Did you clean the carb jets, check the float levels and check the air balance cylinder to cylinder? Dirty or clogged, mal-adjusted carbs on an engine that has been sitting are much more likely to be the source of rough running than anything mentioned in this thread.

Have you looked at the spark plugs to try to read if it running rich or lean? Checked the plug gaps?

Before you messed with the valve springs leakdown was all single digita, now you have over 10% on three of them. Thats not good. Was this check done cold?

If there was a problem with the cam lobes on #6 it should have been noticible while you were adjusting the valve lash.

Leakdown values were determined with the engine cold.
The air flow for all carburetor throats were determined using a Synchrometer and are the same.

synced carb vacuum levels only tells you that the idle air bleed circuits are working. you could be adjusting to cover other problems.I believe you need to rebuild the Webers, at least so you can rule out that as an issue.
unless you have already done so?

On December 1, 2023, I put the left carburetor on the right side of the engine and the right carburetor on the left side of the engine.
The engine speed still increases when I disconnect the spark plug wire from #6 and engine speed decreases when I reconnect the spark plug from number #6. Therefore it seems that the carburetor may not be the problem.

On December 1, 2023, I did a compression test (dry). The results were as follow:
Cylinder #1 144 psi
Cylinder #2 140 psi
Cylinder #3 140 psi
Cylinder #4 149 psi
Cylinder #5 152 psi
Cylinder #6 150 psi

I highly doubt this is an internal engine issue, your comp and leak down numbers don't show anything untoward. Ignition seems most likely. its almost like the connecting of the plug takes spark energy from another cylinder , causing it to run rough.possibly cross firing or random sparking in the cap. I would replace the distributor, leads and coil /CDI box from a Known good car to rule out ignition.

I do not have distributor, leads, and coil/CDI box from a known good car. I have replaced all of these components. The distributor has been rebuilt and has a #1867S Pertronix module, #40611 Pertronix coil, 911.609.06100 spark plug wires, the factory CDI box checked. Spark plugs are NGK BP7ES.

Im all out of ideas with this one.. Im positive I could diagnose it with the car in front of me, but its near impossible to do just exchanging messages. sorry. good luck!

double check all your connections and cap condition.replace coil.try another CD box.
The next step in logical trouble shooting would be remove, thoroughly clean and check the Webers. Make sure all the jets and internal parts are the same.

I have checked all connections. The Bosch cap is new and several caps have been tried. I have tried two different coils. I have tried three different CD boxes.

Lauran

Intake or exhaust valve sticking? Try some Marvel

Got to be driving you crazy

I have added two quarts of Marvel Mystery Oil to my ten quart capacity oil pan and remote oil cooler.
I have also added 3 oz of Marvel Mystery Oil to 7 gallons of gasoline in my fuel tank. The engine seems to be running smoother with the addition of the Mystery Oil. I hopeful that the Marvel Mystery Oil will help with any valve sticking that my be occurring in the engine.

This sounds a bit dumb to me, but i know my cars dont like to idle low, yours seems a bit too low to start with, do you have the same issues if idle is set to 800-900 rpm…

Yes

I believe that adding Marvel Mystery Oil to my crankcase and fuel tank has solved my problem after two years of investigation. Thanks to all for their suggestions. Lauran

Maybe I am the winner, I have seen Marvel work for stubborn issues in the past. Hope she is running good these days.

Could have been a sticking valve