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  6. Running Rich at Idle or Flooding (Carter AFB)

Running Rich at Idle or Flooding (Carter AFB)


  • Test your fuel pump. New fuel pumps are especially troublesome. Check your motors manual for the proper pressure, but for the AFB it should be around 5 lbs. Too much pressure will cause the needle not to shut off the gas.
  • The needle & seat may be damaged. If while adjusting the float, pressure was put on the needle then it could easily have damaged the viton tip. If the needle is new, wipe the viton tip with mineral spirits to wipe off the black residue. Turn the top upside down and while the floats have the needles closed, blow into the fuel inlet (not too hard). Air should not get through.
  • Make sure there is a gasket under the seat and there is only 1 gasket. We have seen old gaskets left behind, then a new one added on.
  • Test your floats for leaks. Heat up some water and immerse the float. Bubbles would indicate a bad float.
  • Check the float level.
  • Your floats should be centered in the float bowls.
  • Make sure there is clearance on each side of the float tangs.
  • Move the float up and down to feel for any resistance, or catching.
AFB Flooding
  • There may be dirt getting into the needle & seat holding the needle open. Filters may be dirty, or perhaps the fuel tank is dirty.

Flooding can be summed up with one idea. Too much fuel is getting into the float bowl.

Running rich after rebuild

  • Since it was running OK before we can rule out the metering rods and main jets. 
  • Make sure the passages from the idle mixture hole and the off idle above it go all the way through the venturi’s. The small holes on top of the venturi is what you are looking for. Look at the off idle video to gain some knowledge on how that works. 
  • If the carburetor was sitting for many months, or years I would highly suspect a plugged passage. Look at every hole you can find and test by blowing air through the hole.
  • Look down the carburetor at idle. Do you see fuel dribbling out of the venturi. If so, then that is probably the problem. The main discharge may be leaking. There will be a check weight in the discharge hole (with pointed end), or perhaps a check ball and a spring above that. Depends on the vintage of the AFB. Take the carburetor off and test the accelerator pump circuit. The small holes on the venturi can get plugged easily.
  • It isn’t unusual for the venturi gaskets to need to be trimmed a bit so they sit flat. If they don’t sit flat fuel will leak into the bore.
  • Test the needle & seat. It is possible that while adjusting the floats pressure was put on the viton tip damaging it. It isn’t unusual to get dirt into the needle & seat after rebuilding.
  • Be sure the floats are centered. Move them up and down to feel any catching.

As with flooding, too much fuel is getting into the float bowl, or extra gas is getting siphoned into the bore because of a plugged air vent.

Updated on 04/05/2022

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