Originally Posted by motolab
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.
Wow, guess I'm really confused. It appears to me there is a small, brass, removable air jet in the throat of the carb on the air cleaner side. It's drilling traces back to a junction with the fuel from the idle fuel jet in the bowl where air and the fuel mix and then their volume is controlled by the air fuel screw. So it looks like there is at least some air fuel mix before the idle air/fuel adjustment screw. Had a friends DRZ run me around in circles for half a day before I found that stopped up idle air jet. He let it set up for a while. Ran really bad. And the tiny (#10 I think) washers that are used to space the needles up are of a small enough outside diameter (same as that little plastic spacer thingy) to not effect the diaphram spring preload. They raise the needle just like moving the clip down on an adjustable needle. At least not the ones that come in those jet kits you can buy. Don't know why anyone would spend $70+ for one of those jet kits for $20 or $30 worth of jets and a couple of washers. As far as drilling the slides and cutting the springs goes, it looks like you might want to be real carefull there. Lots of variables involved there. My FZ1 came with one of those "Jet Kits" when I bought it from the PO. They had it all screwed up. Main jets in the choke/start jet spots, etc. Had to take it all apart and redo everything. When I asked the PO about it he said he took it to the dealer to have the jet kit installed. Hard to believe a real mechanic could mess up like that.