'79 L82 Automatic. Engine stock (I believe) except for intake manifold. All original equipment and emission control devices (except catalytic converter) installed and working. All new vacuum hoses.

Have finished the winter work and took a decent test drive. Even with the idle speed screw backed off the throttle bracket I cannot get idle below about 950 in drive. Only things I did engine wise were to put in the lighter mechanical advance springs recommended by Lars and add a can of seafoam to the gas tank.

In my 3 years of ownership everything I've done engine-wise (plugs, wires, cap, new vacuum hoses, carb cleaning (just spray cleaner), etc. have caused me to back off the idle speed adjustment. I've now run out of adjustment and find no mechanical problem--in other words I appear to be at the end of the possible adjustment.

I haven't looked to see if the anti-tamper caps are in place on the idle mix screws but am I thinking properly that a previous owner likely adjusted the idle mix screws? Perhaps another reason I should send the carb off to Lars? What else could cause a high idle?

 

If it idled fine before I would look at the change you made.

It could be that the mechanical advance springs are too light and allowing advance at idle, this would increase the idle rpm.
Did you put a timing light on it before and after installing springs to see where your timing was and where it is now?

Another common high idle cause is a vacuum leak.

 

I got rid of my timing light decades ago in belief that I'd never need one again. I've had a local Corvette shop set the base timing but that was a few repairs ago.

My experience with this car has been that vacuum leaks cause a low, rough idle. I can start searching again but believe the vacuum system to be extremely tight.

 

Changing the advance springs is not a bad thing, but you still need to verify and confirm they did exactly what you wanted them to do, without going way past the recommended upper advance limit. As REELAV8R pointed out, the light springs could have tossed your idle advance way up the curve and in turn bumped your idle speed.

 

Not clear how you could do a modification to the distributor without using a timing light.
As others stated check for vacuum leak, temporarily remove and cap the lines to the carburetor and dont forget the PCV valve. Make sure the fast idle cam on the carburetor is dropping down when the choke is fully open.

 

You can try putting your original advance springs back in. I think your idle speed will return to normal then. As mentioned above, if you're altering your ignition timing, you need to have access to a timing light.

 

You should be able to kill the engine by closing the throttle plates, regardless of what the timing is and what springs you're running.

An engine cannot idle without air. If the throttle plates are really fully closed, it means that the engine is pulling air from someplace other than through the carb. That means you have a vacuum leak.

First, pull the carb and flip it upside-down to inspect the throttle plates. Verify that both the primary and the secondary throttle plates can be fully closed and seated in their bores with no "light gap" at all around any of the 4 throttle plates. Make sure the link connecting the primary throttle to the secondary throttle has "slop" in it when the primary throttles are fully closed and seated in their bores.

Then, plug every single vacuum port on the carb, including the PCV, power brake, and every other vacuum port.

Verify you have the correct carb base gasket installed and that it provides a full seal against the carb base and the manifold with no leak path.

Then, disconnect every vacuum hose from the intake manifold and plug the ports at the manifold.

Install the carb and start it up, after verifying that the throttle cable is not preventing full closure of the throttle.

Then, determine if you have a fast idle.

Lars

 

Before removing the carb I did some checking with the hand vacuum pump.

Guess what? The only system where I haven't replaced the lines (the HVAC) now has a significant leak at every position of the control switch With the pump attached to the HVAC connection the warm engine idle is now perfect. I'm guessing the switch is at fault as it had a small leak. I ordered a "modified to work properly" switch from a vendor to find that it leaked FAR worse than my original and used some silicon grease on the original to find it sealing perfectly. Something tells me now that such was a stop-gap repair and I must find a replacement switch that actually works.

Just had a test drive with the HVAC vacuum connection plugged and WOW!!!!!!!!!!!!

With wide-open throttle the RPMs ZOOMED up and for the first time I thought it was actually quicker than my '99 STS!

I COMPLETELY believed Lars suggestion and numerous reports of how the light springs in the centrifugal advance will "wake up" later C3s!!!!!!!

A timing light is on my list but I certainly won't replace the dwell meter I sold in the same garage sale as my old (non-inductive) timing light

 

I have a timing light but it is NOT always my tool of first choice. I prefer to use a vacuum gauge. I DO use a timing light but I do not totally get freaked out in what it shows when I check it due to I have encountered so many Corvettes where the outer ring of the harmonic balancer has spun and thus....will give a reading that may cause people to think something is wrong.

This is why IF I am suspect of the timing mark on the balancer is NOT correct...I verify it.

DUB

 

The factory tune on these cars was lousy. Glad you found the problem, it screamed vacuum leak to me too.

 

​​​​​​​Where and how much for the springs!!!!!???? HERE TAKE MY MONEY!!!$ hahahaha

 

The springs are part of a common and very inexpensive kit typically stocked by any decent auto parts store. Look for posts here by "Lars" for the exact name of the kit and which springs to use.

 

Unless you are an expert like me!

Never change more than one thing at a time without a test drive! Change hoses, then test drive! Change dizzy springs, then test drive!

Ps there are some spring kits that are sold as a system with curve charts! Those usually come with aftermarket plates too! MSD, Moroso in the modern?, Accel used to? Mr Gasket not sure, are some that come to mind! But I also find it unusual for anyone in the modern to not own a dial back timing light! Even the cheapest ones can give you more ability to know what and where your engine is doing its thing!

Also make sure those spring plates are lubed and have free movement without hanging up! I polish the sliding overlap edges!

Stepping thru tune of springs is easy!
Start like with heavy heavy, then try heavy medium, then medium medium, then light medium, then light light until you find the one that best performs!

If you started with vacuum leaks affecting your tune, you will have to retune the carb!

 

That's the ONLY way you'll know what effect any single change has made!!!!