71 472 experiencing very hard starting and idling when cold, ok when warm

[Brett]

The car has just come out of 7 months of shop time and near dormancy, but right before everything was great. Spark plug wires, entire distributor, and plugs were changed right before she went under the knife. Anyone have any ideas? Should I change the plugs and see what happens?

[35-709]

I would first suspect the carburetor, especially after sitting that long.  Sounds like the choke is not working properly/sticking.  Judicious application of choke cleaner by someone who knows what they are doing and what to look for may do the trick.

[Stampie]

2nd.

Stampie

[35-709]

    Sometimes there's just no hushing you up, is there Stampie?

[TJ Hopland]

Have you run all the old gas out of it yet?  It may just be bad gas from sitting?

I had one that the choke well in the intake manifold got filled with crud and kept the coil from moving right.   After you get the air cleaner housing off there is only 2 blots that hold it in then its easy to clean.  Is the choke pull off (or I think the manual calls them a vacuum brake) working properly?

If all that checks out ,does a 71 have that thermal switch that changes the dizzy vacuum advance between different sources?  I remember on my 73 I finally got the vacuum advance can replaced on the dizzy but it still didnt start right because that switch/valve was stuck in the wrong position.

[David #19063]

Good morning Brett,

My guess is that your Choke Pull Off diaphram is bad.

I have had to replace this on several cars I have had.

The next time you go to start your engine cold, remove your air cleaner assembly prior to firing the car up. 

After it starts, get out and look at the choke.  If the butterfly is not open, gently push it open with your finger or pen or screwdriver.  The idle should increase and smooth out.  If it does, then you can leave the pen/screwdriver holding it open until the car warms up.

To test the Choke pull Off diamphram do the following.

While the engine is running, remove the vacuum line to the Choke Pull Off and see if you have vacuum on the line.  Of course, making sure the vacuum line is attached and not split or broken.  If it is, reattach or replace it.

The engine needs to be off to check the diaphram itself.  Remove the vacuum line and gently push in the diaphram with your finger and hold it.  Then while holding the diaphram pushed in, tightly cover the diaphrams vacuum's port with another finger.  Release the diaphram while continuing to hold the vacuum port tube port tightly.  If it stays, it is good.  If is moves back out, the diaphram is bad.

Or you can leave the engine running and remove and reattach the vacuum line.  Whenever the vacuum line is attached, the diaphram should suck in.  If it does not, it is bad.

I hope this helps.

David

[Brett]

Thanks,
I will give all this a try shortly. I'm on the home stretch here working out the little bugs. All thats left is this issue, hood cable, A/C, and aiming the headlights (I installed 72 units with marker lights prior to paint).

Interestingly,

I had the car out in the rain the other day. I pulled to a muddy spot inside of the house and the whole car sank down quite a bit...and needed to be pulled out. Really felt the forces of mother nature were trying to swallow up this magnificent beast once and for all    I went inside to call for a tow and when I came back outside I was preparing myself to see the car completely disappeared under the earth. It didn't, I beat the forces again!

[David #19063]

Oh yeah, if you cannot get it to start.  Stick the screw driver or pen in to hold the choke butterfly open a little bit, then try to start.

[EAM-17806]

BRETT!  David is right on the money for all is suggestions.  If there is a vacuum pump available it would be much easier to test the diaphragm on the choke pull-off. Once that is determined to be okay then it would be a good idea to calibrate the butterfly opening in accordance with factory specifications.  GOOD LUCK!  EAM

[Brett]

Thanks, I will check it.

Just to check my downside, Rockauto has two available listed under 'Carburetor Choke Pull Off'

Is this the part in question?

For my own understanding, how does this work? I mean, doesn't the choke come into play when the mixture should be richened when cold? And the choke pull off activates the choke? And if the choke pull off is vacuum controlled, why is there more/less vacuum when the car is cold?  Sorry for the meathead questions...I would just like to understand the car...

Oh...is there a vacuum guage I should by for troubleshooting this, and if so, can the same guage also be used for troubleshooting the A/C controls? Where should I buy one?

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[TJ Hopland]

If its like the later ones the sometimes call them the primary and secondary vacuum brakes.   I think out of my current fleet of 7 70's cadillacs 3 of them worked when I got the cars.  The vacuum advance in the dizzy usually seems to be shot also.  I took this picture for something else.  I think it was off my 75 but it could have been the 78 also.  The one that is circled is the one that was called a secondary.  I have not figured out what the secondary one does but the primary one is what opens the choke a bit as soon as the engine starts.  Older and more simple ones just depend on the airflow to pull the choke open a bit.  I assume the did it with the vacuum can just to make the choke stat a less critical adjustment.  I imagine it was mostly an emissions related thing or they would not have spent that much money for something so slight.  The one on the right is the primary one that opens the choke the screw/spring thing is the adjustment for how far the choke opens at first (before the heat starts to open it).  That one is not attached properly in this photo.  One of them was a stock part at my local store (I think it was the primary one) the other was a special order that took a few weeks for this carb.


As for the tool here is a handy one that comes with a few extras for other jobs:


Here is the Harbor Freight Tool link the picture came from:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92474

[bill henry]

i think you might have some parts missing on the secondary one. i think there should be a rod and a plate that connenct to the slotted plate on the seconday air door shaft just above

[TJ Hopland]

The rod for the secondary top doors is missing in the picture.  On this carb it goes to the front unit which is the one with the adjustment screw on it that acts on the choke blade. That carb is off my 78.  I just confirmed in my 77 manual they refer to those as 'front vacuum brake' and 'rear vacuum' brake.  I think when I was shopping for parts most of the stores called them primary and secondary.  I could see why they (GM) changed the names since the front one acts on the secondaries it may have been confusing.  Didnt some of the earler models have a vacuum operated thing for idle speed?  My 73 only has the front unit so the rear unit must be some sort of emissions add, I seem to recall it was plumbed through some sensors in the air cleaner housing.  We should not have to worry about that with his since he should be dealing with a 71 carb, may be worth checking the numbers to make sure its a 71 Cadillac carb?

[STDog]

I've seen the secondary (rear) pull-off on early years too (like my '69). I think my brother-in-law's '72 had one as well.
More common on the big Fleetwoods with rear AC too.

It's normally used as an idle speed up. I've seen it controlled by a temperature switch and by the AC compressor.


The primary(front) pull off does several things. One is it opens the choke a bit. It also holds the secondary air valve closed, preventing the secondaries from flowing fuel at high speed cruise. You can have the throttle open enough to allow air through the lower throttle plate, but still have enough vacuum to hold the air valve closed. That why it takes a hard WOT blip to open the secondaries in park. No load=high vacuum so the air valve stays closed. You can rev up to 4000 rpm in park and never open the secondaries if the pull-off is working.

[David #19063]

That's a very good explaination.  Thanks STDog.

[eam]

BRETT!  David is right on the money for all his suggestions.  If there is a vacuum pump available it would be much easier to test the diaphragm on the choke pull-off. Once that is determined to be okay then it would be a good idea to calibrate the butterfly opening in accordance with factory specifications.  GOOD LUCK!  EAM