Almost certainly a fuel flow problem. There is a small screen in the T fitting between the carbs where the fuel line attaches and another screen in the tank attached to the control valve. Pull the hose to get at the one in the T fitting, pull the control valve (petcock) to get to the one in the tank.
Also, there is a possibility that the tank vent/overflow hose is pinched or the rollover valve in that hose is partly closed. To test for this, with no more than 1/2 tank of gas, ride untill it acts up and loosen the filler cap. If the problem goes away, the hose or valve are at fault.
