And a big thanx to Bentwrench with his insight to the 650 carb cleaning guide, the three hole mystery is solved!
In the forward end of the carburetor bore, you'll find a number of holes (see Appendix,
item 11). In front of the throttle plate (butterfly), you'll find either one or two large holes
and one small hole. One of the large holes draws mixture from the enricher, and the
other will either be blind or open into the synchronizer attachment port if the carb is so
equipped. Do not apply solvent without removing the choke plunger (see enricher
section). The small hole draws mixture from the fuel screw. Blow aerosol carb cleaner
into the hole until it flows freely from the fuel screw area. Then block the hole with a rag,
insert the tube from a can of carb cleaner in the fuel screw seat area, sealing with a rag,
and blow cleaner in until it flows freely from the hole in the carb body that sits above the
pilot jet (BS38) or the pilot jet casting (BS34). Follow with compressed air.
Behind the throttle plate are more small holes, 2 in BS38's, a cluster of 3 in BS34's (see
Appendix, item 11). In early BS38 carbs these are fed by the bypass pipe, a brass tube
between the slide and the throttle plate in the middle of the bore. Find its source in the
carburetor body and shoot with carb cleaner and air until clear.
In later BS38 carbs and all BS34's, the bypass nozzles are fed from the fuel screw
passage, and no tube is present. Seal the tube on a can of carb cleaner in the fuel
screw hole with a rag or drilled plug, cover the small nozzle in front of the throttle plate
with another rag and hold it in place with your thumb, hold the throttle plate open, and
spray until cleaner emerges from the nozzles. Follow up with compressed air.
