Holley 1940 1 Barrel 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.

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.


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.

FLOODING

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:

When a Holley 1940 single-barrel carburetor refuses to idle, it almost always points to a blockage in the very fine idle circuits, a vacuum leak, or issues with fuel delivery specifically isolated to the low-speed circuit. Because fuel is pulled through a tiny idle tube and routed through microscopic air bleeds and passages when the throttle plate is closed, it doesn't take much varnish or debris to completely kill the idle.

Troubleshoot and fix a Holley 1940 that won't idle using this systematic checklist:

1. Clogged Idle Tube or Internal Passages

The idle circuit begins at the main jet, travels up the idle tube, mixes with air from the idle air bleed, and exits through the idle discharge hole and transfer slot just past the closed throttle plate.

• The Fix: Remove the top cover and inspect the idle tube. Ensure it is completely clear. Use a piece of very thin wire or guitar string paired with aerosol carburetor cleaner and compressed air to physically clear the idle passages.

• Check the Idle Air Bleed: Make sure the fixed idle air bleed hole inside the throat is open. Blow compressed air through it and verify it exits properly. There is also a transfer slot just above the idle screw port that can get plugged.

2. Blocked or Damaged Idle Mixture Screw

If the idle mixture screw circuit is obstructed, the engine will get no fuel at closed throttle.

• The Fix: Completely back out and remove the idle mixture screw. Inspect the tip—if it is grooved, bent, or blunted from being overtightened previously, it needs to be replaced. Match is overall size and tip.

• Spray cleaner and blow compressed air through the mixture screw hole. If you cover the small idle discharge hole in the bore and blow through the mixture screw hole, air should travel freely back into the float bowl.

3. Warped Air Horn / Top Cover (Internal Vacuum Leak)

A notorious design flaw of the Holley 1940 is that the main body/air horn cover tends to warp, usually due to overtightening the perimeter screws or cranking down too hard on the air cleaner wingnut.

• The Symptom: Warping creates an internal vacuum leak across the internal passages right next to the venturi, bypassing the idle circuit entirely.

• The Fix: Lay a straightedge across the top cover mating surface. If it's warped, you may need to carefully flatten it using heat and a vise. A thin layer of Permatex Anarobic can help.

4. Loose Throttle Body Base

• The Symptom: The screws connecting the carburetor's main body to the throttle base plate are prone to working loose over time.

• The Fix: Check these mounting screws. If the throttle body is loose against the main body or the intake manifold, it will pull unmetered air and kill the idle. Snug them down evenly (use a drop of threadlocker like Blue Loctite if they continuously back out).

5. Main Discharge Check Ball Issues (Flooding or Siphoning)

• Check the Check Ball & Weight: The Holley 1940 uses a single 1/8-inch check ball in the main discharge hole, topped by a check weight. If the check ball or weight is missing, fuel can continuously dribble or siphon into the intake manifold at idle, choking or flooding the engine out.

• The Test: Look down the carburetor throat while the engine is idling (or manually feathering it). If you see fuel dribbling out of the main discharge nozzle when it should only be pulling from the idle circuit, your check ball is failing to seat or is missing.

6. External Vacuum Leaks & Base Gasket

• Inspect the base gasket where the carburetor meets the intake manifold. Ensure you used the correct gasket and that it isn't misaligned, torn, or sucking air. Check all vacuum lines connected to the manifold or ported sources.

• Spray carb cleaner around each end of the throttle shaft. Any change in RPM means the throttle shaft will need bushings.

Quick Diagnostic Sequence:

1. Spray test: Try keeping the engine alive by slightly feathering the throttle (which brings the main and transition circuits into play). If it runs fine off-idle but dies the second you let off the gas, your idle circuit is definitively plugged.

2. Clean the circuit: Pull the idle mixture screw, blast carb cleaner and compressed air through the ports, and verify passage clarity.

3. Check for warpage/leaks: Verify the top cover is flat and the throttle body bolts are tight.