Idle problem on a 54 Cadillac

[hearn]

My 54 with its Carter carb starts up and runs just fine except when it has been going down the freeway for appx. 15 minutes or more and then it really doesn't want to idle?  Start it up and run it around town and it idles fine, but after any extended time on the freeway at 60-70 mph, it idles too low and wants to die when back to around town driving.  Thanks for any thoughts on this.  Jim

[D.Yaros]

Start it up and run it around town and it idles fine, but after any extended time on the freeway at 60-70 mph, it idles too low and wants to die when back to around town driving.

It good be as simple as the "slow idle" needing adjustment?  Put a tachometer on the engine.  Let her warm up, and then set the slow idle to factory spec.  Not sure what that is on a '54, but would suspect it is somewhere between 450 and 750 rpm, with car in drive.

If that does not solve your problem, than it may be time to start looking at fuel delivery problems.  Could the fuel bowl be getting too hot and vaporizing the gas?

[R Sotardi #11719]

 Also, check for a vacuum leak. The gasket could be drawing outside air. Not noticeable while under throttle, but when at idle the mixture becomes too lean. Same for a vacuum hose that is defective. Ron

[Fred Pennington 25635]

You should also check the vacuum advance on the distributor. If it is stuck or not working it will cause idle problems. It is also a possible source of a vacuum leak

Fred P.

[R Schroeder]

Don't know if this model has a regular vented gas cap, but it may be plugged if you have that type.
Roy

[hearn]

You should also check the vacuum advance on the distributor. If it is stuck or not working it will cause idle problems. It is also a possible source of a vacuum leak

Fred P.

What would be the best way to check the vacuum advance?

[Jon S]

Easiest way to check the vacuum advance is to pull the vacuum line off once the engine is warm and off fast idle.  You will hear the RPM drop and increase when you re- attach the vacuum line if the vacuum advance unit is functioning properly.  However, I suspect your problem is heat related.  If you have a glass bowl fuel filter, do you see bubbling gasoline when the idle is slow after your highway speed driving?

[Glen]

Jim, correct me if I am wrong but the way I read your post is that the car will run all day long in town and not have a problem, but after it goes out on the freeway it won’t idle correctly. 

It sounds to me like the car never gets up to temperature in town and the choke and/or fast idle cam never comes fully off. 

Just a thought. 

[dplotkin]

I would perform some simple diagnostics before checking & replacing components. When the motor is hot and not wanting to idle, put a vacuum gauge on the engine. Low vacuum will indicate a leak, or bad timing or a valve problem. Then put a timing light on it & see what you have.

I had a similar problem with a 56 and a Carter WCFB (same basic engine & carb as you). In my case it turned out that the idle circuits in the carb became obstructed and I had the carb rebuilt (not just a kit put in it, but completely disassembled, tanked, bushed & reassembled).

Cold engines run richer and mask a lean roll (lumpy or low idle caused by lean mixtures). So they start & run fine until they come to temperature and with the choke fully open, begin to run too lean. Sounds like your problem.

Dan