spray some Starter Fluid around the intake manifold area (not into the carb's intake) If engine rpm increases theres a leak.
And if some Starter Fluid in the carb's intake gets the bike started instantly every time it says that the carb is still a bit dirty or clogged or is not able to deliver fuel for some reason.
The bike could very well be running out of gas.. check fuel flow from the tank to the carb .. fuel should flow in a thick steady stream from the carb's fuel line. If a restricted trickle of fuel into the carb is not fast enough to refill the bowl, the engine will die.
also remove the tank's cap .. if fuel flows better and if the engine no longer dies the cap's air vent's probably clogged. A vaccum builds in the tank if its not vented.
Check float level.. if it's low the bike will run poorly and may cough and sputter as it needs more fuel than the low level can provide.