<VV> Spyder backfire problem

[Doug Mackintosh]

 Hello, Corvair friends!
I have an intermittent engine problem that has me stumped. I hope your collective wisdom can help me!
It is an intermittent problem with my daily driver 1964 Spyder. It starts fine from cold, regardless of ambient temperature (recently 34 to 75 deg F). Up until last week it also ran fine and could be relied on to get me there and back. The only exception was a very consistent rough running / bucking / occasional backfiring the first few minutes of driving for the first time after a rain, particularly under moderate to heavy acceleration; after a few minutes it would smooth out and run normally, and when restarted later there would be no problems.
The problem I need help with started about a week ago. I started the car (it was about 34 deg F), backed out of my driveway, and drove about 5 minutes to my destination. Shortly after I headed down the road it started to putt-putt, blubber and backfire and ran rough enough that I was only able to go about 35 MPH on the 45 MPH highway. By the end of my 5 minute trip it had smoothed out and drove normally, and when I restarted it a few hours later and drove home there were no problems. This was very much like the behavior I am used to after a recent rain, but in this case it was bone dry and had not rained for some days.
The next day, it behaved the same way, except I only made it about a half block down my street before it stalled and would not restart. An hour later, I was able to restart it and drive it back into my driveway.
The next day (about 50 deg F) it started and ran fine (just a little roughness at first, but no backfiring, and was running well within a couple of blocks from home). I drove it 5 minutes to my destination. After several hours it started and ran fine as I drove it back home. Later that day I drove it for about 15 minutes mostly on the highway and it ran fine, including high RPM acceleration and under full boost.
The next morning it started, then ran rough, bumbled, backfired then died less than half a block from my driveway, and would not restart. 2 hours later I was able to start it and drive it back into my driveway.
Here is what I have checked so far:- Cleaned and inspected the distributor cap for cracks or tracking- Twisted the rotor to verify the mechanical advance is working- Verified the correct firing order- Checked that points are opening to a reasonable gap- Checked dwell angle (32.5 deg at 1800 RPM, 30 deg at 4000 RPM)- Removed air filter cover and verified air filter and carb look OK- Verified the choke setting has not moved and the cover is tight
I then started the car and did the following tests:- Let it idle a few minutes (CHT gauge starting to register) then revved it quickly, which resulted in backfire and black smoke out the exhaust; intermittent backfires and black smoke clouds continued as I held the throttle open to maintain high RPM. Repeated quick revving would sometimes be clear, and sometimes produce black smoke.- Continued to let it idle for several minutes and it eventually stalled (which it sometimes does after extended idling)- Restarted it immediately (started on 2nd crank) and it ran OK, revved fine and had no backfires or black smoke- Let it idle a bit, then popped the throttle open and got some black smoke (no backfire), but then it cleared up and revved fine on repeated quick throttle pops- Let it idle a bit, then revved, then idled a bit, revved again - no problems- About 6 hours later (57 deg F) I started the car (1st try), then revved (revved fine). Let idle 5 min, revved again (OK). After another 20 minutes idling, popped the throttle and it stalled. Restarted (took a little cranking with throttle held open), then it revved and idled fine (no backfires, no black smoke clouds)
As you can see it is not consistent. I have not figured out how to "make it do it", but I know there is a fair chance it will do it after running just a minute or so. 
Anyone experienced this problem and know what fixed it? Or have a suggestion to help me diagnose the culprit? Any help will be greatly appreciated!
-- Doug Mackintosh Corsa member since 1996 Corsa/NC member since 1996, Virtual Vairs member Corvair owner 1969-1971 and 1996-on