Adjusting the Carter YF Metering Rod

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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.

12 Responses

  1. Don Alves

    Having trouble at lower speed, seems to miss and has low power on a 71 F-100 300 with a Carter YF clone bought from National Carb. Replaced the accelerator pump and that helped on takeoff but did little for low speed performance. Have put all new ignition parts on and reset timing to man. specs with no real improvement in performance. Runs great at higher speeds but I still feel a slight lag at these speeds. The firing order is 1 thru 6 straight front to rear, the plugs removed from 5 and 6 are very white in color which leads me to a possible lean condition, the rest of the plugs seem a little darker in color as a light tan but no dark or black at all. This carb has no real way to adjust the fuel mix at low speed that I can see.I saw your article on adjusting the metering rod but no info on how to correct a too lean situation. Is there a way to adjust for this or go to a bigger main jet or possibly a smaller metering rod? This is my daily driver, any help would be appreciated.

    • Not sure what clone you got, but if it’s one of the China products then I won’t be able to help you much. Don’t sell them and never had one apart. I find it odd that only 2 plugs are lean. It may be the carburetor is on the edge for rich/lean, but who knows what jets that carburetor would use.

      • Donald Alves

        Mike, I finally took my truck to a professional carburetor tuner and he found a few problems with this carb. First,the bore in the accelerator pump four screw hold down plate was too tight and would not allow the pump arm to move freely as well as the metering rod assembly not moving. The float level was too low as well as the plastic float had a crack in it and was replaced with a brass float. The choke pull down linkage had a non stock bend in it and was returned to a normal function shape. The carb was tuned after a leak in the retard circuit diaphragm at the distributor was repaired and then recurved to complete the repair. The truck now runs like new with none of the original symptom’s described. Hope this helps others with similar problems.

  2. I just rebuilt my YF using one of your kits. I believe it is running too lean. No power when accelerating with choke wide open. If I close the choke 1/2 way when engine is hot it runs much better. I adjusted the metering rod per instructions … Bottom out then full clockwise turn. Would another half turn clockwise on the metering rod adjust cause the carb to run richer? Wondering if I should try this or if I’m way off base here. Accelerator pump is functioning properly

  3. 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?

    • Hi. wondering if the problem on the above post was resolved. I am having the same problem on a carter yf in a 54 International pickup.

  4. 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.

    • I’m not sure exactly what you are doing. Are you referring to the check ball in the accelerator pump cover, or are you testing the discharge check ball?

  5. 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?

  6. 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.

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