Hey guys,

What would cause a car to idle at different speeds? In my particular case, I have a 1980 4 speed, stock 350 with a Quadrajet, and once warmed up the car will idle just fine at a steady rpm. I start driving and after a bit I will come to a stop somewhere and the idle will be different, but consistent. I will drive some more and then the next time I stop the idle might be different, again, but still consistent for as long as I am stopped.

It is frustrating because I will do everything I can to check my timing, adjust my idle mixture screws with a vacuum gauge, and then fast idle screw to get my car to idle correctly, but then the next thing I know I have driven across town and stop at a stoplight, and the idle is high, or low, again. I have checked everything several times over the last few weeks, but the same issue persists.

I'm guessing the answer is going to be a vacuum leak somewhere, but I cannot find it. I have removed the air pump, capped the EGR valve, capped all unused ports on the carb, removed hoses from the carb to check for leaks, and found none... My timing checks out at 16 degrees initial with the vacuum advance can adding another 12-16 (I don't have a dial back timing light so I am guessing). So, any suggestions?

Thanks!

 

Have you tried spraying carb cleaner around the base of the carburetor where it bolts to the intake?
Is your distributor mechanical advance modified that you are running 16 initial?

 

If the carburetor has recently been rebuilt, it's possible the gasket between the carb body and throttle plate is slightly off center and one of the butterflies is catching the edge of an off center gasket. For the record there are gasket aligning pins on the underside of the carb body. There are holes in that gasket that are supposed to fit over those pins, but if broke off (common) the gasket can hang over into the throttle bore and one of the butterflies can hang up on it, making for an inconsistent idle.

Sticking mechanical weights in the distributor can also lead to an inconsistent idle.

 

Yup, that is one of the first things I did...

I have changed the carb base gasket, but did not notice any change.

The only hoses going to the carb are the PCV, vacuum advance to manifold, and brake booster from the rear of the base plate. There is also the vacuum fitting on the top of the intake manifold behind the carb for the headlights and such.

 

The carb was rebuilt it 2016 according to a sticker that is on the side of it. What is the average lifespan of a Quadrajet between rebuilds?

I guess I should go in an check the distributor as well.

 

Throttle plates binding with bore in throttle plate, intake surface, or carb gasket; throttle linkage binding up; accel pedal problem; weak throttle return spring; etc.etc

 

You say it changes, does it stop changing after about 5 to 15 minutes or is it an all day issue.....could be the choke, as it heats up theres different choke speeds depending on the setting. Obviously theres a high choke speed on initial startup, then it steps down as the choke gets hotter and hotter. It could be a vacuum leak that gets bigger as the engiine heats up and the metal expands more. Try tightening all the intake manifold bolts and the carb mounting bolts. Check the hoses from your PCV system, maybe theres a crack

 

Stick a vacuum gauge on it........does the needle bounce? If so, there is a vacuum leak.......
Usual suspects are petrified carb base gasket and/or intake manifold gasket.
The advice on the choke is a good one too if the choke is adjusted in the wrong window on/off it can do the symptoms you described.

Jebby

 

I remember in Cliff Ruggles book him mentioning this could occur it the throttle plate shaft is loose in the carb body. They create a variable vacuum leak essentially.
Originally the shaft has no bushings so it wears on the carb body and gets slopped out from the spring tension from the throttle spring. You can buy bushings for the throttle shaft, drill the body and install the bushings.

 

Next time its playing high-idle games, pop the hood and apply light pressure towards the throttle return. If there is no difference in RPMs, the throttle blades, base gasket, throttle cable, etc are NOT the issue. Look elsewhere.

About the dizzy weights, just lightly clean with a rag and apply a smear of White Lithium Grease (Walmart, red spray can). Grab the rotor, twist, and note how easily it snaps back (or doesn't).

 

Good thought here. I remember mine was sticky once upon a time. Had to dissasemble clean (rust) and add grease

 

Especially of a concern with the lighter / weaker advance springs being used today. As opposed to the stronger ones back in the 1800s wagons.

 

"90% of all carb problems are ignition problems."
Check your timing (with everything hooked up, including vacuum advance) when the engine is idling "normally." Drive around and bring it back to the garage when the idle has changed. Pop the hood, and re-check the timing - I'll bet it's changed...

 

I agree with the scenario presented by Lars, that timing changes will be associated with this idle speed variation problem. However, it is unclear whether that variation is the "result" of an ignition problem or whether it is the "cause" of that timing change (yet to be proved, but very possible). If there is binding in the throttle apply system, that would mean that throttle plate position would be changing when engine is idling. This, of course, would cause engine vacuum to vary and thus vacuum advance to change. So timing would change with idle rpm differences...as it should. The 'root' cause of this still needs to be established and the OP must do some sleuthing to determine that cause.