The other Item You might look at is the Electric Tach. I believe You added One to replace the original ? Anything that might affect  the Ignition Circuit is worth looking at.

  

If returning to points corrects it, the IGN I can be affected by an electric TACH. There are service fixes @ their site. If bad, replace with IGN II.

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seems odd but me and 2 others this side of the border use ford dura spark,converting the ford dist,[buying new coil and module,we all run tacks. get base parts for dist from early 302 ford dist, at wrecker. bit of a chore to wire,no issues from idea to valve float. petroxl seems to spell issues.

DURASPARK II is the logical choice but is hard to hide and requires extra effort to install. PERTONIX is hidden and draws no distraction to the appearance of the engine. IGN I is old tech and has a lot of problems. They should have discontinued it when IGN II was introduced (IMO).

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It was a long day, but the mystery miss is gone. Removed the Pertronix module and installed new points and condenser. No change.  Disconnected the tach and O/D wires.  No change.  Changed coils.  No change.   Opened up the Holley carb and found it was full of dirt. Cleaned out the carb and installed new needles and seats along with a new power valve.  Installed a new fuel filter.  Plugged the vacuum advance, set the inital timing to 10 degrees . 36 degrees total on my timing light.  Installed new plugs.  Nothing seemed to help.

Finally I drove down to the gas station and bought 5 gallons of high test. No change at first.   After a few minutes the miss went away completely.  Crappy fuel.....

This just proves the old adage "90% of electrical problems are carburetor related"  An vice versa.  Pete

... hmmpf ...

How long does the car sit between drives? Do you use a fuel conditioner?

How much 'crap' was in the carb? Sediment? Fuel filter?

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I have had the TBird for about a year and a half.  I drive this car at least once a week on short trips.   The car had been sitting all summer while we rebuilt he engine.  I didn't have any problems with the fuel until a few days ago.  I took it out on a country road and went through the gears hard. The fuel gauge was reading low, but we had 1/4 left.  As I slowed back down, I smelled gas.  The car kept running, but started to miss. I thought I might have pulled off a push rod, but the engine idled OK.   We made it all the way back home, but the motor wasn't happy.

The dirt in the carburetor was dark red in color. Very fine like powder.  It flushed out easily with carb cleaner. I installed new needles and seats because I thought they might be hanging up which is why we smelled gas.  I think the low fuel level caused some water or rust to get in the gas when it sloshed around on a power run. I have a fuel filter on this engine.

I'm going to keep the tank topped off from now on.  If I have any more problems, I'm going to swap out the gas tank. Is there some gas additive that will remove water from the gas?  What should I use?

I am really confused about the vacuum advance thing. I was always under the impression that vacuum advance aided the centrifugal advance, not added to it.  This means the cars are running 50 degrees advance with both working.  Makes no sense, but there you go.  I disconnected mine and the car runs fine without it.

They do work together though completely independent systems. Vacuum advance is strictly load based, while the distributor centrifugal weights and advance mechanism is strictly RPM based. Both systems are constantly adding in or taking out timing degrees depending on whatever the specific engine load and RPM combination are at any given moment, so they complement each other.

The only time the very highest levels of timing lead like that are seen is under very specific conditions as when steady cruising on level ground. Engine load is low. Engine will run a fair bit cooler with vacuum advance among other things, there's no good reason not to utilize it on a street driven car, it will only run better. The manufacturers in fact would have left it out altogether if it wasn't a good idea.

When cruising on the highway in high gear the timing is actually quite a bit retarded relative to optimum and will run hot. Fuel mixtures are relatively lean and take longer to ignite. Lots of ignition lead is beneficial under those specific conditions. The manifold vacuum is very high at this time, conveniently. It is an ingenious system. At the time it was the only way to get more timing pulled in under those conditions.

When we see things like "36 degrees mechanical timing" remember that's a number seen only at a bit higher RPM, cruising down the highway the mechanical advance timing will be a fair bit less. Lightly loaded, part throttle, lean fuel mixtures will tolerate relatively high levels of ignition advance. This also happens to improve fuel economy, maybe 15%, something like that. It is an interesting subject and widely misunderstood, plumbing a vacuum gauge into the cabin will go a long way to helping your tuning.

Tedster gave a very good explanation at the beginning of this thread about the vacuum advance. I have seen engines overheat from an inoperative vacuum advance. It seems that my slow typing posted this after the fact.