CARBURETOR 68

CHAPTER FOUR


CARBURETOR

DOUBLE NEEDLE MODELS

Pilot Metering System

1. The pilot metering system operates from 0 to 1/8 throttle opening.  The fuel is metered by the pilot fuel jet (see Figure 1) and the air by the pilot air jet and throttle slide.  The low speed mixture screw meters pre-mixed fuel and air into the carburetor venturi.

2. With the throttle closed and engine idling, the up stroke of the piston creates a vacuum in the intake port.  This causes air intake through the small left hole under the carburetor mouth.  It goes through the pilot air jet and over the pilot fuel jet, causing the fuel to mix with air.

3. The pre-mixed fuel flows past the low speed mixture screw into the carburetor bore through a hole in front of the slide, and into the engine.

4. With slightly opened throttle, more fuel enters the engine through a hole inside the front edge of the slide.  With the slide raised slightly, more air flows in to mix with the fuel.

5. The low speed mixture can be changed by changing the pilot fuel jet or the pilot air jet, or by adjusting the low speed mixture screw on the right side.  Turn the screw clockwise for leaner and counterclockwise for richer.

6. Correct adjustment of this system is import­ant to all engines.  It may hesitate when the throttle is opened, if the mixture is too lean, or load up if it is too rich.  This applies to competition engines also.

7. The pilot metering system continues to operate at wider throttle openings, but has less influence on performance.

Slide Cutaway and Mid-Range Needle

1. This system operates from 1/8 to 1/4 throttle opening.  The carburetor slide is cut away on the air intake side.

2. Two tubes protrude from the venturi bottom (see Figure 2).  The shortest one is the mid-range needle jet.  The long one is the high speed needle jet.  The slide's tapered needles insert into these tubes, metering the flow as the slide is raised and the needle withdrawn.

3. From 1/8 to 1/4 throttle opening, the slide cutaway directs air down against the mouth of the mid-range needle jet to decrease the vacuum and the amount of fuel withdrawn.
 

4. The height of the slide cutaway is stamped in millimeters on the top.  Higher cutaway causes leaner mixture, and lower cutaway richer mixture.

5. The mixture is adjusted by changing the cutaway or by raising or lowering the needles.  The latter method is preferred.


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