Carburetor Rebuild, Choke Plate WARNING

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TonyC
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Carburetor Rebuild, Choke Plate WARNING

Post by TonyC »

Hi, all,

Today, trying to revive some motivation to work on the comatose car, I finally fixed a problem in my carburetor that I've had for several years. I want to say this happened about 2016, but I can't really remember now how long I had been riding with this problem. During a rebuild, I made the mistake of trying to loosen the two small screws that hold the choke plate onto the pivot arm. In doing so, I broke one of the screws, prompting me to not attempt anything with the other screw. Yesterday I finally confirmed the size of screw I needed to replace the broken one, and today I put it in. I'm not sure how much difference that will make in the carburetor's performance, but at least it's something I no longer have hanging over my head.

So, as a lesson learned, I want to share this experience with others who may want to attempt a rebuild as opposed to a replacement and all the associated complex mods that would go with a replacement. Keep in mind that this applies to the four-barrel Carter AFB carburetor used from '63 thru mid-'68, and not necessarily the case with other models. The screws that hold the choke plate are staked on the ends; it was trying to twist the screw off with that staking still in place that broke it. Being made of brass, the screws can break easily. Today I saw that the shop manual instructs not to try disassembling the choke plate unless parts on it have to be replaced, which is hardly the case with a regular rebuild. If it must be disassembled, the stake marks have to be removed before trying to unscrew the screws. Now I know why, and so do you. Take this as an object lesson: Do not try taking the choke-plate mechanism apart except for the aforementioned replacement of anything associated with it that may need replacing.

If you do make the mistake of fiddling with the choke plate and break a screw, there is a way to fix it. First, you need to take the air horn off the main body, invert it, then use a thin flat-head screwdriver to unscrew the broken part from the pivot rod. With the air horn inverted, turn the screwdriver counterclockwise, and the stud will come out (it will turn in the same direction as if you were tightening the screw with the air horn upright). Finding replacement screws of the proper size can be tricky if one does not know the size; these screws are Size No. 3-48 x 1/4" (possibly 3/16", but 1/4" is good). These screws can be found in good hardware stores, if you look carefully; I did not find this size right away, but Waters True Value does have them, in that size and of various lengths. However, since I did not find them immediately last week, and honestly I did not really know the size at the time, in the local hobby store I found a large selection of very small screws used for model trains or R/C-cars; I went back yesterday and bought a package of No. 3-48 x 1/2" screws with nuts and washers (a very old package, I have to say); they were half an inch long, which is very long for this job, but they were the only length of that size available and were a perfect fit in the threaded hole. As I needed only one, I used only one and trimmed off the screw after installing it. The deformed end after cutting should serve as a stake mark to keep that screw from ever falling out, and for good measure I applied a drop of super-glue to the threads as I tightened it down. I don't think the extra length would have interfered with anything, but I didn't want to take the chance, so I cut it down to about 1/4". The other original screw is still intact, so I left it alone.

So, if you cannot find a No. 3-48 screw in your hardware store (though you should if you look carefully enough), check your hobby stores. Remember that it must be that size, and a length of 1/4" if you can find them that short; otherwise, go with whatever length you can find and trim accordingly (after you have it screwed into the rod). To use a larger (e.g., 4-40) screw, you'd have to tap the hole(s) in the rod to accommodate that size screw, which can be tricky; you should not have to tap it if you're careful in removing the broken stud. So that was my hard lesson learned, here's hoping others don't go through what I did.

UPDATE, 20 May: Having come to terms that my original carburetor may be toast, especially since I have damaged a bore for one of the rear jets which I also wrecked trying to remove for inspection and cleaning, I was forced into buying a replacement; fortunately for me, John Brewer was selling one in good shape, so I got it and have started going through it and prepping it to go onto the engine. I noticed that this one also had a broken screw in the choke plate, so I removed and replaced it. This means that I'm not the only one to have made this mistake in the past. Hopefully, this write-up will help others who may make that mistake or come across somebody who has.

---Tony
"Don't believe everything you read on the Internet, just because there is a picture with a quote next to it." (Abraham Lincoln, 1866)
"Question Authority!"

1966 Continental Sedan, affectionately known as "Frankenstein" until body restoration is done (to be renamed "General Sherman" on that event)
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