1967 Deville: How to cure rough idle?

[76eldo]

I had some 4 barrel carbs rebuilt for a 55 I was working on with dual quads.  The carbs were all plugged up from sitting.  I used Bowen Carburetors in Levittown, PA and I have always been happy with their work.  The cost was $300 each for the 4 barrel carbs.

Brian

[35-709]

Great video at the end of this description of how to use a vacuum gauge.

http://www.secondchancegarage.com/public/186.cfm

[clarkent5477]

Hello: what do you use to reseal? Hi temp RTV or?

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.

[James Landi]

Again, I mention the issue of "fuel separation" issues with carburetors... a brown dirt sinks to the bottom of the carburetor as old fuel evaporates, and the "dirt" lodges in the low speed idle jet "tunnels" that are in the bottom casting that are, obviously, invisible. I have removed and rebuilt any number of carburetors over my 7 decades of dty shade tree experiences, and in the "early days" believed spray GUM OUT would clear these tiny low speed idle "tunnels."  I found, after much painful work," that one needs to use compressed air through the adjustment screws at the base of  the carburetor... works like magic.   James

[Cadman-iac]

 Jeff,
I've had older vehicles that didn't idle well. It is due to carbon buildup in most cases when everything else has been checked and or corrected. What I have done for the ones that didn't improve with the usual methods,  is to add a product called "44K" from a company called "BG". You have to get it at a mechanic's shop as it's not sold directly to the public.  I get it from a shop that does my alignments.
You can add it either by pouring it into the tank, or by running it full strength thru the carb via a vacuum hose.  Preferably one that draws from both halves of the manifold. It can also be poured directly into the carb if you don't have,  or are not sure if there's a vac hose for both sides.
Going through the carb, you want to keep your RPM's up to prevent it from stalling until it's almost all used, then quickly pour the rest into the carb and allow it to stall out. It works by dissolving the carbon deposits, but you need to let it set for about half an hour to let it work.
Once it's sat for 30 minutes or so,  restart it.  It will smoke like a locomotive for a few minutes then will clear up.  Rev it up several times to blow the carbon out. It  should run much better.
This can also be done with water,  but I don't recommend it unless you have alot of experience. It can damage your engine if not done properly.
But using it in your gas tank will work as well,  it just takes a little longer. However it will also clean up the whole fuel system that way, and if you're apprehensive about going through the carb, this will work just fine. You will want to only have at most a half a tank of gas because of the size of a Cadillac tank. It dilutes too much if the tank is too full.
Now if your car has an oil consumption problem,  this will only work for a while before the carbon will return.
Hope you find this helpful.
Rick

[TJ Hopland]

The Quadrajets have what I think is a unique to them design feature where the fuel passages come off the very bottom of the fuel bowl.   Most other carbs that I can think of the fuel passages are just near the bottom, not actually the bottom.   Think of the drain in a bathtub.   This means any crap that makes it up there or that gets created from sitting goes 'down the drain' rather than just builds up at the bottom.   Once it goes down the drain there are some twists and turns and small orifices involved in getting the fuel where its supposed to go so it doesn't take much to start plugging things up.  At least with other designs there is a chance the crap will just stay at the bottom.   

[dadscad]

A lot of good information and ideas in this thread.  Just curious because it hasn't been mentioned, have you checked the timing chain for excessive slack? If it is still the original gear set it may be becoming loose enough to cause running issues. Even though the engine is low mileage,  the plastic coated cam gear deteriorates with age. One indication would be the timing mark dancing up and down while viewing with the timing light. Or, removing the distributor cap, then rocking the engine crankshaft CW & CCW while observing the rotor movement compared to the timing mark degrees. HTH.

[hornetball]

Speaking of timing chains, when I went into my engine to replace it, I found that my camshaft had 3 badly worn (but not completely wiped out) lobes.  That would also produce these symptoms.

http://forums.cadillaclasalleclub.org/index.php?topic=154660.msg425803#msg425803

[Cadman-iac]

True, cam or gear, either can be a cause for your problem. It's easy enough to check for the chain slack by rocking the crank back and forth,  but the gear issue may not show up as easily if it hasn't actually begun to come apart.
As for the cam lobes, a slight wear shouldn't cause much in the way of roughness,  but if one or more are worn too far, you will get first of all a noisy valve train and it's not adjustable,  and the hydraulic lifters will only take up so much slack. Secondly,  a backfiring under load, especially if it's an exhaust valve. It will backfire thru the carb. If it's an intake, most times you only have a weak cylinder, but it would show up more at a higher RPM, as there's less time to charge the cylinder.
But  consider how much the car was driven in it's life, and by whom, and how so. A lot of low mileage cars were only driven to church on Sunday's by little old ladies, (as the old salesman's line goes).
You get a lot of carbon build up that way if it was in fact driven that way.
Just something else to consider.
Good luck with finding the problem. It will be interesting to know what you find.
Rick

[TJ Hopland]

Ya good point about the carbon.   I don't have the photo handy anymore but I have been into a couple of these motors were the carbon build up was almost the size of the valve.  Could still see the corsshatch in the bores so wasn't likely that many miles on it.   I'm sure it was a just to church and the video store sort of a deal.