Fuel starvation. Empty the tank - or very carefully turn the tank on it´s side away from the petrol tap - once you have removed the tank and pipes. DO NOT DO THIS WITH A FULL TANK OF FUEL! Ideally empty the tank. Remove the bolts holding the petrol tap on and extract the tap and filter. If gunked up clean it thoroughly before replacing it with a new or cleaned gasket. If this fails then check the carburettor float heights - easily adjusted by removing the float bowls. If this fails check that the fuel tank cap/breather is clear too! Phew - good luck!