You haven't said what model Walbro carburetor it is. If it is a WA, the nominal starting point for tuning is with both jets open one and a quarter turns. From there, set a sensible idle speed with the throttle stop screw. Tune the idle jet by turning it clockwise (leaner) which will make it speed up, then reduce the idle speed setting, then turn the mixture clockwise again and slow down the idle speed with the stop screw. Keep repeating until you find that any more clockwise movement of the mixture screw causes it to slow down instead of speed up. Go just a smidgen richer from there, and you're all set. For the high speed jet, run it flat out and listen to the exhaust. It will be four-stroking, so it will sound rough. Turn the high speed screw clockwise until it speeds up and smooths out. Then turn it anticlockwise just a little until it is on the verge of four stroking. That should be it.
