)

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 low speed air screw and the carburetor slide.

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 left-hand hole beneath the intake mouth of the carburetor. This air flows through a passage that leads to the low speed air screw. This screw meters the amount of air available to flow past a small orifice that connects to the pilot fuel jet.

3. At this orifice, the fuel and air are mixed together and then flow into 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 another small hole 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 changing the pilot fuel jet. A larger number jet will make the mixture richer: a small number jet will make it leaner.

6. You can also vary the mixture from 0 to 1/8 throttle by adjusting the low speed air screw. Turning this screw clockwise makes the mixture richer; turning it counter-clockwise makes it leaner.

7. Correct adjustment of the pilot metering system is a must for a trials motorcycle. Since throttle control and engine response play a major role in trials riding, particular attention should be paid to the pilot metering system to insure that no power loss or hesitation is present.

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

Page 103