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1979 Coupe DeVille - Rough Idle on Cold Start

Started by 0maha, March 13, 2021, 02:49:27 PM

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0maha

Hello, gentlemen! Haven't posted for a while.

My '79 Coupe DeVille has a (severe, by my reckoning) rough idle on cold starts problem.

The problem only happens when it is fairly cool out. It doesn't happen (or at least happens less severely) if it is warmer. Like 70 degrees warm.

This morning I decided to make a video. It was about 45 degrees out, and the car hadn't been run for two days.

Here is the link to the YouTube video I put up: https://youtu.be/kY9oHzOoXFg

Once it is up to full operating temperature, it runs fine. Starts right up. Idles smooth. Perfect.

I'm trying to decide what, if anything, I should do about this. My general sense is that the carburation/intake/timing/ignition systems on these was something of a hot mess from the factory. They were trying to comply with a bunch of new emissions rules and really didn't know how.

So, what I'm thinking is if this has a simple and fairly easy fix, I may dig in and do it. Otherwise, unless someone tells me otherwise, I think I just live with it. The other option is to swap out everything and put a FI unit and a new ignition on it, but that seems like a lot of $$$$ for a fairly minor problem.

Any thoughts? Thanks!

Jeff

The Tassie Devil(le)

G'day Jeff,

Sounds like you could be having a problem with the choke not being properly adjusted, a Vacuum leak, and yes, a mixture problem as well.

When starting cold, the choke should be holding the revs higher than you have, plus, the "hunting" could be from a lean mixture, plus, the Vacuum leak could be self-sealing as the engine warms up, and the various materials stabilise.

The idea of the choke it to artificially richen the fuel/air mixture, whilst holding the throttle open to allow the engine to spin over well above idle.

Then, as the engine warms up, the choke plate opens up, thus leaning out the mixture, but, unless the throttle is "kicked", the fast idle part of the choke will still be holding the throttle open, hence the higher revolutions.   Kicking the throttle releases the fast idle operation, allowing the engine to idle as set when the engine is warm.

The rough-running when cold sounds like what I get when the choke is wired open.   (I don't use a choke on my car, and it acts a bit like yours until it is warmed up.)   The reason I don't use a choke is simply because when the car cools down after being used, and I restart it the choke comes back on, which I don't want to happen multiple times during the day.   The Choke is there for compensation for a myriad of conditions that an engine will get from Summer, Winter, Freezing to boiling hot, and millions of different conditions.   Notice how single use engines, as in race cars, Stationary Engines, etc, most times there is no choke, as there is no need for them.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

"Cadillac Kid" Greg Surfas 15364

Jeff
The first thing I would heck is the choke pull off. That is the little vacuum actuator at the front passenger side corner of the carburetor. It is connected directly to a vacuum port on the carb by a short vacuum hose. Check the hose first then check the pull off by depressing the plunger with the vacuum hose disconnected then placing a finger on the pull off vacuum inlet to
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

79 Eldorado

My first instinct would be the choke as others have mentioned already. The one other thing I noticed in the video, but don't really have experience with how the car would run if it wasn't functioning properly, is that vacuum operated baffle on the snorkel. Does that also interact with the heat riser tube on the exhaust manifold? I thought I recall that being attached to the bottom of the snorkel... cannot recall.

Scott

Matti R

Can you take a video of cold start without air cleaner so we could see if the choke is working properly?
Best regards,
Matti
CLC #33333
67 DeVille convertible Sudan beige
79 Seville
64 Sedan DeVille
66 Calais Coupe

TJ Hopland

The fact that it starts out sounding ok does point to the choke.  Mixture is fine for those first few seconds but then probably too rich.  A look at the choke blade would help.   Like Greg suggested if that pulloff is having issues that could explain it and we may see the blade moving. 

Its maybe leaking and can't quite pull it open but eventually vacuum builds and it starts to open.  When it starts to open vacuum is reduced so its no longer strong enough to hold it.  Cycle repeats.       

Seems a little strange that its better when its warmer,  usually warmer means it needs less choke which not more which seems like the case here.  Maybe the cool weather is enough to make the rubber in the weak pull off stiffer so it just doesn't have enough pull to do its thing?
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

0maha

Appreciate all the responses. Thank you all very much.

I'll see what I can do about posting another video without the air cleaner housing.

wheikkila

I watched the video. I think the problem is the dash pot that pulls the choke open might be bad. Take the just the top off and do a cold start see if the choke opens. The dash pot is on the passenger side at the top. When the car starts the vacuum pulls the lever forward. It is a quick check. Please keep us informed.
Thanks Wayne

Koa

Hi Jeff,
my suggestions (when not the choke itself): 
ignition setting, spark plugs,
fuel filter, fuel pump
and of course the carburetor setting - the most complicated task in my eyes

Question: except the "lazy" start, does everything work properly? Is the power on WOT, what it should be?
77 Coupe DeVille 425 ci - federal, red+ white interior
78 Fleetwood Brougham 425 ci - California, white + red int.
98 DeVille with the famous Northstar engine
32 Tatra 57  -  do You know that?