OS venturi 20-25 FP-S, stock # 22312000, is suitable for FSR25 with OS nva 20-25 FP-S, stock # 22311000,.
The venturi is about 6.5-6.6mm ID and the nva is about 3.5mm OD.
I prefer Nelson nva, about 4.0mmOD, so I use 6.8-6.9mm venturi on my FSR25 for Doodlebug440.
It runs very well.
Aki