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Holley 1940 Carburetor Troubleshooting

Which check ball does the 1940 use?

The 1940 uses 1 check ball which is 1/8″ diameter. This is put in the main discharge hole and then the check weight goes over the ball. Be sure you have room for the check weight to move up. About 1/16″
I have a customer that wants to put a 1940 on his econoline but he is going to use a DUI distributor and he want to know what to do with the spark control valve. Someone told him to put a power valve where the spark control valve goes.

Any ideas on this one?

No need to do that, just leave it or if he wants he can plug it. Should work just fine.

Check Weight w/o Check Ball

The check ball must be there. If missing gas will be siphoned into the intake manifold at idle and probably after the engine is shut off. The check ball is used to block gas from going through the main discharge. 

If you idle your engine and look down the carburetor throat there should not be any gas dribbling out of the main discharge.
Is the Holley 1940 correct for my vehicle?

The Holley 1940 was used as a replacement carburetor for the Autolite 1100 carburetor. The 1940 was also used on a lot of industrial application. Wood chippers seem to come up a lot. There is nothing wrong with using the 1940. In fact in some respects it is a simpler working carburetor than the Autolite 1100 is.
I have a 1940 Holly, have completely rebuilt it thanks to your excellent video..Had a “professional” take a look even.. However, it will not idle at all?? Seems like the only fuel it gets is from the main jet and acc pump-nothing else??

Make sure the idle mixture screw passage is clear. Might be clogged somewhere in the Idle mixture circuit. Take the screw out and blow through it. Can you feel air through the small hole in the bore?

Next cover the hole in the bore and blow through the screw hole. Air should go through into the float bowl.

Get some thin wire and clean out the passages.
1940 Spark Control Valve

Not all Holley 1940 carburetors have a spark control valve. For those that don’t you may find the recess for the valve, but there will be no threads.
Accelerator Pump Does Not Return

When pumping the accelerator pump, the pump does not return. This is most likely due to the spring being installed incorrectly.

1940 Accelerator Pump
Assuming you are using the new spring in our premium kit, it should look like the one in the illustration. There is a recess that the pump moves up and down in and you want to make sure the spring sits on top of the recess and isn’t going down into the hole.

The other thing that might be a problem is the pump cup may have curled up when being installed. That will cause it to bind. Try spraying some silicon spray lubricant into the well when installed so that the pump slides in easily.

Carburetor isn’t getting enough fuel.

1940 Fuel
Test the fuel pump – new pumps are especially suspect. Should be around 4.5 lbs.Look for any kinks in the fuel line.If the vehicle has been sitting for more than a few months, then the gas has probably turned and coated the inside of the carburetor with varnish. This means the small passages may be plugged.Check the float level – Different specifications for different engines, so look at the instruction sheet of the carburetor kit, or your motor manual.There is also a float drop. Be sure that is set. I have seen them set so little that the needle doesn’t have any room to open.Check the float to make sure it isn’t rubbing on the side. They fit fairly snug in the float bowl.With the top off, turn it upside down and see if the needle falls down easily. Any sticking is a problem.Be sure the retainer is in place. The retainer holds the float in position.
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Holley 1940 Carburetor Flooding, or Too Rich Trouble Shooting:

Flooding, or running too rich is technically two different things, but in both cases too much fuel is getting dumped into the engine. When gas is coming out of the top of the carburetor, or from the throttle shaft end, then that is flooding.
If you are getting black smoke from the tailpipe, or the engine smells gassy, then that is too rich. Also when the spark plugs burn black indicated a too rich situation.

Electrical problems can also cause a too rich situation, but we will concentrate on the carburetor here.

The main jet can cause a too rich situation, otherwise the list below could be the cause of either problem. Look at the spark plugs to see what coler they are burning. Gray is good, white is too lean, black is too rich. When rich reduce the jets 2 sizes and try again. If too lean, then increase the size by 2 and try again.

Check the float for leaks. If you have the black nitrophyl float either replace it, or weigh it to see if it has absorbed any fuel. Weight should be 12 grams. For brass floats – immerse in hot water and look for any bubbles.

Check the float level.

The float pontoon should be level with each other.

Move the float up and down by hand to see if it is binding, or catching.

Any dirt on the bottom of the float bowl could mean dirt got into the needle & seat. Change your fuel filter and clean the gas tank.

Test the fuel pressure, especially if the fuel pump is new. Your motors manual should have the correct pressure, but around 4 lbs would be OK.

Make sure the gasket under the needle & seat was cleaned off totally. Make sure there is only 1 gasket.

Look at the needle to see if there are any lines, or marks on the viton tip. If there is, then it was damaged when installing. Any pressure on the needle while adjusting the float will damage it. 

Look for any cracks around the needle & seat inlet – You don’t want any fuel to leak around the needle and seat. 

Power piston should be free to move up and down easily. If stuck open, the power valve will be open all of the time. allowing too much fuel through. 

Check the main discharge. There should be a check ball and a check weight on top of it.

Check weight missing? You can use a 2nd check ball instead.

Check the main discharge to see if fuel is dribbling out while idling. That would indicate a check ball problem.

Watch a video about the 1940 flooding:

Why Does My Car Die at Idle, or Low RPM

There can be many reasons for this. Here is a list, not in any order of importance.

  • When Cold
    • Choke thermostat isn’t closing. Do you press on the gas pedal once before cranking the engine. This allows the choke to close and provides a squirt of gas from the accelerator pump. A cold engine needs a bit of extra fuel to get started.
    • Fast idle may not be set correctly allowing the RPM to fall too low.
    • Choke thermostat not adjusted properly.
  • When Hot
    • Choke thermostat not opening. If the choke valve is closed when the engine is warmed up, then the engine will get too much fuel and die out.
  • Cold or Hot
    • Look for a vacuum leak. Spray carburetor spray cleaner around the base of the carburetor and at any vacuum ports and hoses. Also test the intake manifold. If the RPM smooths out, then you have found the leak.
    • Ignition problems. Check spark plugs, wires and distributor.
    • Idle mixture screws are out of adjustment.
    • Check the float level. While doing that, shake the float to be sure it isn’t leaking. Nitrophyl floats should be replaced with our brass float.
    • The idle mixture passage may be plugged. Follow it all the way from the throttle body up to the venturi. Blow air into one end and make sure it comes out the other end.

Why Does My Carburetor Die Off Idle?

  • Accelerator pump not working. Look down the carburetor and with engine off, pump the gas once. You should get 1 good squirt out of the main discharge into the bore. More on the accelerator pump circuit here.
  • Float level many be off not allowing the float bowl to fill.
  • Float pontoon could be rubbing on the side of the carburetor bowl. There isn’t much room between the pontoon and the float bowl wall.
  • Distributor advance not working properly. You can check this with a timing light and revving the engine. The timing mark will move around.

Why Does My Carburetor Only Run With Choke On?

There are several possible solutions, not in any order:

  • Vacuum leak – check carburetor mounting, throttle body gasket, intake manifold, vacuum hoses, etc.
  • Not enough fuel in the float bowl – test fuel pump pressure, float level, sticking needle and seat.
  • Most likely a passage is plugged. Squirt carb cleaner into each passage to make sure it goes through to the other end.
  • If the vehicle sat for several months, the gas may have turned and coated the gas tank and carburetor. Fresh gas will not take care of the problem.
  • In conclusion there is not enough fuel, or too much air.
Updated on 04/24/2021

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