Originally Posted by visovm
Or would it be better to do similar mods at the BST40 (spacer under needle, adjust idle screw, slide drilled) to get better throttle response?
As shimming the needle clip preloads the slide spring beyond standard, it results in lowering the slide rather than raising the needle (except when the slide is against the stops). I would not shim the needle clip unless the goal is actually to lower the slide for a given intersection of throttle angle and rpm. Note that the additional preload from shimming the needle will also make the slide come off the stop at a later point in terms of rpm and throttle position (i.e. more velocity will be required to get it to come up off of the stop).
BST carbs have slide guide, emulsion tube, slide and jet needle wear issues that are exacerbated by the enlargement of slide lift hole area. I'm also not convinced that there is any performance benefit to be had from slide drilling. In fact, dyno testing I did on a BST40 equipped KTM made made the following gains from going from the double lift hole KTM slide to a single lift-hole DR650 slide:
WOT from 4K up, with a max gain of 1.3 HP at 7K rpm.
Across the entire range at 1/4 opening, with a max gain in torque of 1.4 ft-lbs at 3K rpm.
Across the board at 1/8 opening, with a max gain in torque of 2.6 ft-lbs at 2K rpm, along with the usable rev range extended from 3.4K to 5K rpm.
Across the board at 1/16 opening, with a max gain in torque of 1.2 ft-lbs at 2.5K rpm, along with the usable rev range extended from 2.5K to 3K rpm.