Autolite 4100 troubleshooting


15 Year Member
Oct 3, 2003
Bethesda, MD
I had started to rebuild my 4100, but after starting to take it apart, and people saying that it looked very good, I decided to put it back together and start with a good dose of carb cleaner.

At first, it ran really rough, and kept wanting to die. I noticed that one of the bleed holes in the secondary booster was clogged. After clearing that out, it idles much better and doesn't want to die anymore.

So here is what I am seeing now: it is very rich at idle (warm and cold). I can screw the idle adjustment screws in all the way, and it still runs. I know that indicates that fuel is getting in there from somewhere else. I have a hunch it is from the secondary circuit.

After I shut the engine off, if I look down into the carb, there is gas pooling on top of the driver's side secondary throttle plate, and it appears to be dripping from the booster. But when running and idling, the plates are closed, and no fuel drips from the booster.

And if I quickly take the air horn after shutting the engine off, the secondary bowl is almost empty. The primary bowl is pretty much full.

So I am confident that the fuel is dumping through the secondary circuit. But I don't know how to fix it.

When I had the carb on the stand, the secondary throttle plate moved freely, and it appears to close completely. But when the engine is running, it is difficult to open it by hand. I know the primary throttle plates need to be open first because of that mechanism that keeps the secondary from opening, but even with the primary open, it is hard to open the secondary plates by hand. Is that normal?

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Mar 8, 2007
Just take it completely apart, soak it over night in carb cleaner. Put a kit into it and try again. The smallest amount of dirt can cause all kinds of trouble. If the idle screws can be in tight and it still runs, there is a problem elsewhere in the carb. You are chasing symptoms not the root of the problem. It is normal for the secondaries to open very hard when the carb is on a running engine. If the 4100 is from a 289 engine it should be calibrated so you can see the secondaries open if you have the engine running and you go to WOT for about 2 seconds. If the carb is from a big block and its on a 289, they may never open.