I'd imagine that you have all those sneaky hoses for vacuum this and vacuum that and it really could be one of them misbehaving. Just for the heck of it, check your fuel filter. You should have two of them on that rig, one before the pump and one after the pump that splits and sends fuel to the carb and the other back to the tank.... Make sure that both are clear. Easy test is to blow into the inlet side and make sure that air will pass through them with little effort. On the one with two outlets, check for both sides. If you are running too rich, could be because your return line is clogged and too much fuel is getting forced into the carb. As for the starting up okay, idle okay, then when you try to run it and it starts running rough, this is a prime reason for that to happen. There is enough fuel getting by to idle okay, then when you need more gas, it doesn't get it. I was reading an article yesterday about another problem, the idle ports inside this carb get clogged sometimes, and dribble fuel at the wrong time into the throat of the carb. I can look that up if you want to persue that. I'm still leaning to the vacuum issue, that can be a result of so many hoses or the components that they go to.... Never really know without being there though.