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The piston in the housing of the automatic choke on my Carter AFB has frozen in place. I tried a spray lubricant and few whacks with a wrench to dislodge the piston, but to no avail. I imagine if I could somehow remove the cap at the bottom of the cylinder bore I would have a better shot at freeing up the piston. Does anyone have any suggestions on how best to remove the cap, or do I have to drill it out? If drilling is the answer, are replacement caps available? If there is an easier alternative to free the piston, I sure would like to know about it. I think the piston became frozen in the first place because the tube going into the choke went directly into the manifold subjecting it too nasty exhaust gases. Apparently, there should be some type of "clean air tube" involved with the choke, but the diagrams I have seen are not very clear, and may not apply to a 1964 with A/C. I would really appreciate any advice!
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Re: Carter AFB Choke--1964 327/300 w/ air
Once you remove the choke cover you should be able to see that the choke housing can be removed from the carburetor body. The '63 Corvette Shop Manual, which is the basic manual for your car (plus the '64 Supplement) has a very good section on the AFB.
The choke functions via a vacuum passage in the carb body that continously draws clean air from the carburetor air horn through the exhaust manifold passage and then to the coil and choke piston to the vacuum passage in the carb body. The warm air expands the thermostatic coil to release the choke valve as the engine warms. Once the engine starts, manifold vacuum pulls the choke piston to open the choke slightly so as not to starve the engine of air. This is known as the choke vacuum break. The vacuum port continuously draws air when the engine is operating to keep the thermostatic coil warm, and over time these passages get dirty along with the choke piston, which will cause it to stick causing improper choke operation. If any of the clean air tubes rust through, particularly the tube inside the exhaust manifold, the choke system will quickly become clogged up and jamb.
The internal tube in the exhaust manifold keeps choke air separated from exhaust gas. Prior to this in the flow path there is a steel tube that connects to the rubber hose from the air horn nipple that wraps behind and up into the manifold. There's a good chance that this could be corroded through. From here the air traverses the internal tube in the manifold and then out the chrome plated tube to the choke housing. The tubes should be available from the usual Corvette parts supply houses and other aftermarket sources. Whether you will be able to replace them without removing the exhaust manifold depends on how badly the old ones are corroded into the manifold.
Soak the choke housing and piston in carburetor cleaner, and they should break free. Also shoot some carb cleaner into the choke clean air vacuum passage in the carb body to clean any debris out of it. When installing the choke housing back on the carb body, be sure to replace the O-ring that seals the choke clean air passage to the carb body. These should be in zip kits, or you may have to match the old one from a commercial supply house. If the O-ring is missing or doesn't seal you will have a vacuum and dirt leak.
The AFB is the simplest, most reliable four-barrel carburetor on the planet ,but the choke clean air system is its weakest link, and whenever the choke shows signs of sticking, the choke housing should be removed, cleaned, and all clean air tubes checked for integrity.
Duke -
Re: Carter AFB Choke--1964 327/300 w/ air
Duke,
Thanks so much for your help and excellent explanation. Am I right in understanding that there are two metal tubes in the choke system? One that connects to the rubber hose from the carburetor/air horn (Tube #1) and a second tube (Tube #2)that on one end connects to the end of Tube #1 extending from the exhaust manifold, with the other end connecting to the choke assembly. Or, is there yet another tube/nipple that goes through the exhaust manifold that joins Tubes #1 and #2?
I could only see one hole in the exhaust manifold. Based on your explanation, my guess is that there must be another hole in the manifold that is not easily seen. If this is true, where is the other located?
Thanks again for your help!Comment
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Re: Carter AFB Choke--1964 327/300 w/ air
Here's the fitting on the carb:
Here's where the clean air tube attaches to the bottom of the passenger intake manifold:
My AFB set-up lacked the clean air tube and the choke was all gummed up even though the tube inside the manifold was still intact.
The metal tube thatattaches to the choke housing from the top of the manifold is called the "choke tube" and the one that fits into the bottom of the manifold is referred to as the "clean air tube".
....yeah I know R43s are bad plugs to use on a street engine. I learned the hard way.
BrianComment
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Re: Carter AFB Choke--1964 327/300 w/ air
There are THREE tubes. The "inlet tube" routes between the manifold and head.valve cover, plus the two exterior tubes. The inlet tube connects to the rubber hose that routes to the air horn nipple. This plain steel tube routes behind the manifold and then loops around to connect under the manifold to the interior manifold tube. It is tough so see the portion of this tube under the manifold, but the top part protrudes between the manifold and head/valve cover about an inch or two aft of the front outside valve cover screw. A welded bracket on this tube ties to the #3 exhaust manifold bolt that has a stud extention to hold the bracket, lock washer, and attaching nut. It's possible that this tube is completely missing from some cars because they rust out. I don't think too many understand how the AFB choke clean air system works or it's proper configuration. I could not find a replacement for this inlet tube years ago, so I made one out of stainless steel. Looks just like the original, exept for the material. Damn the points, it will never rust out!
The third tube is the chrome plated "outlet tube" from the exhaust manifold to the choke housing. If you take this off and look carfully, you can see the boundary between the interior manifold tube and the manifold material.
The purpose of this system is to heat air for the choke thermostatic spring and a small vacuum passage in the carb continuously draws a small amount of filtered air from the air horn through the manifold to heat it and thence into the choke housing to the piston and into the inlet manifold through the vacuum port in the AFB.
DukeComment
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Paging Joe L.
Joe - I show a 3814026 tube in Gr. 3.603, which I think is the interior manifold tube. I can't find either the inlet or outlet tubes. Where these parts for the AFB choke clean air system ever serviced?
Can you find part numbers and what group are they in?
DukeComment
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Re: Paging Joe L.
Paragon, LICS, CC, and the Doc have all these pipes, tubes, and hoses.Comment
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Re: Carter AFB Choke--1964 327/300 w/ air
Bruce: I encountered the same problem with the frozen choke piston. I was able to eventually free the piston up by first cleaning it up with carb cleaner and then numerous applications of penetrating oil. It takes a little time, but it will eventually break loose. Good luck....LenComment
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Re: Paging Joe L.
Duke-----
Although specific part-numbered inlet and outlet tubes were used in PRODUCTION, they were never available in SERVICE. For SERVICE, instructions were to use bulk tubing from the Standard Parts Group and cut/configure, as necessary. The same thing has been true for most all fuel lines and brake lines. GM rarely SERVICED any with the PRODUCTION, pre-formed pieces. Bulk tubing and fittings through Standard Parts were all that was offerred.
This is an area where reproduction sources have it all over GM. For years, exact reproduction fuel, brake and choke lines have been available from reproduction sources and virtually none of these were ever available from GM.In Appreciation of John HinckleyComment
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100 PSI of air pressure will blow of the end cap
pressurize thru the vacuum hole in the choke housing. start off with 50# and work your way up. been there done that.Comment
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