Question on 1968 472 vacuum line
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500, 472, 425, 368 Discussion, Question on 1968 472 vacuum line in Cadillac Engine Technical Discussion; As some of you may know I recently bought a 1968 calais that had been sitting in a garage since ...
  1. #1
    PsychoBillyCadillac is offline Cadillac Owners Member
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    Question on 1968 472 vacuum line

    As some of you may know I recently bought a 1968 calais that had been sitting in a garage since 1975. One of my first jobs was to rebuild the original Q-jet as it had seized up from sitting for so many years. When I put the fresh carb back on the engine, I noticed there was a vacuum line leading from the air pump which had apparently not been hooked up. I can tell it goes on the carb someplace as the line has a brass end in it. It doesn't appear to have been attached where the small line runs between the carb body and the pull off for the secondaries. Does anyone know what this line does and where it is properly supposed to be attached? Or is it possible someone just bypassed it at some point? Thanks in advance.

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    steelybill is offline Cadillac Owners Enthusiast
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    Re: Question on 1968 472 vacuum line

    If it's the inlet line to the air pump, maybe it went to the air cleaner on the carb, so that filtered air goes into the pump. You might look at the air cleaner for a place for the line. Later cars had a separate filter canister for the air pump inlet.

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    PsychoBillyCadillac is offline Cadillac Owners Member
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    Re: Question on 1968 472 vacuum line

    It's the only line I can see coming off from the air pump on the passenger side. I will check, but it doesn't appear that the line goes into the air cleaner. The way the line is curved, just from being in the same spot for so many years, puts the brass end of it right near the body of the carb someplace. Does anyone have a diagram they can check for this year caddy?

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    Re: Question on 1968 472 vacuum line

    The short vacuum line that runs from a manifold vacuum port on the carb to the vacuum break, should have a T in it. The tee goes off to the AIR pump. It operates the diverter valve in the AIR pump. The purpose is to... Aw what the hell, here it is from the manual:

    The diverter valve is attached to the side of the pump. It senses manifold vacuum through a 3/16" fitting at the carburetor. During sudden deceleration, vacuum increases causes the valve to open, allowing air from the air injection pump to pass through the valve and silencer to the atmosphere. Approximate duration of the valve opening is 4 seconds. This valve also controls air pressure within the system by diverting excessive pump output to the atmosphere through the silencer.

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    PsychoBillyCadillac is offline Cadillac Owners Member
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    Re: Question on 1968 472 vacuum line

    Not sure exactly what a "vacuum break" is. I'm guessing that is the technical term for the round vacuum operated device that pulls on a linkage to the secondaries?? It is connected via a short vacuum line to the body of the carb. So there should be a T-there? If the vacuum line to the air pump is not hooked up, if I just leave it the way it is, will it have an adverse effect on the engine or driveability?

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    Brother_B's Avatar
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    Re: Question on 1968 472 vacuum line

    Yeah, the vacuum break is the diaphragm that connects with the secondaries through that linkage (on passenger side). There should be a T in the short vacuum hose between the vacuum break and the carb. Personally, I think I would put in the T and hook up the AIR pump. I don't think it would have much effect on performance to leave the vacuum line coming out of the AIR pump loose. But I really don't know, sorry.
    If you do a search, you may find some good old posts about removing the AIR pump entirely if you are interested. I think it was cadillacmike that described the process on his '68, and I asked him a question or two about it. So far, I've kept the AIR pumps on both of my vehicles, but someday I might ditch 'em. The way I understand it, the AIR pump only interacts with the exhaust to improve emissions, so I think leaving it unhooked would not affect performance. Removing it entirely would free up some working room under the hood and that's one less thing running off a belt.

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    Will68 is offline Cadillac Owners Member
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    Re: Question on 1968 472 vacuum line

    There are two configurations for the '68 air pump. 'Brother B' describes the factory setup.

    The second arrangement, which I think is aftermarket, leaves the air pump vacuum line disconnected. A vacuum bypass configuration rerouting the vacuum lines is installed to the right and just below the distributor. I can find no materials on this arrangement but it is in my '68 along with the factory air pump which has the dangling disconnected vacuum line.

    I would really like to find how and why the vacuum bypass arrangement was introduced/installed and what it does in place of the factory setup.

    All comments gratefully received!

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    Re: Question on 1968 472 vacuum line

    Hey Will68, thanks for chiming in, can you describe the aftermarket set-up in more detail? When you say "to the right and just below the distributor", do you mean passenger or driver side? That would sound to me like the thermal vacuum switch that creates more vacuum advance when the engine temp gets high.

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    carnut is offline Cadillac Owners Master
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    Re: Question on 1968 472 vacuum line

    If the vacuum line is not connected to the diverter valve, then all air produced by the smog pump is sent thru the exhaust manifolds. Thats fine until one day the extra air that should have diverted to the air cleaner on deceleration blows the muffler off! If it were mine, I'd remove the smog pump, open it up and remove the vanes that pump the air. The older smog pumps came apart as simply as an alternator. Once the vanes are gone, no air is produced, check valves dont stick and no air is pumped thru the exhaust. Hence no potential for the muffler blowing off. Put the pump back on, cap off the nipple on the diverter valve reconnect the belt and drive it. Brother B is correct on its function, and I've never seen this vac hose connected to a thermal switch until much later years when the smog pump was used to blow air thru the cat converter to speed its warming up. Once warm and the engine in closed loop mode the thermal switch redirected the air produced into the air cleaner. Your era of smog pump operation was to lower NOX and HC by blowing air at the exhaust valves.

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