Adjusting the Carter YF Metering Rod

YF Metering Rod Adjustment

YF Metering Rod Adjustment

The adjustment below is for early Carter YF carburetors that don’t have an adjusting screw at the top of the metering rod. 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. With the throttle valve seated (this means the idle screw is not holding the valve open) in the bore of carburetor, press down on upper end of diaphragm shaft (C) until diaphragm bottoms in vacuum chamber. On carburetors which have the projection (D) on the pump lifter link, metering rod should contact the bottom of the metering rod well (E), and metering rod arm (F) should contact lifter link (G) between springs and at supporting lug (D). Adjust by bending lip (H) up or down. In the illustration there is a tool that can be used to hold down the diaphragm, but chances are you won’t have one of those and it isn’t that big of a deal to hold it down.

On models that do not have the projection (D) on pump lifter link, the metering rod should bottom in the metering rod well (E) and flat of metering rod arm (F) should make parallel contact with flat of pump lifter link (G). Adjust by bending the lip (H) up or down.

6 thoughts on “Adjusting the Carter YF Metering Rod

  1. Just got on this blog and have never subscribed to one before so forgive me if do something out of line and let me know correct etiquette. My 1956 Willys with Carter YF 938SA has a very bad flat spot when getting on the gas between shifts. I’ve updated to the 75-1500 rod from the 75-966 with no luck. My pump discharge circuit is updated to a weighted needle from the ball and weight. What is correct size of ball for this and is it aluminum or steel? Also between 1800-2300 RPM under load at WOT in 2nd and 3rd gear there is a bad surging or bucking that goes away above 2300 and stops if I let up on the throttle.

    • This is the response I got from one of my expert consultants.

      Presumably the owner knows the procedure/dimension to set the float for both a solid inlet needle or a spring-loaded needle (the latter is recommended). So:

      1) Intake check ball I (if any) is medium size aluminum.

      2) Discharge check ball is medium or large steel (I forget which).

      3) Be certain that bowl has not been resurfaced so many times that weight on top of ball is “bottoming” against lid, allowing no gasoline to pass ball. Solution: shorten weight.

      4) Re-set both seats using a sacrificial steel ball (NOT aluminum!) of the correct size. Rap the ball a couple of times with a hammer and a punch of appropriate diameter. Discard sacrificial balls which now have a flat spot on one side and a score mark on the other and replace with balls of appropriate size/material.

      5) Check to be sure that pump lifter link and metering rod hanger are adjusted to specs.

      6) Oversize the current accel. pump discharge orifice by .001″ – .002″ to compensate for “lighter” characteristics of modern gasohol.

      7) If carb leaks down while parked, suspect bad pump diaphragm or warped pump cover. Also inspect for warped bowl at base mating surface preventing vacuum from reaching pump diaphragm.

      Now, all of that applies only if it is indeed the carburetor!

      It can also be:

      1) Fuel hose liner collapsing under vacuum and restricting delivery to carb.

      2) Faulty fuel pump.

      3) Clogged in-tank fuel filter.

      4) Clogged/nonexistent fuel tank vent.

      5) Clogged fuel filter.

      6) Rubber bowl gasket in Carter-type fuel filter “swollen shut” due ethanol contact (replace with cork style).

      7) Bad/disconnected vacuum advance.

      8) Fault located elsewhere in ignition system.

  2. Thank you very much. Spring loaded needle and seat assembly float at 15/32″ right? Needle and seat assembly orifice size for my 1956 938SA is .076 correct? Seems kits have too large of orifices. I opened up the discharge jet .002″. Steel ball size for this passage is .125 correct?

  3. Mike, when I test the checkball like in your video I push the accelerator pump down and hold the checkball weight down with screwdriver tip. When I let go of the pump it rises up quickly and bubbles come out of the pinhole in the top of the accelerator pump casting cap. This is a 938SA. Never known there to be a ball in the cap.

  4. Like your video attached to the YF kit. When I push down on the accelerator pump to check the discharge ball seat you say the pump should stay down if hold slight pressure down on the checkball weight. When I push it down air escapes from under the diaphram cap through the pinhole in top of cap. Is 31-500 assembly spring loaded?

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