Rough Idle Solution

[Dave J]

Having exhausted all of the simple fixes for a rough idle from a cracked manifold to bad fuel, without success, I have finally accepted the solution that has been staring me in the face all along.  My vacuum gauge tells the tale.  There are about 8 different scenerios  in reading one of these.  Below are cut-outs from 2 different vacuum gauge websites showing symptoms and cause.  My gauge points to worn valve guides every time.

1) Worn valve guides can be detected by a vacuum reading that vibrates back and forth between 14 and 19 or so very quickly.
2) Excessive Vibration that Steadies as RPM Increases   Worn valve guides.

The decision now, is whether to fix it or drive the car as is?  The car runs great otherwise so if I decide to pull the heads it definitely won't be until the cruise season is over.

If anyone else has been chasing this same problem, try a vacuum gauge, it can tell you a lot.

Dave Johnson

[The Tassie Devil(le)]

I agree...... The Vacuum Gauge is a good Diagnostic tool, and an engine tuning tool as well.

Bruce.

[russ austin]

Have you checked for bent pushrods?

[Tito Sobrinho]

Dave. since you have a low engine mileage and to me, a worn valve guide is low in my list. I would check for a leaky engine valve due to carbon accumulation, sticking valve, broken valve spring, and tappet clearance.
I received an advice from an old mechanic fellow when working at a car shop during my college years.
Get Mystery Oil mixed 1/2 with warm  water in a spray bottle and apply this mixture spraying down the carburetor throat at a fast idle. At this point, you don't have nothing to lose i.e  before you open the engine. For the hydraulic tappet, I'm sure you are using a detergent oil. For a broken spring and a defective hydraulic tappet including its clearance, of course, you will have to open the engine.
Hope this helps.

[Scot Minesinger]

Cadillac year and engine?  Vacuum leaks are a very common cause of rough idle in the 1960's and 1970's Cadillacs

[wrench]

The way I handle this question is as follows:

1) open a bottle of beer
2) take a sip
3) place the bottle on top of the air cleaner at idle
4) no foam = smooth running engine
5) some foam = try a tune up
6) lots of foam or the bottle falls off = tear down

[Dave J]

Thanks you guys for the input.

My car is a 1956 Coupe de Ville.  Because the car runs very smooth when choked during warm up and because the vacuum gauge is vibrating, I'm thinking the rich mix at warm up is compensating for a vacuum leak.

I can check out your suggestions by removing the valve covers before pulling the heads.

I agree that worn valve guides seems unlikely with such low mileage, unless it had poor maintenance.  But my compression test came in at 180 lbs on all cylinders and so did the one at the repair shop.  I think that would eliminate a leaky valve or tappet clearance.

Dave Johnson

[dplotkin]

Compression numbers like that wouldn't immediately bring to mind internal problems. I think you have a vacuum leak or as in my case with my 56; a plugged idle circuit in the WCFB. Your guage will show low or irratic vacuum I'd think every time a cylinder misfires because of the lean mixture. You might see if it straightens out by choking the carb with your hand. That will tell you if its a "lean roll" or something else.

Dan

[Roger Zimmermann]

Dave, the engines from my 2 '56 Cads have been rebuild and are running well. At idle, I have also that vibrating needle. I don't care anymore...

[Richardonly]

Stupidest thing happened a while back with my flathead.

Car started to idle and run fast (rpm's) for no reason.  Lifted air cleaner a "bit" higher on carb, and it stopped.  Drove it, it settled to the lower position again and it went back to an accelerated rpm. Lifted it a bit again, but this time placed a clamp around it to hold it in place and the problem was solved.

Inspecting it, it was NOT touching any linkage to cause this problem.

Once again, this is a flathead with a totaly different air cleaner system.

[Tito Sobrinho]

Richard, seems to me that your carburetor has a problem with the air-gas mixture. Or,  do you have a bath oil air cleaner? If so, the mesh could be dirt and has to be cleaned with gasoline

[Tito Sobrinho]

Art Archbolo, did you do the same in your '49? I always wanted to change the oil bath air filter for a paper element.

[Dave J]

Roger,
I had the 365 engine rebuilt on my 58 and still had a rough idle and vibrating vacuum gauge. After exhausting all options, I swapped the original Carter for a new Edelbrock carb.  It didn't solve the problem but it had better idle adjustments to compensate for the problem. Eventually I gave up and drove it with the Edelbrock for 10 years.

The guy who rebuilt my engine was a young man, so I always questioned if he did the job right.  I still believe that the gauge needle should be steady.

[Viator Trudeau]

I have a question on the use of s vacuum gauge?, on my '74 ELDO.
Where do you hook up the vacuum gauge??
AT one of the active ports on the carb, or at the fitting at rear of intake where hoses go into the fire wall.  The carb location givea a general idea of overall vacuum thru out the manifold.  no needle flutter.
On my car the fitting at the rear of the manifold shows a fluttering of the needle where cyl. #3 & 4
are weak at idle. With a little increase in idle speed fluttering dissappears. Opening the idle screws  enriching mixture for all cylinders im proves idle??? in general.
V.Trudeau  P. S.  The numbers on my carb indicate the carb is for California usage.
Would this be the cause of my idle problem?

[R Schroeder]

Viator
I have my vacuum gage hooked up all the time. I have a tee in my vacuum line to the vacuum advance. My vacuum line comes right out of the manifold.
I believe yours , being a California car would hook up on the carb, above the throttle plates.
This is not a good set up, as it keeps the engine retarded at idle.
See if you have a port in the manifold up front near the distributor that you could plug into. Then block off the one on your carb.
To set up carb, you adjust the air mixture screws until you get the highest vacuum reading. you may have to idle it down to do this.
If the vacuum advance is plugged into the carb, above throttle plates, it wont work there.
If you want, use a tee at the back of the carb for the testing.
As you can see, mine pulls 21.5 inches of vacuum at an idle. This is the best you can get with a stock set up. Also a steady needle is preferred.
Roy