Another 1955 question, need help with a vacuum line?

[bullit]

I have a line that comes off the automatic choke on the carter that just is hanging and connected to nothing?  Where is it supposed to go and what is its function?
Thanks!

[Jon S]

"sounds like" the climatic control choke stove pipe; not a vacuum line.

[John Washburn CLC 1067 Sadly deceased.]

The line from the choke goes to the rear of the exhaust manifold. There is a plate on the rear with a hole in it that the line goes into, there is also a bolt that holds a fitting that clamps the vacuum/heat line from the choke to the casting. The line is flared so that the fitting can hold it in place.

The Johnny

[Caddy Wizard]

The concept behind this line is that it is a vacuum leak (intended).  The carb draws in a little fresh air through this "choke tube", over the choke element.  The air is first drawn into a maze formed on the top of the passenger side exhaust manifold and eventually into the end of the tube, and from there it is delivered to the choke.  As the engine is first fired up, this exhaust manifold heats up faster than the engine itself and so the air being drawn through the maze (or is that technically a labyrinth? -- note to self, look that up) gets heated and the warmed air is delivered to the choke, where it warms the choke element and allows the choke plate to open.

The maze (labyrinth?) is covered by a small sheet metal plate, so you don't normally see its inner workings.  Overall, it is a simple and reliable system for warming the choke element.

[Jon S]

The heat tube from the exhaust manifold to the Climatic Control heats a bi-metallic spring inside the Climatic Control which opens the choke plate on the carburetor.  There is no vacuum involved . . . just exhaust gas heat. 

[Caddy Wizard]

Jon,


I believe you will find that the carb has a small vacuum bore drawing the heated air from the manifold through the choke element and into the carb. 

[J. Gomez]

Jon,

Art is correct; see the attached picture from the Rochester 4GC manual.

[Caddy Wizard]

Well, I was wrong about at least one thing -- it is a labyrinth, not a maze...

[Walter Youshock]

The choke pipe should also have a woven asbestos sheath to better insulate it and keep the heat in the pipe.  It aids in opening the choke even faster.

The end going into the choke stove in the exhaust manifold is cut at a 45 degree angle and should be turned short angle side up so that it will draw the warm air into it.  Otherwise,  the warm exhaust has no way of traveling up the pipe to open the choke, except for the little bit of vacuum coming from under the carb.

[RobertM]

Here's the plate it goes to and the labyrinth hidden beneath

[Caddy Wizard]

The little crease at the corner of the plate, near the bolt hole, is the air inlet for the labyrinth...

[Caddy Wizard]

Well, just to show how significant this is, consider the following.  Over the last few days my 55 has been giving me trouble.  It was idling too fast -- the choke was not releasing until I had driven the car about 20 minutes.  Not only did that make the car want to creep at red lights, but it made the downshifting as you approach red lights a lot rougher.  Last night I opened the hood to see the choke heat riser tube had popped out of the hole in the labyrinth.  So it was sucking cool air into the choke, not the heated air drawn through the labyrinth.  As soon as I popped the little tube back into the hole and secured it with the clamp, all was right again.


Little details make a big difference.  Folks often overlook how well engineered/designed these cars were.  They are a lot more sophisticated than you might guess from a first glance.  The challenge often is getting the subtle, sophisiticated systems back to working as new.  Modern mechanics and owners often have no understanding of what lies beneath...