The Carter YH metering rod is dual-controlled by both mechanical throttle linkage and manifold vacuum via the diaphragm. Proper adjustment is critical for transition and power enrichmentâif set incorrectly, the engine will either run excessively rich or suffer severe lean bogs during acceleration.
Here is the exact procedure to set the metering rod height.
Back out the curb idle adjusting screw until it no longer touches the throttle lever. Ensure the fast idle cam is disengaged. The throttle valve must be perfectly closed and seated tightly against the inner bore.
With the bowl cover removed, press down firmly on the diaphragm shaft until it bottoms out completely inside the vacuum chamber. You must maintain this downward pressure for the duration of the measurement.
While holding the diaphragm shaft down, check the linkage. The metering rod arm must be making solid physical contact with the lifter link at the diaphragm stem.
Remove the metering rod and drop the Carter metering rod gauge (Tool T109-104) into the jet. With the stem still bottomed out, the pin on the metering rod arm should rest lightly against the notch on the gauge.
If you do not have the original Carter gauge, leave the metering rod installed. With the diaphragm fully bottomed, the metering rod should be completely seated at the bottom of the main metering jet. Additionally, the flat top of the metering rod arm should sit perfectly level with the top machined edge of the carburetor casting.
If the arm does not align with the gauge notch (or sits too high/low without a gauge), carefully bend the lip of the metering rod arm where the rod hangs. Bend it slightly up or down until it rests perfectly in spec. Never bend the brass metering rod itself or torque the diaphragm stem.
Key insight: The YH diaphragm also acts as your accelerator pump. Before adjusting the metering rod, inspect the diaphragm material carefully. If the diaphragm is stiff, wrinkled, or compromised by modern ethanol fuels, it won't pull the metering rod down fully under high manifold vacuum, causing the carburetor to run permanently rich.
