I've been suspecting that the timing chain and gears need to be replaced and looking for a little confirmation or ways to test and confirm as well.

I just finished balancing the throttle bodies and went to go time with a timing light (EST was disconnected), and the timing mark would bounce from 6 to 10 degrees BTDC. No matter which way I turn the distributor, the timing would continue to bounce. Which leads me to believe it's the timing chain and gears. Thoughts? Anything else I can check? Short of pulling the timing cover of course.

 

Any extra play in the chain will always show up at the distributor rotor.

Remove four plugwires from the dist cap standing on the passenger side. This will allow the cap to lay on its side.
Take a piece of masking tape and a Sharpie and mark on the dist body where the rotor is pointing.

Now, turn the crank backwards with a breaker bar and socket and note the amount of degrees at the harmonic balancer before the rotor also begins to move backwards.
That is your chain slop.

For what its worth, your erratic timing light jumping around is somewhat common on HEI.
Some timing lights have great difficulty monitoring actual firing of a plugwire. So, it appears to dance around on the balancer. Consider it a glitch.

 

I would say, as above remove distributor cap so rotor can be observed. Remove spark plugs so engine can be turned by hand. Rotate engine just enough to get rotor to move. Rotate engine in the opposite direction. How far in the opposite direction did you turn it to get rotor to move? Do this several times. What do you think? A really tight chain and that rotor moves within a degree or 2. A really loose chain? .......
Timing chain is affordable. It's pulling the oil pan and harmonic balancer and water pump that's a bit of work. And gaskets and fluids
So, yes I'd check that chain for play first.

 

The timing chain is likely the issue causing the timing variances that you are seeing. Having an HEI distributor makes life even more challenging as the hot sparks it makes can leak causing the nearby wires to pick up the noise. When working on the timing on my C4 with it's HEI, I tend to put the #1 cylinder "spark pick-up" as far away from other wires as is possible to keep the signal strong for the number 1 cylinder. Any wires that touch or even run nearby can pick up electrical noise.

If your engine came with the nylon coated timing gears then as it wears off, it can make the timing very unstable. I don't know exactly when GM started with the nylon coated timing gears but it was another bad idea as it wore off way too soon and made problems for Corvette owners like yourself. I used a heavy duty double roller chain and it has worked well for me.

Before taking the engine apart I would double check the parts inside the distributor to be sure that something inside of it is not making the problems. A loose base plate or centrifugal advance inside the distributor could cause the same type of issues that you are seeing. On my MSD distributor I have the vacuum advance and a centrifugal advance and either one could make issues for you if it got too loose.

The last time I had to have my oil pan dropped I took my C3 to Tony's Corvette Shop in Maryland. They replaced the timing gear cover gasket and the oil pan gasket and neither has leaked in decades after the pro's re-installed the oil pan and timing cover gaskets. There are times when a professional Corvette mechanic might be able to save you time and seal your engine more effectively than we can doing it on Jack stands. Their experience is worth paying for at times when you need a bit of extra help.

 

Thanks for the pointers on identifying play without needing to disassemble. I will look to do that this weekend. I suspect I'll see play but definitely want to confirm before I go digging in.

With my 82 it's computer advanced so no centrifugal or vacuum advance for me. I also try to put the timing light pickup as far away from other wires as much as possible. I believe that the engine has the nylon coated gear but won't be able to confirm until I drop the oil pan and remove the cover. Looking to do that in the new year. My oil pan is pretty beat up so I want to look at getting a new one put on (has a decent dent where a previous owner hit something).

 

Your 82 came factory with the nylon coated gears. Something GM did for many years.
And if your pan is beat up, yes now is the time to replace it!

 

Have you checked the harmonic balancer? The rubber can go bad and the outside will slip.

 

After reinvestigating everything it turns out that when I balanced the TBI's I did not back the throttle stop on the front TBI out far enough to let it close properly. Once I backed that out, I rebalanced the TBIs per shop manual instructions and timing has been dead on.

I do not believe that the harmonic balancer has gone bad yet. I checked it a few months ago and found that when I had the engine at TDC (verified independently of the balancer) the line on the balancer lined up perfectly with the TDC pointer on the timing cover.

I've driven it a few times since and the idle feels much better.