Autolite 4100 4 Barrel Troubleshooting

Autolite 4100, 4 barrel carburetor troubleshooting. This is where we try to address customer questions and provide the best solution to their problems.

Baffle Plate
I am rebuilding a 4100. It has the spring mounted in the choke case. It does not have a choke baffle plate. I assume all should have one to control heat. is this correct and are they available?

The baffle you refer to is used to keep the heat in the choke thermostat. It would be nice to have the baffle, but they are often left out because they are unavailable. It isn’t that hard to make one from a piece of tin.


There are several things that make a 4100 4 barrel flood. Here is a list of things you can check, not in any order of importance.
Bad needle – Any pressure put on the viton tip will damage the needle. Dirt in the needle & seat – Many good rebuilds go bad due to dirt in the gas tank. Even fuel filters can’t keep it all out. Try blowing some air into the inlet to clear any dirt particles. If it keeps happening, then you probably need to clean the gas tank. Test the float for any leaks. Heat up some water just prior to boiling and immerse the float. As the hot water heats the inside of the float, it will expand and blow air through any hole. Check the float level. Set as per the instruction sheet. Do not fudge the float trying to correct richness. You are just hiding the real problem. Be sure the clip that snaps onto the float pin and fits over the seat is there. A missing clip, or wrong clip will allow the float to float up via the pin allowing too much fuel into the carburetor. Check the float to see if it is catching when going up and down. Test the fuel pump pressure for 4 – 4.5 lbs. Add a regulator between the fuel pump and carburetor if necessary. New fuel pumps are especially suspect as they are not always made well. The 4100 is susceptible to warpage which allows fuel to slop over the float bowl and into the carburetor bore. This can show up when going up and down hills. Use some flat stock to check for warpage. You can try a 2nd float bowl gasket. Do not use any sealer. Look for extra gasket material under the seat. Fuel might be leaking past the threads. A ruptured power valve can cause a rich condition. You can use a vacuum tester with some sort of attachment that fits one side of the power valve. You can also put one side or the other in your mouth and see if you can blow through it. See if fuel is dribbling out of the main discharge at idle, or when 1st shutting the engine off. The discharge may be leaking. There is information here on how to test the discharge.
Black Smoke – too rich Check your spark plugs for the correct burn color. Black could mean the jets are too big. Reduce by .002, white is lean and the jets should be increased by .002. Gray or tan is perfect. Leave the jets alone. This assumes the ignition is working properly. If the carburetor has been on the vehicle for some time and running OK previously, then your problem is not the jets. They don’t all of a sudden change size.

Too much fuel?
Hi,I’m working on a 66 autolite 4100. It’s a 1.12 in a 66 galaxie 500 with a 390. The carb was “rebuilt” when my neighbor bought the car. The carb is very clean and appears recently overhauled. It currently starts hard. You must hold the accelerator to the floor to get it to start. then when you shut the car off the carb dumps about 1/8″ fuel on the secondary throttle plates. The float adjustments seem to be in order. However they installed a different needle and seat combination than what you sell. the needles have flat bottoms with a small pad on them. THe fast idle lever was broken on the choke, and I purchased that from you about a week ago. That helped a little but the the fuel dump on the secondary’s seems to be a major issue. Any thoughts you can offer are appreciated. Thanks!

You are flooding (getting too much gas in the float bowl).
High fuel pump pressure (test the pressure and compare with specs from your motors manual)
Needle & seat not closing.
Float leaking and is too heavy.

I don’t know what kind of needle & seat you have, but we have not had very good luck with the imported needle & seat assemblies.

Another Answer:
The needle & seat as he describes are made by XXXXX and were in most XXXXX kits. I have seen the black rubber button come out, flip over sideways and be loose in the seat. any of these conditions would cause overfilling or flooding. Another problem is the tab on the brass floats. The original OEM floats tabs were straight and extended out over the contact point on the needle. The replacement floats that are available now are bent up at 90 degree angle on the end of the tab. I straighten the tab on to get better contact. I would think that condition would be more of a problem with Tomco needle & seats. Two possibilities: the adjustment of the sec. link and if the plastic sec, arm was installed correctly (this would cause the butterflys not to close all the way) also the sec. booster (and primary) will drip fuel if the air bleeds are plugged and/or dirty. (remove booster and blow out passages in booster and main body).

Idles too fast. Can’t get it to idle down?
It could be that the throttle plates are not closing completely. This is especially suspect if you had them off of the carburetor during a rebuild.Throttle plates not closing would be the same as if you were pressing on the gas pedal a bit.

What float setting do I use?
I have 1965 T-bird with a 1962 4100 carb. Would I use the 1962 specs for the carb or the 1965 specs? 

Use the setting closest to the engine size that you have.

My throttle shafts are loose. How do I fix those?
You can rebush the shafts, but you should know what you are doing. It is easy to ruin the carburetor. We know a few people that can do this for you.

Diaphragm install order
Disphragm, gasket, then cover.

Where does the 2nd check ball go?
One check ball is used in the 4100 and that is in the main discharge. If you got 2 in the carburetor kit, then put the larger one in the main discharge.
The 2nd check ball is intended for the secondary diaphragm, but it isn’t necessary. You can leave it out.
Fast Idle

4100 Fast Idle
Vacuum Advance Port
Autolite 4100 Plug
A customer needed to block this port. This is what he did:

In case anyone else ever is looking for one last night I picked up a 1/8 “ brass stub, cut it short and back filled with solder. The I ground it down and cut a slot to look like the OEM plug. Unless you look real close and notice the different color of the solder (in the center), you would never know.

Gas dribbles out of the venturi at idle
Most likely the issue is that the vacuum of the motor is pulling fuel out of the float bowl. No vent to the float bowl. Hard to block the vents unless someone has purposely done it. If the check ball and check weight in the main discharge isn’t sealing, then gas will dribble out. This can be cause by a dirty carburetor. Has it been sitting for over 6 months? The gas may have coated the inside of the carburetor blocking the small passages.

What If My Flange Isn’t Flat, or It Is Warped?

You can run it through a flat disc sander. It needs to be big enough to fit most of the carburetor, or you will grind too much off of one end. Do not take off anymore than necessary. A good re-builder will re-surface a carburetor flange when rebuilding.

Carburetor Seems To Draw Fuel From Secondary and Not Primary

Check your throttle valves. They may be open due to mal adjusted secondary’s.

Pops Through The Carburetor

This can be caused by a bad intake valve but also by a very lean condition.


  • Vacuum leak (especially if you hear a whistle coming from engine)- check mounting gasket, intake and all vacuum lines.
  • Not enough fuel
    • Float level
    • Weak fuel pump – test pressure. Should be around 4-5 lbs.
    • If the engine has been sitting for months, the gas may have turned and varnished the inside of the carburetor. Happens often on classic cars because they sit so much. You can usually smell varnish.
    • Fuel line restriction – non injection fuel line can get eaten by ethanol and/or collapse inside.
    • Sticking needle & seat.

After Sitting Overnight the Bowl Empties

  • After turning off the warmed up engine look down the carburetor. If you see gas seeping out of the main discharge, then you found the problem. Gas is being siphoned out of the carburetor and into the intake. It is probably leaking from the discharge check ball. Be sure there is a check ball and check weight. The check weight should have small amount of room to raise up.
    Test the pump circuit: Remove the venturi, gently hold down the check ball with a drift punch and pump the gas. No gas should come out of the discharge. If it does, tap the check ball gently to form a good trough for the ball. Test again.
  • The gas cap may not be vented. This will create a siphoning of the gas.
  • Perhaps the gas is leaking out of the bottom of the carburetor. Remove the carburetor and place it on some paper with gas in the bowl. See if it drains out.
  • Percolation is a possibility. Turn off hot engine and the gas boils. Some call this heat soak. Maybe the engine is running too hot.
  • Check any vents on the carburetor top. Any vents should be open.
  • If the gas tank happens to have a vent line running to the engine, or fuel pump, then make sure it blows through.
Updated on 01/07/2021

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