I had some difficulties recently with the Holley 1946 carburetor on my straight-six 78 Fairmont. Figured I'd write up what I found in case anyone else has similar problems.

The initial problem was a stumble/bog or outright stall when trying to accelerate quickly from a stop. It seemed worse when the engine was cold. It would also bog if I planted it full-throttle while cruising along.

This pointed to an issue with the accelerator pump in the carb. It's a remanufactured carb with a fresh Autoline tag, but I don't have much history as I just bought the car in Oct 2021, and it seemed like the car had sat a while. So I took it apart to check out the accelerator pump interals.

It's a fairly simple system in the Holley 1946, but it took me a while to figure out and I wish I'd had the full picture when I started looking into this, so here's a diagram and explanation:
- As the throttle opens, the linkage (31) is released, which allows the rod (17) to rotate and the piston (23) to be pushed down by the spring (24) (note that it's not that the throttle drives the piston down, but that it allows the spring tension to be released).
- The rubber cup (25) on the piston forces fuel through a hole in the bottom of the pump well.
- From here the fuel travels up a bore to the check-ball (39) and weight (38 ) check valve, with the pressure pushing the ball up and allowing fuel past it and the weight.
- Just above the weight, there's a tiny hole that runs on a downward angle through to the internal wall of the carb air passage.
- The air-horn gasket (29) seals the bore so that the fuel is forced out of this tiny hole and sprays into the main intake air passage.
- When the throttle closes again, it pulls on the linkage, which rotates the rod, and draws the piston back up.
- The check-ball drops as the piston goes up, preventing air from being drawn into the well.
- I think the fuel refills the well through the hole in the piston cup where it attaches to the piston stem (this hole is effectively sealed while pumping, but loose otherwise).

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I initially got a rebuild kit and replaced the rubber piston cup and the float, and gave it all a good clean-out with carb cleaner. There was quite a lot of silt in there and the old float was bubbly. I added an extra fuel filter just before the fuel pump to help keep out any more silt. This seemed to fix the problem for a couple of weeks, but it eventually returned (though not as bad as it had been originally).

Looking under the air cleaner, I noticed that the accelerator pump rod was stuck. It wouldn't rotate more than a couple of mm as the throttle opened, and it was difficult to force it to move by hand. I ended up taking the carb on and off a bunch of times after this, thinking the problem was something to do with the piston cup getting stuck in the well, so adjusting various things by trial-and-error. During reassembly, I noticed that the pump mechanism would move normally when the air-horn was lightly tightened down to the main body, but would start getting stuck as I tightened it down further.

I eventually found that the tiny hole I mentioned earlier above the check ball and weight was completely clogged (I could hardly even tell it was there). This made sense: when the air-horn wasn't tightened down or didn't have a gasket, the air pushed down the well by the piston was able to travel freely out the top of the bore. But as that path was sealed by the gasket, the air was stuck with nowhere to go because of the clogged hole. This restiction was felt all the way back to the rod on the outside of the carb.

So I pushed the crud out of the hole with some garden wire, then sprayed it out with carb cleaner, reassembled everything, and it all seems to be running well again. I can also now clearly see the jet of fuel when looking down the carb as I open the throttle.

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(pic of my spare carb showing the bore & hole)

Got there eventually thanks to various posts on this forum and that Eric the Car Guy rebuild video on youtube. Cheers!