<VV> Carburetor Mystery

[Brian]

Over the years while working on Corvairs I've encountered a lot of mysterious things.  But in every case the mystery was ultimately solved.  Solved either by extensive research into shop manuals, input from other Corvair guys, or just plain figuring it out by myself over time.  However, now I've got a mystery  that may be unsolvable.  I'm sure hoping it's not but I am baffled.
Here's the setup.  I have two engines I use in my track car.  The first is a176 cubic inch one using 3 9/16 pistons.  The reciprocating assembly is balanced and it has hot cam.  The second one is  3.1 liter using Volkswagen Porsche pistons and cylinders.  Like the first engine, everything is balanced and it has a cam.  Each engine has its own ignition system.  The only common item to both engines are the modified Rochester carburetors.  The Venturi has been poured out to an inch and a half and the Venturi cluster arms extended to gain the correct distance from the Venturi walls.  These carburetors have worked for many years on both engines.
Last summer something mysterious happened.  Neither engine would easily start and when they were finally running they would only run for a couple of seconds and then die as if they were out of gas.  This was regardless of the throttle position.  After the engines would die, operating the throttle shaft would result in for very strong accelerator pump shots so it was obvious there was gas in the carburetors.  Nothing I did would correct the problem.  Hours and hours went by with no success.
Recently I tried again.  This time using the run stand my club built.  This made it much easier.  I didn't have to swap engines in and out of the track car.  Using the1 76 in.³ engine, I first installed four stock carburetors.  The engine started and ran perfectly responding correctly the throttle position.  I then installed the modified carburetors and got the same results as last year.  Hard starting and only running for a few seconds before dying as if out ofa gas.  I took the carburetors off and made sure every passage was clear and the fuel bowls were full.  I even tried different main Jets sizes although I had very little hope that that would affect anything.
After checking each one of the carburetors I put them back on the engine and experienced the same old results.  This is a mystery that I cannot fathom.  Why suddenly after years of working perfectly did these carburetors now refuse to work?  I should mention that while I had each carburetor apart I filled the bowl with water and using my air supply blew air down through the Venturi with the throttle plate open.  In all four cases I did get a fine mist of water on my hand which I was holding on to the throttle plate.
What a mystery !  Why did these carburetors suddenly not work?  Why when all the passages a clear and I can get water to spray on my hand as if the carburetors were on the engine why do they not work when installed  on the engine?  Absolutely nothing was changed on the engine between the runs made with stock carburetors and the attempts to use the modified carburetors.
To say I am baffled is an understatement.  Does anyone have any ideas?