Carburetor Troubleshooting

A lot of

Motorcraft 2100 Troubleshooting

1st off, there is seldom just one thing that can cause a problem in your carburetor. You may need to look at several items before finding the problem.

Do not adjust your carburetor until you have found the real problem. Making adjustment may just cover up the real problem and cause other problems.

What Make my Carburetor Flood

Needle & Seat Leaking
  • 1st check the needle & seat for leaks. Blow thru the inlet, or hook up a pressure tester to the inlet. Don’t go over 5 lbs of pressure. The viton tip on the needle may have been damaged by putting pressure on it when adjusting the float.
  • Your fuel pump might be putting out too much pressure. This is especially true of new fuel pumps. Test the fuel pump for 4 to 4.5 lbs of pressure. Check your motors manual for the exact specification.
  • Check the float for leaks. Shake the float to see if there is fluid inside. If the engine has been sitting for an extended period of time then the gas inside the float may have evaporated so heat up some water just prior to boiling and immerse the brass float in the water. For Nitrophyl floats you will need to weigh the float with a gram scale.
  • Inspect the float pin for any wear. Move the float up and down by hand so you can feel any catching.
Plugged Passageway
  • If the engine has been sitting for months, the gas may have turned into varnish coating the inside of the carburetor, blocking passages and keeping the float needle from sealing.

Runs Rich

Reasons for flooding can also be the same reasons for running rich.

Black smoke out of the tailpipe indicates a rich condition. A strong gas smell is another indication.

Carburetor Running Rich


Jets – If you have been running the same carburetor and it was running OK then it isn’t the jet size. Now if you picked up a different carburetor then there is a good chance the jet size is wrong. 2100 carburetors are easily swapped around, but that doesn’t mean it is correct for your engine.

Motorcraft 2100 venturi size also matters. A 1.21 meant for a 390 will not work as well on a 289.

Can’t get the carburetor to idle, or rough idle.

  • Adjust the idle mixture.
  • Check for vacuum leaks. Check around the mounting gasket, vacuum hoses and intake manifold. Spraying carburetor cleaner around the gaskets can help. Any RPM change would indicate you found the vacuum leak.
Idle Mixture Screw
  • Check the idle mixture screw for scoring, or bending.
  • Spray carb cleaner thru each passage to make sure the passage is open throughout.
  • Above the idle mixture discharge inside the bore there will be a transfer slot, or hole. This is used to provide extra gas to the idle system as RPM moves up. This transfer is connected to the idle passage and idle mixture. Any carbon buildup, or dirt will affect the idle.
Carburetor Throttle Bore
  • Throttle bore channels are dirty, or carboned build up.

Hard starting after turning the engine off.

  • Look down the carburetor after turning the engine off. Gas should not be dribbling out of the venturi, or main discharge. If it is then the check ball may not be sealing, or because of bad venting the float bowl has pressure, forcing gas out of the discharge.

No power

  • Power valve bad
  • Float level is off
  • Fuel filter is dirty
  • Fuel hose is kinked, or if a rubber hose may have been eaten by ethanol and will collapse inside.

Carburetor Vacuum Channels
  • Vacuum channels, or power restriction are plugged, carboned up, or some other obstruction.

Carburetor Power Piston

Can’t Get Idle Down to Specifications

  • Power piston is scored, dirty, or bent.
  • Throttle valves are installed wrong, upside down, or switched between bores. Always mark the throttle valve before removing to make sure they are put back the same.
  • Back out the idle screw and open the choke valve. The throttle plates should be closed.
  • Is the throttle linkage binding? Disconnect the linkage to see if the symptom goes away.
  • Check the mechanical advance in the distributor. Are the springs intact?
  • Check the vacuum lines for leaks. Replace any rubber that has gotten hard.

Hesitates or Bogs

  • With engine off look down the carburetor and pump the gas once. You should see 2 strong squirts coming out of the venturi.
Carburetor Check Ball
  • The inlet or outlet check ball may not be sealing, or someone put in the wrong size.
  • Test the accelerator pump system by putting fluid in the float bowl, we use mineral spirits, and pulling up on the accelerator pump to see if the pump well is filling up.
  • Press down on the pump and fluid should come out of the main discharge.
  • Hold down the check ball in the main discharge an push down on the pump. You should feel a slight pressure. If not, then tap lightly on the check ball to form a better seal.

Why Does My Carburetor Backfire?

This is caused most likely by a very lean condition. Either there is a vacuum leak, causing too much air, or there isn’t enough fuel getting to the intake manifold.

  • Check the carburetor mounting gasket, where the throttle body connects to the float bowl, vacuum hoses and fittings and the intake manifold.
Updated on 10/10/2021

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