Well, the carb woulda been to easy, that's my kinda luck...
I would go ahead and change the pickup coil. P/u coils very rarely go bad, But, they do go bad. I have had it happen to me twice. However, both times the symptoms where extremely rich conditions, followed closely by plug fouling. I'm still inclined to believe it's a timing issue of some sort. The blow back thru the carb, depending on the amount, seems to indicate a timing issue, or a valve adjustment issue. Your discription of how you adjusted the valves, sounds exactly like how I do it. It's a pain, but it might be worth rechecking. I always wait a few minutes between turning the engine over to do the next set of valves, to let the lifters bleed down. Just a little thing you have to do with Harley's to keep from bending pushrods, so it can't hurt on a chevy.....
FYI...Just saying, that kinda attitude won't getcha much help...I'm just trying to help ya out....
