125 don't use Mikuni's do they? Should have a Dellorto carb, the one with the massive idle screw?
If it is Dellorto, check your floats aren't sticking on the poles. The tiniest amount of dirt in between the pole and the float will cause it to stick and thus not shut the valve off. I'd also recommend removing your existing float valve, hold it up to the light and see if there is any marks/imperfections etc. The smallest of marks here will allow fuel to bypass the valve.
Before you buy anything, strip everything and examine very closely. Get a needle or very fine flat head screwdriver and clean everything!
Dan
my beta is using the mikuni carb.
thanks for the advice
jack