Many thanks for the quick suggestions.
@WE b OLD: I guess it could still be running rich, but when I first started chasing it, I leaned out the mixture as much as I could. Prior to that the plugs were all a little sooty. I haven't thought about a stronger return spring. Will investigate that.
@Joe H and justhorsenround: Checking valve adjustment was next on my list of things to try, just haven't had time to work it. Never done it, so any advice here beyond what I've read already would be appreciated too.
@52Carl and ndkid275: I'm not familiar with the function of the heat riser valve two of you reference. I'll pull out the shop manual and do some reading, but if you know a good way to test functionality and fix if broken, please advise. Carb was rebuilt approximately a year ago and it's never had ethanol gas in it, but it's possible that the pump has a pinhole, also something to check.
Again, many thanks. This site is invaluable for those of us who have worked on vehicles for many years, but new to these specific trucks.
