1964 Cadillac AC with a 7 port Vacuum control valve

Is anyone having the same problem I'm having ? In park with AC on no vacuum at the fast idle diaphragm, ( so the fast idle diaphragm is out and bumping up the idle) when I put it in drive I'm getting vacuum there. So in park with the AC on the engine is idling at 900 RPM's put in drive it drops down to 450 600 ish. So when your sitting at a red light it feels like the engine is starving for fuel not to mention it's running hotter than it should.

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Comment by Tony and Ginny 429 on August 24, 2017 at 5:36pm
Joe D
Not familiar with the Rochester myself

Need to know from someone if you have an idle mixture adjustment on the Rodchester that can be adjusted

Russ may know that answer

Comment by Joe DiIaconi on August 24, 2017 at 3:15pm
Mine is a Rochester
Comment by Tony and Ginny 429 on August 24, 2017 at 2:14pm
Joe D

You are reminding me of an issue that Russ just discovered when checking his car with the air sensor in his exhaust for correct fuel mixture at the carb
His mixture was correct everywhere but at an idle when tested
He made an adjustment to his idle mixture only as it was lean as i recall
Lean would make the engine run hot at idle and make the engine feel as if it is starving for fuel which you say are the symptoms that you are experiencing at an idle yourself
I think it is the nob in the center front of the stock Carter carb that would be adjusted for the idle mixture only
Russ would be best to address this
Just something that may be adding to your AC idle issue and idling in general
A shop can check the idle mixture with a sensor placed in your exhaust pipe

Checking exhaust emissions was one of the required yearly test done in NY state
You had to pass for the yearly inspection sticker that was required by law yearly

When i removed my two rotory valves to clean them years ago i found some debri in both so be sure they are clean and pathways are not plugged And allowing vacuum to pass freely

My idle up for the AC has been disconnected for many years
The AC always performed as it should at the cars regular 480 RPM idle when at the average stopping interval
No loss of cooling power noted at a stop
Most times you are only at a stop but for a very brief period of time
The extreme would be a slow moving parade for example which is not the norm for me
Increasing the idle to 900 RPMS at a stop would be burning a bit more fuel and the engine SB working against the trans torque converter to move the car foward requiring you to use the brakes now working against the torque converter wanting to move the car foward
You would be --stressing the trans in my opinion --for something not required anyway for my purpose
Bottom line is the AC works as it should without the idle up feature
No see no down side having it disconnected
Plug the line if you do not use it

Just my 2 cents

We do all need to have our AC creature comfort to be working as it should so we can fully enjoy the ride of these Super Great Machines
Could this AC system be any more complicated than it is WOW !!

Comment by Joe DiIaconi on August 24, 2017 at 10:45am
I have done both of them in the past using Tim Groves book.
I fear I may not have done the one on the dash side correctly
Comment by Clovis on August 24, 2017 at 10:45am
I don't have a carburetor idle up dashpot (diaphragm). Does the dashpot increase idle when a vacuum is present or is it low idle when a vacuum is present. I slightly remember mine pushing on the throttle when no vacuum is present and when the vacuum is present it pulled back and let the engine idle at 480 rpm's.
While looking at the shop manual I see they the dashpot has vacuum from the brake at low vacuum and at high vacuum at the power servo. This info is on page 13-20 of the shop manual. So it appears the when the car is in gear it allows vacuum from port six to pull from port five on the servo. From there it is directed to port six on the dash control and then it pulls from seven which is the fast idle diaphragm.
Comment by Anders on August 24, 2017 at 10:34am

Suggest to remove the rotary valve from the servo and clean it. Check to see if all ports are opened, could be blocked. Then you need to make sure both halves are clean and level. That is done by a wet high grit sandpaper. Start with 800 and finish with 2000. Make sure the surface you put the sandpaper on is perfectly level. Its recommended to use glass! When both halves level, clean them out thoroughly. Then apply some vacuum grease or white lithium.

The dash rotary valve is done the same way, but you need to drill out the rivets. When reinstall use a screw and washer. Use loctite on the thread and don't overtighten. Make sure the halves can move freely.

Comment by Joe DiIaconi on August 24, 2017 at 5:43am

Thank you Anders, I have done all you mentioned, I'm not hearing any leaks & not getting 900 RPM's in drive with the AC on. So I'm starting to think it must be the dash control valve.

Comment by Anders on August 24, 2017 at 4:18am

Did you fit the replacement for the vacuum check valve correctly? It could very easy be done the incorrect way. So just double check. The vacuum check valve is there to keep vacuum in the Comfort Control System even if the engine does not pull vacuum (as in acceleration/deceleration).

Vacuum check valve correctly fitted? If so its time for some more serious investigations.

Remove the vacuum hose from the emergency brake release. Plug that hose so it doesn't leak vacuum. Then park the car with the parking brake and extra stops at the tires. Have someone at the driver seat ready to brake if necessary. In parking (with A/C on) you should now have 900 rpm, as the fast idle diaphragm is pulled. Ask your assistant to put the gear into drive. See whats happen with the fast idle. It should still be pulled by the vacuum and you should have 900 rpm.

If not then something is wrong with the NSS or the black hose to the 7-port valve. If not any of those you might have an issue with the 7-port valve in itself, not directing vacuum properly or any of the hoses after that. But my guess is that you have an issue with the NSS or even the brake release diaphragm. Do you hear any "air" being sucked from NSS or brake release?

Keep the engine in idle with the gear in drive (helper still in drivers position ready to brake). Listen! Do you hear any suction in the wrong location? You just have to keep listening.

Finally in the bible, Comfort Control by Tim Groves page 6 bottom, its stated; "both the dash control and servo rotary valves are major causes of Comfort Control problems". So if your NSS, brake release diaphragm and vacuum hoses are in good condition, then you have issues with the dash control valve or the servo rotary valve.


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