DM engine playing silly *

Try loosening the fuel cap, don't run it with it removed, just loosen it and let the engine run for a bit. Sometimes they don't breath correctly if sat around for a while. That's the most simple thing to try first.
The next step is to remove the carb/ tank assembly and take the tank off. On the side of the assembly is a rectangular plate held on by four screws. Behind that is a diaphragm that becomes quite "baggy" after a time. While the plate is off, you will see two holes. One of these goes straight down the fuel pick up pipe. A good blow down that with an air line, will remove any "stuff" that is on the pick up filter on the end of the pipe.
A new diaphragm should cost you no more than £1. I would ALWAYS replace the gasket between the carb and the tank with a new one. For safety's sake you don't want spilt fuel getting onto that nice hot exhaust, do you?