The first place you should look is the spark plugs. Pop them out and check the colour. If they're new then they should still be rather shiney/clean. If they are white, then it's running far too lean, and therefore you may have a fuel line blockage or a very lean idle mixture. If they are black and have a powdery texture, then it's running too rich, so there is too much fuel entering the engine per unit of air, and this will soot up the spark plugs.
If they are black, (and I'm on about the electrode tip and base plate) then get a brass wire brush and give them a good clean. Stick them back in and see how she runs.
Do you have any problem while riding? Any bogging down, stuttering, inability to rev above "x" revs?
When it was running well, what sort of idle speed is it set at?
What aftermarket modifications have been made to the fuel/exhaust system. Different air filters, exhausts, holes in the airbox, etc will all have a big impact.
All the best,
Bullet5.