Motorcraft 2100 Carburetor Troubleshooting

Have 65 mustang w/ 2100 carb. Floods so bad car will not run watched your video on rebuild and order a new float… Is this the way you would go?

*If you have a black nitrophyl float, then by all means replace it. For brass, heat up a pan of water just prior to boiling. Immerse the float. Any crack, or hole will show up with bubbles.

*Check the fuel pump pressure. Compare with specifications from your engine manual. New pumps are especially suspect.

*Check the needle & seat – look under the seat and make sure there isn’t an extra washer, or residue under the gasket. Oh and yes, did you remember to put a gasket under the seat. Make sure it isn’t cracked. Test your needle & seat for leaks.

*Gently hold down the needle into the seat and blow into the inlet hole. Air should not get through. The needle has a viton tip and is easily damaged if you put any pressure on it while adjusting the float level.

*Check the float level.
*Make sure the float isn’t rubbing on anything. It could be catching on the red rubber check valve. Cut the stem off if it is catching.

Runs Bad Over 1/2 Throttle

Question:The engine bangs out really bad as if it’s not getting enough fuel. I don’t know the history on this engine, it has been rebuilt, not sure if the cam in stock, it does have headers on it. Is it possible that I need even larger mains?

A new ignition control module fixed it!!

What Does The Float Spring Do?

Question: I have a damper spring on my float pivot rod. Currently it is oriented with the short side under the float hinge and the long side against the back of the float bowl (behind the needle) …in effect putting slight up pressure on the float. The spring is in its natural state, i.e. not wound around giving more tension. Is this correct?

Answer: The spring is used to keep the float from bouncing. A small amount of tension is desirable. If it is flopping around then wind it one turn. From what you describe you have it in the right position.

Vent TubeQuestion:Where can I get the vent tubes and how do I replace them?Answer:The tubes can be found here:
These tubes are pressed in. You should be able to move the tubes back and forth while pulling on them. You can also drill them out, just be careful not to drill too large. Install by tapping them in gently.

In general:
Vents are used so that the float bowl doesn’t create a vacuum and siphon gas.
Older carburetors simply vented the fuel bowl to the air. Later, the vent was routed back to the engine so that the gas fumes were burned (pollution control).
For the type that vented back to the engine, it can be left open with nothing connected. If gas comes out of the vent then you are flooding (too much gas) On some models the vent is opened and closed via a rubber stopper at the vent hole and it is opened and closed via the accelerator linkage. This type should work as designed.

Power Valve Bad

Carburetor Power Piston

My thoughts are they installed the power valve incorrectly. The newer power valves use a large gasket that fits into a grove a the edge of the power valve sealing surface. I hold the carburetor upright as I screw the power valve and gasket into the main body. This operation is complete when you see the gasket is securely seated into the grove on the lip of the power valve. If not seated correctly it will damage the gasket and the power valve will leak like it is ruptured!! The same thing will happen if they use the wrong gasket for the two different power valves with different sealing surfaces. The newer power valves use the perimeter thick gasket and the older ones use two different types. One gasket is thick and triangular on the inside ( probably the correct one for Ford 2100 &2150) and round on the outside. The other is thinner in thickness and diameter and has three or four tabs on the inside to keep centered on the power valve. (used more often on spark valve like the 1100 carb) You look at both older gaskets to see if they seal to the mating surface on the main body. Possibility that the venture cluster air bleeds are plugged. Remove the carb air horn then the cluster and blow out passages in main body and the cluster assembly, Don’t forget to remove the pump discharge check ball and weight!! Maybe a bad power valve if installed correctly?

Red valve

Question: I had gotten a 4008 kit from you for my 2100 autolite 2bbl. I was wondering which hole the little red plug goes into in the acc pump location. Mine has a center hole and one right under it..I think it goes in center hole.

Answer: The red silicone rubber (inlet check) goes in the center hole of the main body. The other hole is the inlet for fuel to refill the accelerator pump cavity. After the pump discharges thru the squirt nozzle the lip of the red silicone check lifts and fuel refills the pump cavity. Caution when installing the rubber check by lubricating the stem or it might break!! Thanks JIM L

Accelerator Pump Cover Leaks
Cover may be warped. Put it in a vise and heat it up. Squeeze so it becomes flat.

Float Bowl Runs Out of Gas When Sitting

Gas these days has a low boiling point and evaporates much quicker than it used to.

  • Check any vent line and gas cap going to the fuel tank. If plugged a vacuum will be created that sucks the gas out.
  • Make sure the vent on the top of the float bowl is not blocked.
  • Pull the carburetor off, sit it on paper, fill the bowl with fluid. Look for leaks out the bottom overnight.
  • May be percolating. Engine gets hot, turned off, the fuel boils and evaporates out. Try other brands of gas. Look for ways to cool the gas. Rerouting gas lines, etc.
  • Needle may not be seating. If new wipe the viton tip with mineral spirits.
  • Test the fuel pump for the correct pressure. Around 4.5 lbs would be good, but refer to your motors manual.
  • Installing an electric fuel pump with a regulator would be the last thing to try.

Where can I Get the Vent Tubes & How Do I Replace Them?
The tubes can be found here: These tubes are pressed in. You should be able to move the tubes back and forth while pulling on them. You can also drill them out, just be careful not to drill too large. Install by tapping them in gently.

Power Valve Vent

Motorcraft 2100 Carburetor Power Valve Vent.

When I pour gas in the fuel bowl in order to test the pump, all the fuel leaks out the bottom. Is that supposed to happen??

Is the power valve installed. It resided on the bottom. If it is installed, then it might be leaking.

Hard Starting after engine is warmed up.

If this was a problem before tinkering with the carburetor, then make sure the electrical system is in good shape. The carburetor is blamed for many problems when in fact the electrical system is most likely at fault.
Check to make sure the choke valve is open completely when warmed up. If it isn’t opening, then the fuel mixture will be too rich for starting.
This could be due to gas leaking into the engine after the engine is turned off, or at idle. At idle look down the carburetor and see if gas is dribbling out of the venturi. If so, then you have a main discharge problem. The 2100 used a check ball and check weight in the discharge. Be sure they are there. Check out this page on how the accelerator pump circuit works and how to test it.
The check ball in the discharge may be leaking. Tap it lightly to form a good fit around the ball. Not too hard or you won’t get it out.
Gas dribbling out of the venturi can also be caused by gas being siphoned as with the above. Non vented gas cap. Remove the cap and see if there is suction. If so, then the gas cap needs to be replace by a vented cap.
Is the vent on top of the float bowl open? It must be opened to the air, or in some cases a rubber hose ran back to the engine.
Hard starting can be caused by too much fuel when hot, or not enough. Is the fuel boiling out after turning the engine off. One way to figure out if its too much fuel is to hold the throttle all the way to the floor when starting hot. That will introduce much more air to the mixture. If it start that way, then you know it’s too much fuel.
Do not pump the gas when the engine is warm, or press on the gas pedal slightly.

No Fuel Out Venturi

My motorcraft 2100 serial D2PF GB for a 1964 1/2 mustang 260 does not send gas from the venturi into the 2 barrel. When the venturi screw is removed there is gas when the accelerator pump is manually pumped. Also when screw is loose there is gas. Engine fires with starting fluid every time. I can’t understand why it doesn’t flow. Help! I love your helpful videos but I haven’t found anything about this. Seems like some sort of adjustment. 

  • Venturi passages are plugged
  • There should be a check ball and a check weight in the main discharge hole. The hollow screw goes over the check weight. If for some reason the screw is holding down the check weight (maybe too long), it would cut off the fuel.

Springs for the choke pull-down

These are not available, however we do have two possible replacements, these were recommended by another rebuilder. We haven’t tried them ourselves.

Motorcraft 2100 2 Barrel Carburetor Pull-Down Spring
Motorcraft 2100 2 barrel carburetor pull-down spring.

What is the felt gasket for?

That goes on the choke rod just underneath the top. The types that need this will have a clip under the top to hold this in place.

Rich Condition
A rich condition can be caused by any of the items above along with the following:
Gas may be getting siphoned through the main discharge. Watch the venturi at idle. Do you see gas leaking into the bore? It should not be. This can be caused by the float bowl vent being plugged off, gas tank vent plugged, venturi gasket not fitting flat, which can hold up the venturi, or perhaps the main discharge not installed correctly. Most have a check weight installed. Take a look at this page about how the accelerator pump circuit works. There is information there about the main discharge.
Main jets might be too big. This might happen if you installed a different carburetor on the vehicle. The replacement carburetor may not be jetted for your engine.

What Makes 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.
  • Test the fuel pump. Pressure should be 4-4.5 lbs, or as per your motors manual. A new fuel pump is especially suspect. Worst case is to put a regulator between the fuel pump and the carburetor. Excess pressure will hold the needle open.
  • Some 2100 carburetors have the rubber check valve in the wall of the float bowl. Be sure the stem is cut off so that it doesn’t interfere with the float.
  • Move the float up and down by hand gently, to see if it catches. A worn float pin can bind if worn.
  • The power valve might be ruptured allowing excess fuel into the main circuit.
  • Test the float. Heat up some water and immerse the brass float. Any bubbles out of the float is a leak. Nitrophyl floats (like plastic) can only be weighed in grams.
  • Has the car been sitting for an extended period of time? If so, then the gas has probably turned and varnished the inside of the carburetor. The problem is if you clean the carburetor it will happen again because the varnish in the gas tank will end up in the carburetor again.


In order to start cold and idle correctly all of the choke components have to be working together, correctly.

To start a carbureted engine:
Press the gas pedal once. This will use the accelerator pump to squirt extra gas into the intake. Look down the carburetor when cold and engine off and pump the throttle once. You should see 2 squirts of gas. If not then you have accelerator pump problems.

At this point, the choke valve should close and stay mostly closed while starting. The choke valve should start opening slowing while warming up.

With the choke closed, the fast idle speed should pick up. See 2100 adjustments.

Can I have 3 Fuel Filters?

Every added fuel filter will cut down on fuel flow. One filter is enough.

Will a 302 Carburetor Work on a 351?

No, not correctly. The venturi size is different.

Pops Through The Carburetor

  • Most likely electrical
  • Carburetor could be running lean
    • Float level off, or needle not opening enough.
    • Fuel line kinked, or plugged
    • Fuel filter plugged. You need 1 fuel filter between the fuel pump and carburetor.
    • Fuel pump weak – test for 4.5 lbs prox.
    • Foreign matter in the needle and seat. This happens often when teflon tape is used on the carb fittings.
  • If by chance this carburetor is a recent transplant then it may not be jetted for the engine.
  • A transplanted carburetor may not have the correct venturi size.
    • 1.08 – 289, 302
    • 1.21 – 351, 390
    • 1.28 – 390, 400
    • NOTE: These are approximate sizes. Look on the side of the float bowl for the venturi size, usually cast in a circle.

Only Runs With Choke Closed or Partially Closed

This means there is too much air and not enough fuel. Here are some ideas not in any order.

  • Clogged fuel filter.
  • Gas line crimped.
  • Rubber gas hose has deteriorated because of the ethanol. Use fuel injector hose. You will usually see black flakes in the bottom of the fuel bowl.
  • Float level way off not allowing enough fuel into the carburetor.
  • Float may be sticking. Manually move the float up and down to see if there is any binding.
  • There might be a vacuum leak. Check base of carburetor mounting, intake manifold and any vacuum lines leading from the carburetor.
  • Restriction in the fuel inlet, or needle & seat. I once found an earwig bug in a fuel inlet restricting the gas flow.
  • Float valve (needle) sticking closed. The viton tip may have been damaged.
  • If the vehicle has been sitting for months then the gas may have turned into varnish coating the inside of the carburetor. You can usually smell the varnish. Remove tank and professionally clean the inside. Clean all fuel lines. Clean the carburetor.
  • One of the passages may be plugged – Use compressed air, or carb spray cleaner to spray through each passage and make sure it comes out the other end.

Idles Too Fast With the Idle Screw All the Way Out And Choke Open

The only way to get a fast idle is if the throttle valve is open, or with a vacuum leak.

  • With a vacuum leak the engine would probably run rough. Often the leak will produce a squeal.
  • Disconnect the throttle linkage 1st to make sure it isn’t holding the throttle open.
  • The throttle valve may not be closing completely. This usually happens when someone puts them in wrong, or they got bent. Take the carburetor off and see if you see light around the throttle valve.
  • Corrosion, or carbon build up is holding the throttle valves open.


Updated on 02/09/2022

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