The main fuel metering system supplies the fuel required for the engine operation during the cruise and part-throttle range. The position of the metering rod in the metering rod jet, controls the amount of fuel admitted to the high speed nozzle. The position of the metering rod is dual controlled: mechanically, by movement of the throttle, and by manifold vacuum applied to the diaphragm. During part-throttle operation, manifold vacuum pulls the diaphragm assembly down, holding the metering rod arm against the pump lifter link. Movement of the metering rod is then controlled by the pump lifter link connected to the throttle shaft. This is true at all times when vacuum under the diaphragm is strong enough to overcome the tension of the pump lower diaphragm spring.

The upper spring serves as a bumper upon deceleration and as a delayed action spring on acceleration. Under any operating condition, when the tension of the pump diaphragm lower spring overcomes the pull of vacuum under the diaphragm, the metering rod will move toward the wide-open throttle position. The restriction and air bleed in the vacuum passage provides a lower and more uniform vacuum condition in the chamber below the diaphragm. To prevent heat-caused vapor bubbles in the nozzle passage and low-speed well from forcing fuel out of the nozzle, high-speed air bleeds are used. The purpose of the air bleeds is to vent the vapors and relieve the pressure before it is sufficient to force the fuel out of the nozzle and into the intake manifold. High speed air bleeds and the main nozzle are permanently installed and must not be removed in service.

ADJUSTMENTS

https://youtu.be/AQq1EWFz_c0?si=Kw3Z4GPfbhrlzLaH

LATE MODEL YFA (ALUMINUM BASE)

• Most YFA carburetors would have the star type of adjusting screw. This adjustment is important and should be performed anytime the carburetor is rebuilt, or the metering rod replaced.
• Remove the carburetor. We do not recommend working on your carburetor while on the car. This would chance dropping something into the engine.
• Back out the idle screw until the throttle plate is completely closed.
• Bottom the diaphragm by pressing down on the diaphragm shaft.
• The metering rod should contact the bottom of the well.
• For models with an adjusting screw, turn the screw until the metering rod bottoms in the well.
• Turn the screw one more turn.
• For models without an adjusting screw bend the lever the metering rod is attached to until the rod bottoms out.

CUTAWAY DIAGRAMS AND IMAGES

This cutaway view should give you a better idea of what the metering rod system looks like. Disregard the TOOL as pictured. The tool has been out of production for many years. Use the instructions above.