There are a few possibilities-
first, put in a brand new spark plug if you haven't already.
Check your gas tank and petcock for rust- flakes could be leaking into your carb and gumming it up. You have to take apart the carb and clean it extremely well with carb cleaner and compressed air, especially if it's been sitting for a long time. Get some new fuel line and an inline fuel filter to keep the particles out of your carb, and replace the petcock if it needs it.
Check to see if your gas cap vent is clogged- when it starts to die, open up the gas cap and see if that helps. If it does, clean out the tiny hole that vents your cap.
The aveage for most idle screw adjustments is between 2-2 1/2 turns out. Set the idle with the bike on the stand and a warm engine. It should be just high enough to keep it from stalling.
Your compression could be low- check your compression, it should be above 120psi. Lack of compression could be from an air leak, or worn piston or rings.
If you cleaned the carb with acid, did you replace all the rubber or paper gaskets that would be in there? Did you take out the jet, and blow compressed air through it, and all the little air passages?
Is your choke opening and closing properly?
Did you clean the points?
Check out Fred's guide in the articles section of this website for details on how to get your ped running in top shape.