Strange running behavior/erratic timing

[bhobbs]

So, it turns out, that it's the simplest of all problems. After checking the wiring thoroughly, the voltage drop across everything in the system, spark-plug color/condition(fluffy black deposits on nearly new plugs), and confirming that there weren't leaks around the carb or manifold, and that I'd successfully found TDC on cyl 1 on the compression stroke the first time through, and I had tolerable compression for such an old engine, I decided to really carefully inspect my hoses. I'd replaced several already that were obviously cracked. I discovered that the one running from the driver's side rocker cover(with the valve in it, which I'd confirmed was working), had a lengthwise crack in it that I'd never noticed before. I think that I'd managed to break it even further when I reassembled it the last time, since it looked freshly split all the way up to the carb.

Replace tube, and voila, it runs just a little bit better than it did before I fried the wiring a few years ago. Timing is reasonable(I can make it better, but for right now, it's close enough), and it actually idles pretty well.

What's the function of that tube wrt a typical 4bbl carburetor operation? It's fascinating that I had such an odd behavior for something so simple. I'm guessing that it was generating sufficient vacuum during acceleration that it could suck in fuel and run, but at idle/constant rpm, it was using a different set of jets that had no vacuum. That's just postulation though.

Thanks for all the suggestions guys. It gave me enough ideas to go through and at least be more methodical about getting it going. Now I can upgrade/replace individual pieces and know that it was working tolerably prior to replacement, and the city will get off my back about having a non-working truck in my driveway(despite my custom-fit cover and lack of HOA for my neighborhood).

Barry