Originally Posted by Brad_SwiftFur
GM's are notorious for the plastic fuel caps not venting after some years.
That would make my day if that's all it was. I will check that out, I also believe that this tank is not the stock tank and the cap is not vented, so I will explore that and see what it looks like.
It reminds me of all the caps on the diesel fire trucks/ambulances I used to drive, simple steel tank with hard steel cap. Maybe my post count is high enough I can post some pictures too.
I also noted that the new fuel line/filter that the guy who changed the carb installed lays right on the heater hose so perhaps vapor lock is still an issue.