Look for a Mikuni, super BN owners manual. I don’t think anything in that valve block moves.
Also look at Mikidymac’s carburetor, rebuilding tutorial. It can be found as a sticky in the front of the two-stroke section.
Probably not a problem with the flow. I would think the water will slobber out of a 3/8 hole. If you stick with the quarter inch you will have a stream.