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Vacuum lines

Started by Jvteach07, March 12, 2023, 07:00:11 PM

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Jvteach07

Can anyone identify this part? My problem is when the vacuum line goes to this my transmission won't shift. When I removed this line and have the vacuum line only going from the carb to transmission it shifts fine. Should I replace this part?9282F2FA-56FE-4794-A8AA-A02A2EE8EB78.jpeg
Joe Tortora
1970 Cadillac Devill Convertible

Daryl Chesterman

Joe, that item that is circled is the vacuum advance unit on the distributor.  Depending on what year car it is, the hose from the vacuum advance should go to the base of the carburetor (for ported vacuum if above the throttle plate) or to a fitting on the intake manifold for direct vacuum.  Where does the hose currently go—vacuum advance to where???  Do you have a shop manual to look at?  You can test the vacuum advance for a ruptured diaphragm by using a hand vacuum pump or if you don't have one just connect a length of the proper size vacuum hose and suck on the end of the hose.  With the hand pump, just pump it up to 20 inches of vacuum and see if it will hold that reading.  If it doesn't, the advance unit is bad.  No hand pump—just suck on the end of the hose.  If you are drawing air through the hose, the advance diaphragm is bad.  If you suck on it and stick the end of your tongue on it and it holds vacuum, the advance diaphragm is good.  Post the year of your car and maybe someone else can address the problem of vacuum and your problem with the transmission shifting.

Daryl Chesterman

Jvteach07

1970 deville. It's currently connected in the back of the carburetor but like I said the transmission won't shift when it's connected.
Joe Tortora
1970 Cadillac Devill Convertible

klinebau

It sounds like maybe you have some uncapped vacuum ports on the carb or a vacuum leak.

There are two vacuum ports on back of carb.  The big one in middle is for brake booster.  The smaller one connects to the transmission modulator.  There is another port in the back on the top part of the carb which is a vent and can be ignored.

The There are several vacuum ports on the front of the carburetor.  The big center port goes to the PCV valve.  The port on the left (drivers side) is for the vacuum advance and is ported (full vacuum only when throttle opened).  The port on the right (passenger side) goes to the vacuum break diaphragm (sometimes called choke pulloff) on the carburetor.

The port on the intake manifold drives the climate control, parking brake and thermac air cleaner.  If a leak is suspected here, you can cap this off temporarily.

This is a bit of an over-simplification, but with this setup, the engine and transmission should be happy.  There is additional vacuum routing for the TCS solenoid and thermal vacuum switch, but can be ignored until you get the basic functionality working.
1970 Cadillac Deville Convertible
Detroit, MI

Jvteach07

klinebau- thank u so much for this simplified version!
Joe Tortora
1970 Cadillac Devill Convertible