Engine: 61 Lark Issues

[drnittler]

My 61 lark six cylinder, automatic, has the follow problem. At a car show in May I bought some bad fuel and that resulted in the car running badly and quitting. Any way I took care of the problem, including removing the bad fuel, and it worked until today. (But without much use)
Today the car would sputter and barely move or accelerate. I did get it home, and checked the usual items: the carb, fuel filter, plugs, points distributor, etc. I started it up and it ran badly and then it picked up and ran fine.
My conclusion is that the fuel must be absorbing moisture and causing problems.
So my questions would be what additives(s) would you recommend so the I can cure this problem?
What are your opinions about Marvel Mystery Oil, Sea Foam, Heet, and so on.
Notes: The motor is rebuilt with 7000 problem free miles on it. The valves were checked and adjusted, and I do not think any "big stuff" is messed up.
Your thoughts are appreciated.

[drnittler]

Update: 6/24/14
The car had been running ok. Today it is running poorly again; sputtering, no acceleration, etc. Before today the car would do this and sometimes return to normal. In 23 years of ownership I never had this problem before,
I checked/changed fuel lines, plugs, cleaned the carb, replaced the after market fuel filter (Nothing was collecting in the filter), checked the fuel pump, distributor, wires, connections, etc. The engine was rebuilt 7000 miles ago. Plugs are clean with number 6 a little carboney. I have added Stabol, Seafoam and Heet. The tank was cleaned out years ago.
I would appreciate any ideas. I am running out of stuff to check. (Note: The car always starts up and the timing seems ok) Thanks for the help.

[studerodder]

try a different coil.

[drnittler]

I believe I will and a condenser Thanks

[EssexExport]

What happened to me recently was the sputtering. Had had just about everything done to the engine. Turns out it was the gas cap no longer acting vented. Purchased a new vented one at Napa & haven't had any problems since. The old one had have been around for 50 years: they do wear out.

[StudeRich]

A '61 Lark does use a vented gas Cap, so if it is not, or it is plugged, it has to be replaced.

[drnittler]

I ordered a gas cap.