the 4 i measured were .038". ordered a bag of #10 washers from macmaster and measured with calipers, dug up 4 pretty quick that were exact.
it just richens the fuel when its on the needle circuit. more 'meat' in the throttle response down low and in the mid-range, def winds up quicker. I did the diaphragms 1st, was better, but the shims gave it a bit 'more' there and def better mid-range. had my friend David ride behind me on his new Z900 and said it didn't 'smell' any richer at all that he could tell, and hes super sensitive to it. did a bunch of mid-range runs and really good. will repeat this on the red 2000. they both ran good, but this is a def improvement.