Needle and seat sizes?????

Does anyone know the correct needle seat size (Also known as the float/needle valve) for a 88 YZ250 38mm roundslide Mikuni? I just ordered one that they say fits this carb, it is a 1.5...but the one in my carb is a 3.5. Which is the correct one and can someone explain the difference/performance aspect of the smaller/larger opening? With the 1.5 installed, it seems to be starving for fuel. While occasionally the 3.5 overdumps fuel causing flooding.

Yeah, I tried a 2.5 and it was better than the 1.5 but was still lacking. I put the 3.5 back in after polishing the seat, reset the float and now it's getting all it seems to need.