1967 Deville: How to cure rough idle?

[MikeLawson]

My recently acquired 1967 Deville runs great on the road but I cannot get it to idle as smooth as it should.  Here is a summary of the things I've done in an effort to correct the problem:

Removed the plugs to inspect them and they all looked fine with no signs of any abnormal conditions. Gaps were all correct.
New ignition wires were installed by the previous owner, they look perfect and all connections are tight.
Replaced distributor cap, the spark plug contact posts on the original were pretty worn looking.
Replaced points, condenser and rotor.  Old point set had pitted contact surfaces.
Checked and reset dwell to 30 degrees.
Replaced air filter, fuel filter and pcv valve and connecting hoses.
Checked to verify that vacuum advance diaphraghm is not leaking.
Tightened carburetor mounting bolts.
Tried adjusting idle mixture screws which helped somewhat. Carburetor is original and does not look like it was ever rebuilt.
I have a set of carburetor mounting gaskets and heat shield plate on order and will be replacing those as well.

With the engine running in drive at 600 RPM, the vacuum gauge reads 18 but the needle is not steady. It vibrates constantly a very small amount, about 1/2 point on the gauge. At higher RPM's everything smooths out and the vacuum gauge reading is steady.  The car has only 45,000 miles and runs fine otherwise.

I won't be happy until I get this right so I'd greatly appreciate any suggestions.

["Cadillac Kid" Greg Surfas 15364]

If the switch pitch is working the. I woul look to doing a compression test and suspect the valves
Greg Surfas

[James Landi]

Greg's diagnoses is based on your vacuum gauge dancing at idle; however,what is odd is that leaky valves would likely keep that needle dancing up through the rpms.   Have you tested your vacuum advance on the distributor to make sure it does not have a ruptured vacuum chamber?  The vacuum advance has a limited service life, and a non functional one can create bad idle, poor gas mileage, and loss of engine responsiveness. Other vacuum "appliances," such as the vacuum brake on the carburetor, the vacuum line to air cleaner,  your vacuum actuated power door locks and your automatic brake release might be causing a problem for you.  Before you pull the plugs and perform a compression tests, I suggest that you carefully remove the vacuum lines to the intake manifold and close off the manifold to ensure there are no leaks -- then re-adjust the idle air screws .    James

[cadillactim]

Could be worn throttle shafts in your carb, allowing too much air at idle. At off idle speeds the increased fuel balances things out.

Tim

[MikeLawson]

Here is some further information on this problem.  I took the suggestion and did a compression check. The results were 180-185 psi for all 8 cylinders. Based on that I think the mechanical condition of the engine is fine. When I took my vacuum readings, I  disconnected the vacuum source hose for the vehicle accessory systems and connected my gauge directly to the manifold port. The vacuum advance and choke pull offs diaphraghms are all good. My best guess now is that I have some type of carburetor issue. Anybody have a recommendation for a good place to rebuild it?

[35-709]

There a several good places around the country that have been recommended here .  Here is the one I have listed on several occasions ---

https://daytonaparts.com/

[Scot Minesinger]

I second Jeoff's recommendation on Dayton parts for carbs, they have a new needle and seat design that works way better.  Also when ordering the rebuild kit, always buy the float too.  Check the carb to be sure the shafts do not need to be re-bushed. 

Usually rough idle is carb build and/or vacuum leak (the carb rebuild often cures vacuum leaks).  There are numerous vacuum actuators, storage tanks and the like to be checked, especially if it is a power lock car, remote trunk release, climate control and etc.  Check vacuum actuators at e-brake release, and all vacuum actuated devices.

[Jeff Rosansky CLC #28373]

I have been fighting a lousy idle for years. I pulled the carb and disconnected the linkages and was surprised how much slop was in the throttle plate bushings. So I second what Tim said and have them checked. It made a huge difference for me but the idle is still lousy. So, I am on the waiting list for a local Quadrajet guy that the local car guys really recommend---- I have done all I can with it so time to farm it out.
Good luck

[Julien Abrahams]

Intoo chased a bad idle for quite some time. In my case there were several causes (for example a bad gasket inside the carb, bad vacuum hoses and condenser which had died). But on top of that, myintake manifold had a hairline crack ausing a leak. Because it is virtually impossible formoocal guys to weld it I "fixed" it by using black liquid sealant. That has held for the last 5 years and isstill on there. So ifyou have the carboff, just have a close look at your intake manifold as well.

["Cadillac Kid" Greg Surfas 15364]

I would next check the carb base gasket. 67 QJ has a unique gasket and (I believe) shim and insulator. This is susceptible to blowing out with age. Results in a vacuum leak. Worth checking out
Greg Surfas

[wrefakis]

fuel
my 67 runs like swiss watch on 100 0ctane
no high compression Cadillac engine will run 100% on 93 or less octane

[Jeff Rosansky CLC #28373]

Intoo  myintake manifold had a hairline crack ausing a leak.

May I ask how you found the crack and where on the manifold was it?
Jeff

[MikeLawson]

Thanks for all the ideas and suggestions so far.  Today I removed the carburetor, cleaned the gasket surfaces and reinstalled the original heat shield plate with new gaskets.  It appears the carburetor had never been touched and the gasket surfaces definitely needed some attention.  The car now runs a little better but still not perfect. At 1000 rpm, the vacuum gauge is rock steady at 22. At 600 rpm the vacuum reading is 18 and the needle still vibrates in a very narrow range, maybe a half point on the gauge. While I had the carburetor off, I checked the throttle plate shaft for looseness and it feels OK.

I contacted Daytona Parts and they quoted $329 to rebuild my carburetor, seems kind of high. I'd hate to spend that amount of money only to find out that it did not cure the problem.  Is there any particular place where the intake manifold is most likely to develop a crack?  I might consider pulling the manifold and to check for cracks and replace the gaskets if I can get motivated enough for that. 

["Cadillac Kid" Greg Surfas 15364]

Sounds like the gasket solved some of the problem. I would suggest you try slaying carburetor cleaner along the edge of the intake where it Mates to the head with the engine idling. A leak will show up as an increase in speed. I would also heck both the PCV hoses and fittings as well as the power brake booster fittings and hoses.
Greg Surfas

[James Landi]

Mike, congrats on the compression test-- bellissimo!  here's another suggestion--- have you blown compressed air through the idle control adjustment screw holes?  I had a situation with a big block V8 that I could not get to idle well.   I sprayed "Gum Out " through the control screws and through every port on my disassembled  carburetor.   It was not until I blew compressed air into the idle adjustment screw holes that I detected brown "gunk" being shot out of the idle jets.  I believe the gunk was due to fuel separation that occurs with ethanol that creates these blocking deposits in the idle passages.   James

[Scot Minesinger]

Jeff,

Did you reseal the intake manifold?  It has to be sealed all the way around (not just in the 8 intake ports), and the corners have to be sealed really well too.  It is a very common engine rebuild mistake to just seal the 8 intake ports.  The intake manifold on my 1970 Caddy was not sealed correctly (just on the 8 ports) and I had to reseal it to get the idle smooth, which also made it able to be timed correctly.  Now it is nice and smooth.

[Jeff Rosansky CLC #28373]

Yes,
I resealed everywhere I could find. The ports, rails and corners. Made a difference but still not right. I am going to bite the bullet and have the carb redone by someone else. I need to get that eliminated once and for all.
Jeff

["Cadillac Kid" Greg Surfas 15364]

Jeff
I’m late in the game here but if you haven’t yet CAREFULLY check the power brake fittings, hose and check valve. Greg Surfas

[Jeff Rosansky CLC #28373]

Yep. I actually disconnected (and plugged) the booster line.
Actually for anyone reading this (I think I kind of hijacked this guy's thread-sorry) I had the plastic check valve crack and it ran even worse. So, check that.
Jeff

[savemy67]

Hello Mike,

There are several websites and other documentation that explain how to read a vacuum gauge. From what I recall, a slightly fluctuating needle could be indicative of sticky valves, worn valve guides, or an ignition problem.  You may want to look at the references for vacuum gauge results to see which matches your gauge's readings.

From your post, it looks like you have replaced all the ignition components, so it seems a safe bet to rule out an ignition problem (although this is not 100 percent conclusive).  I would next try Greg's carb-cleaner spray test to verify that the manifold and its connections are good.  I would also check that the vacuum gauge and its connections are good.  If the manifold and gauge pass the tests, you may want to consider a cylinder leak-down test.  Despite the good results from your compression test, a leak-down test may reveal an issue with the valves.

$329 may be a fair price to have your carburetor rebuilt by a firm with lots of experience.  Is it guaranteed to smooth out your idle?  No.  You will have a good carburetor, but no matter how good the rebuild is - or even if you install a new carburetor - your engine is 52 years old with 45,000 miles on it, and it is possible that there is some wear that will preclude you from getting a smooth idle despite a change in the carburetor.  When I rebuilt my Q-jet on my '67, I used a dial indicator to measure the side play in the throttle shafts (see attached photo).  I measured both the vertical and horizontal play, and found enough to warrant re-bushing my primary throttle shaft.

While we may tend to think of carburetors as mundane components, they are, I think, fairly sensitive instruments for metering fuel and air.  Based on your engine, I can't be sure where your throttle plates are at 600 and 1000 RPM relative to the idle and off-idle discharge ports.  You may want to connect your vacuum gauge, and adjust the curb idle screw with the engine running so that you can observe the gauge as you adjust the idle speed from a low of 400 to a high of 1200 RPM.  If the needle fluctuation increases at lower engine speed, there may be a problem with the valves.

While problems like this are a bit difficult to sort out, I am fairly confident that the vacuum gauge is telling you something.  How much time, effort, and money you want to put into resolving the issue is something only you can judge.  Good luck.

Respectfully submitted,

Christopher Winter