"Drilling the slide" means enlarging the vacuum port, the vertical "hole" in the slide connecting the venturi with the volume above the diaphragm. Take a look at, "22-cent mod" for more details. The purpose of drilling the slide (enlarging vacuum port) is: Promoting faster throttle response.
7/64" appears appropriate for KLR650s; 1/8", though common with Harleys, may cause some slide flutter on KLRs.
Back to your problem: Maybe nothing's wrong with the hole in the slide; the problem may be from a fuel-lean condition at mid-throttle. SHIMMING your needle, I'd recommend ONE # 4 washer (some say, "TWO") under your needle, this procedure will fuel-richen mid-throttle mixture somewhat, perhaps curing the problem ("22-cent mod" research discusses needle shimming).
Last edited by Damocles; 09-07-2016 at 07:20 PM.