Thank you i will have a look at whot you have said.& i will post the out come thanks againHi Hursty, I am not familiar with the xr 125 carb, but if the plug black is sooty, then obviously too much (rich) fuel to air ratio. The first obvious check is the cold start choke mechanism, Is it stuck on? If this has a plunger valve to do this it may be stuck open, or leaking past the bottom of it. Other than that, check the needle and jet for damage or wear. If the needle is adjustable (notches and a tiny circlip ) try dropping the needle down 1 notch at a time. If you are not sure about all this, seek help from a motorcycle mechanic!!
If the plug is oily black (accompanied by blue smoke when running) then you have a more serious engine problem,likely to be a Piston/rings problem, requiring re-bore and new piston.