While I'm posting resolutions to old problems... here's a theory to once and for all (famous last words) put a stake in the heart of my vacuum/carb stumble problems. And this will be
important to folks with 1963 or 1964 260's and original aluminum carb spacers that have rear port PCV and maybe not anybody else.
If you look at the previous page you can see photos of my carb spacer and the pooling gas/oil underneath in the intake crevices. If somebody can tell me why those crevices under the carb are there on the 260 intake, that's an automatic 5 point bonus.
Anyway...
My issue was always when I slowed down quickly from high speeds (like at a stop sign coming down from 40 or higher) the car would stumble and almost die, idle would drop to near nothing like you had turned off the engine... then it would recover and run fine. I kept checking the brake booster thinking I had a vacuum leak that correlated braking with power loss. I'd disconnect the booster, plug the vacuum, etc. In fact further up in the thread, you see where my daughter found a leak in the booster T and we thought that solved the problem, but it didn't last.
Here's a 64 aluminum spacer
Here's the oil/gas that pools underneath
And here's a standard store bought paper gasket
Now what you can't see and will just have to take my word for cause I'm not gonna tear this all apart just to take a photo is that...
If you laid that paper gasket on the bottom of the aluminum spacer you'd see that the opening in the gasket is almost exactly the same size as the OUTER DIAMETER of the intake opening in the bottom of the spacer. In fact, it's so close that if you softened the gasket just a little bit (say with hot gasoline/crankcase ventilation or the like) you'd be able to slide the paper gasket right past the side wall of that intake spacer effectively opening up a hole between the intake and those crevices cast into the spacer and the intake manifold.
So when the gasket is new, everything is fine, but as it wears, and slips past the outer side wall of the spacer it create a kind of variable vacuum leak. When you're driving the intake draft sucks all the air out of those gaps in the spacer and the intake. When you hit the bakes at high speed, intake velocity drops and those previously negative air passages suck air/fuel mixture back into the crevices, etc. like a reverse power valve. This leans out the air/fuel mixture (robs the intake of fuel) and causes the stumble. Once they reach equilibrium, the fuel mixture recovers and things are fine... Of course when you take off, the vacuum can suck that extra fuel back into the intake which should be like a bonus accelerator pump, except it's raw oily fuel in the throttle rather than above where it can be atomized.
You would think this would have been resolved when I made my own gaskets, except that every time I made my own, I used the paper gasket as a template, which replicated the problem. The last (and hopefully final) time I made one, I used the aluminum spacer and took care to make the gasket opening match the INNER DIAMETER of the spacer. I have not had any stumble problems since.
I assume that the standard Autolite 2100 carb gasket is made to fit all the later model carbs which probably have slightly larger throttle openings for 289 and later, which probably have slightly larger intake openings. Perhaps the 65 and later PCV spacer is different as well?
Ok, so that's the theory. Let's hear any rebuttal. Be gentle, I only had one physics class and that was more than 30 years ago.