Check the vacuum line that connects to the petcock valve. It's possible that there is not enough vacuum to open the valve and provide fuel flow. The petcock valve is a rubber diaphragm that opens a valve in the presence of suction. You can test the valve by sucking on the tube and seeing if fuel flows from the other tube. If not you may have a bad valve. If the valve works then you may have a clogged float valve. Sometime debris clogs the float valve and the only fuel you have to run on is what's in the bowl. Once the bowl is empty the engine dies and the bowl is refilling too slowly to sustain the engine.

Check the connections and wires under the front headlight few are probably loose or too tight since they don't work all of the time.

Hi i have a superbyke m-ped 50cc qmb139 an ive had the same problem ive done everthing to try fix includeing changeing carb then today a freind suggested checking my valves my exsaust valve was too tight so i put it at the right gap an now it runs great doing 40mph no probs on flat so try your valve gaps

I need a diagrame of the placement of the fuel lines on the pecock can anyone help