On some 312's (maybe all?), there is an item referred to as Automatic Choke Vent Tube.  It has a special press-in 90° elbow fitting connected to the manifold just below the carburetor (mine is a Carter--don't know if that matters).  From the elbow, the tube travels up to where it is inserted in a grommet in the carburetor body.
It can be seen behind the throttle linkage in the highlighted portion of this photo.

Mine is damaged, but I have not found a parts place or mechanic who is even familiar with this.  Does anyone have a suggestion on where to find or if this part is even needed?  Maybe an idea on how I can fabricate one?  If it helps, this engine is from a 1956 Mercury.

THERMOSTATIC CHOKE CONTROL TUBE - INLET

1956 MERC 312 CARTER

ECK 9820-C

- https://nospartsltd.com/product/eck9820c/

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Before you install that replacement part, run the engine & make sure that exhaust fumes are not coming from the connection hole in the intake?  If that happens to be the case, (hopefully not) it would indicate that the tube in the intake "crossover" is also rusted out & needs replacement. Maybe Kultutz can provide a part # for that too..  Up here in west coast Canada, some time back, I could not find a listing for that part.. I made my own, but that's another story..       


Paul

Paul, the crossover and end fittings are readily available at most vendors. The WCFB is unique in it's inlet tube design.

But if one needs the OEM PN's, I will provide them.

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Is it a big problem if the crossover tube in the intake manifold is corroded or missing?   Can you tell by having the exhaust pulse out the attachment hole? 
Maybe if the tube is gone, you have created your own EGR (exhaust gas recirculation) valve and updated it to a more modern emissions system.  
Ralph

CASCO has  the tube than runs thru the manifold if your is bad.   tube 9890 if a 56 or 57


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

The choke tube that goes through the heated crossover passage in the 1956 & 1957 intake manifolds is replaceable when it corrodes.  Paul gives both a source and a part number above.  If wanting to retain the tube as originally designed, then the old tube can be pressed out and a new one installed but this will require removal of the intake manifold to do so.

If exhaust gasses do get into that tube due to corrosion, then it plays havoc with the factory choke assembly at the carburetor.  That hot exhaust at the choke coil will initially corrode up the internal workings of the choke and eventually destroy the assembly itself.

If going with a choke that does not require that particular air source (ie. Electric) then the choke tube holes on each side of the intake can be plugged thus eliminating what would be an exhaust leak.  When doing away with the choke tube, then tapping those holes to a 3/8-16 thread and putting set screws or bolts in those holes suffices for plugging those holes.

Lorena, Texas (South of Waco)

As others have said you may have a lousy cross-over tube, which is fixable as described in previous posts. On the verticle tube connected to the carb, I would simply bend one up from the proper brake line tubing. The connections are not high-pressure tight, so this should be easy enough to fabricate.
Replacing the cross-over tube is more involved and requires removing the intake. After removing the intake manifold, use the proper size punch and other to drive out the remnants of the old tube. It doesn't really hurt to leave them inside the manifold if you have to (they may be rolling around in there already). Then install the new cross-over tube, maybe use some grease or wax to ease installation with a wood block and mallet. Oh, and you will need to replace the intake manifold gaskets.

       A  L  A  N   F  R  A  K  E  S   ~  Tulsa, OK    

In looking at my '57 intake manifold, I do not see a hole on the driver's side of the manifold where the heat tube is to be pushed through.  What am I missing?  Do some manifolds only have a hole on the passenger side?  If so, the tube would be hard to replace. 

Thanks, Ralph

rponfick,
I’ve got a few questions about your carb picture.   The first is if you have any history on this carb?  Does it look original to the engine?    What year and model Merc is it in?  The tube coming up from the manifold seems to be going inside the air horn of the carb.  Where does it go to after entry into the carb?   Could I see I pic of the carb throat to try to figure out why it’s routed where it is?  Does the existing choke tube look homemade?  Can I get a pic of the automatic choke assembly?  If it has a manual choke that would indicate it's not the original carb. 
I did some checking in the 1956 and 57 Ford Factory service manuals and the 1956 shows only one four barrel, the teapot.  The 1957 shows one Carter four barrel, and that appears to be an AFB.  It also shows a Holley and a Ford available.  It looks like only Mercury used a WCFB or someone grafted it on.
Over the years I have rebuilt quite a few AFB and WCFB carbs.  Most have been Chrysler and a few  GM’s and I don’t remember seeing a choke tube crossing over the air horn to get to the choke assembly on  the other side.  
I’ve attached some info on WCFB chokes I ran across.  HTH
Don
EDIT 
I finally found some Mercury carb info and it does show a WCFB used on the 312 in 1956.  The carb number stamped on the base of carb should be 2361.  Chances are it'ts the orginal carb.  I still would like to see how they got the exhaust across the carb to the choke.