Hi ddangus,
It sounds to me like the jets in the carburettor have varnished. Unbolt the carburettor from the engine ( including removing the cables, and take photos before you undo anything so you know how it goes back together ) and remove the screws holding the carburettor together. Gently lift the top off, and look inside.
You'll be able to remove the gunge at the bottom of the carburettor bowl, and if you get a can of carb and jet cleaner from your local motor factor, spray everything inside a couple of times, letting the cleaner evaporate off between each time. DO NOT take wire or small drill bits to the brass jets, you'll mess up the carburation if you do.
Put the two halves back together, refit the screws, bolt it back on to the engine, reattach the cables and that should be it!
Dan.