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Caburetor Rebuild Questions/Problems

I’m undertaking one of the most exciting/complicated vehicle repairs on my 76 Lincoln Mark IV, rebuilding the carburetor. I got it all taken apart and cleaned pretty easily, but I have a few questions/issues in putting it back together. When I was taking it apart, the float and the pin that holds it were sitting in the fuel bowl. I assume this happened as I was taking it apart, but this means I’m not sure that whole assembly is supposed to work. In that same area, I made the mistake of not measuring the vacuum piston setting before I took it apart, and I thought I had to loosen the screws to get it out, which I’ve since learned was not what I should have done. How can I figure out where to set this?

I’ve put some pictures at the end of this post if they will help at all. The carb is a Motorcraft 4350.


Here’s an example how the pin holds the float to the top of the carb.

The pin is the hinge for the float, that allows the float to move up and down thereby allowing the opening and closing of the float valve.

As far is the vacuum piston setting, I’d have to look that up in my old Chilton’s manual. Which isn’t at hand at this moment.


Have you googled ‘how to rebuild 4350 Motorcraft’? Lots of links…

I got the float figured out. My only question now is how to set the vacuum piston metering rods. All of the literature I’m finding just says not to adjust them. Is there a way I can figure out where to set them?

Your rebuild kit came with an instruction brochure, right? Difficult to understand with lots of diagrams & numbers which may or may not apply to your particular version of the carb? Or doesn’t the brochure describe how to set the metering rod mechanism? When I was rebuilding my Ford truck carb (2150) I found a couple of websites which had different versions of the rebuild instructions in pdf format. Comparing one to the other was helpful, as some ambiguities were cleared up. Maybe do a little web surfing for “Ford 4350 carb rebuild instructions” etc.

I googled ‘Ford 4350 metering rods’, found this. Does it help?$PDF$

@GeorgeSanJose & @texases, The instructions say to “use clearance measured on disassembly”, which I was stupid and didn’t do. From what I’ve been able to find, this was set by the factory and varies by each individual carb, so I’m not sure if I can figure that out somehow.

From @texases reference, here’s the metering rods (the two needle-looking things hanging straight down):

Picture 1

It looks like at high vacuum, the rods are pulled down into the main jets (48) to lean out the mix. What clearance is adjustable, and how do you adjust it (and by how much…maybe it doesn’t make much difference…put it in the middle)?

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Here’s one way they adjust:

The link @texases provided summarizes it pretty well. There is probably 1/2 of an inch of adjustability, adjusted with screws on top of the rods.

Looks to me like the main adjustment is a single screw the moves the rods holder up and down.

There is a main adjustment there as well, but I didn’t touch that one, so it’s still at factory spec.

From the link:

the wifes finger… you have to adjust the center screw so the rod holder can fully seat… then turn the side screws so the metering rods are just touching /bottomed out in the jets… this equalizes the heights… makes for a better running motor.

Since the the center screw is “right” it looks like you raise the needles with their individual screws so you can “seat” the rod holder (gently push it down with your finger), then lower each until just touching as described in the quote.

You’re right. I didn’t read that page carefully enough. I’ll try that and see how it goes.

Beyond the metering rod adjustment – which I’m certain you can solve, don’t panic on that – here’s a couple of tips I learned last time I rebuilt my truck’s carb.

  • Wear eye protection , preferably a full face shield, any time you are spraying anything, including plain air, into the carb or its orifices. With the various tubes and chambers inside, no telling where the spray will come back out, and often it will shoot forcefully right back at your face.

  • The disassembled carb can be cleaned with a spray can of carb cleaner, or soaking it in a bucket of carb cleaner. The soaking method does a better job.

  • After the cleaning is complete, spray all the orifices again with a compressed air jet, about 15 psi.

  • The fuel level height in the bowl is very critical to the drivability and engine performance. 2 or 3 mm makes a significant difference. The dry setting method as posted by Tester above is a good start, but do a wet check too; i.e. with the fuel bowl full of fuel. I do this with the carb installed on the vehicle.

  • Last but not least, after the rebuild is nearly complete fill the fuel bowl while the carb is on the bench to make sure no gas is leaking out the bottom. I had a problem with a leaking power valve which I would have easily discovered had I done this.

I got the carb put back on, and it seems to have a massive fuel leak. I didn’t take the advice to test-fill the carb before I did that, as I just really wanted to get it done. Stupid in hindsight. The leaking seems to be happening from both the air horn gasket and the bottom of the fuel bowl. I’m guessing I should take the air horn off and test-fill the bowl to see where the bottom leak is coming from, but what would cause the upper leak? Shouldn’t the gas not be getting that high? Or is my understanding of the fuel bowl flawed?

Leaking into the intake manifold?

Certainly not in the bowl. Possibly, the accelerator pump is leaking from the top somewhere? Otherwise the float valve isn’t sealing.

Here is a video of what is happening. I tried putting teflon tape on the threads of the fuel filter, per the suggestion of a co-worker of mine with carb-rebuilding-experience, but I’m having the same problems.
(Also sorry for the vertical video).

That looks like the float/inlet valve mechanism may not be working. It’s supposed to cut off fuel flow into the carb once the fuel level reaches the set point. But if the float is sticking, or the inlet valve isn’t seating properly, the gas will overflow the bowl. Be very cautious with this problem, keep a big fire extinguisher on hand, and don’t do this work near any buildings, just in case. Best bet is probably to remove the carb and see if you can figure out on the bench using the test fill method. It’s pretty easy to get the configuration of the inlet valve clips wrong.

Teflon tape on the threads of the output end of the filter can cause a lot of carburetor problems including the one seen in the video. The tape can shred and a small piece can cause the needle to be blocked from seating which results in the flooding seen in the link or the tape can plug the inlet on the seat resulting in no fuel entering the carburetor.

Pull the carburetor off and tear it down again.