Thanks Larry and Dan. Dan, now I know why you switched to Webers! I'm going to do a complete tear-down (again) tonight on the rear carb, which is the one I re-cleaned right before all of this happened. I have a feeling that it's something really dumb and obvious.
Dan, as far as the carb balancing goes, I bought a single balance meter and rigged a plastic margarine tub to fit over the carb. JC Whitney also carries a carb balancer for about $20-30 and they claim really does the trick - any experience with these? Any other tips so that I can rule-out mis-tuned carbs?
Larry, I've been thinking of the enrichening system in this way (since nothing obvious from my re-build memory comes to mind): the system would require a vacuum passage and a fuel passage. The best vacuum source and fuel mixture outlet I would think would be at the throttle body, but I just can't recall a fuel source from the bowl, or even a fuel/air mixture from primary or idle jets, that's connected to the throttle body. There's a passage for the idle fuel cut-off solenoid, but nothing else is jumping out at me. Looks like it's time to go back and map all of the passages because this is one area where Hayne's just doesn't cut it!