Right then So the throttle (butterfly) controls the air into the engine. As the throttle is opened, and engine speed increases, there is a pressure differential between the throat of the carb (the bottom of the slide) and the area above the slide's diaphram. This vacuum, (that's the V in CV carbs to some people, or the Depression in CD carbs) lifts the slide, this slide lifting, pulls the needle up, the tapered needle allows more fuel to flow up the emulsion tube and into the intake stream. The amount of fuel flowing into the stream is controlled by this slide and needle combination. (This is assuming that we have moved off of fuel being metered by the idle circuit.) The amount of air is controlled by the throttle (that's the butterfly) As the throttle is opened more, the engine speed increases, as the speed increases more air is flowing through the throat of the carb, as more air moves, the slide moves up and as a result more fuel is flowing into the intake stream. Close the throttle and air is reduced, the slide drops the needle moves deeper into the emulsion tube the amount of fuel is reduced. A heavier slide will fall faster, shutting off the fuel flow quicker. Frankly this discussion is about CV carb'd bikes not FI but since you bring it up. I don't know about the VFR1200 but I do know that the VFR800FI Does not have a CV device, It has four "traditional" (has FI been around that long, we can now call it traditional?) FI throttle bodies with butterfly valves and injectors. There is a desire by manufactures to make bikes easy to ride. By smoothing out the powerband and making the relationship between throttle inputs and reactions of the bike easy to get the hang of. Some bikes (most notable is Suzuki's line of GSXR and DL's) employ a second throttle body, this is to change the intake velocity allowing for a better more complete intake charge at the smallest of throttle openings. This is also done to keep intake noise down. Many riders have reported disconnecting this device with no ill effects. But those second throttle butterflies are not a CV or CD type device. The CV or CD describes a Carburetor that has a slide that lifts by way of Vacuum (or Depression if you will), This separates from a Slide Carburetor (like what's on the R90S stock) which have slides that are lifted directly by the throttle, or Fixed Venturi Carburetor (older Harleys and many lawnmowers) that don't have a venturi that changes sizes. Not surprising since BMW made a number of different bikes for a number of different uses. I've never said that heavier slides don't effect the way an engine responds. Only that heavier slides don't make a bike more rich. For instance if you want more lower end tractable power, it's easy to see that slightly slower reacting carbs would be a bonus, smoothing out abrupt throttle inputs and the like. The drawback is on closed throttle there tends to be an increase in engine braking the faster fuel is shut to the cylinders. Where as on a sporty bike ridden on road you might want faster reaction to throttle inputs, give the bike gas it goes roll off it slows... Again, the lighter slides allow for a slower closing of the throttle and a little less compression braking. Also different countries have different tastes when it comes to engine response.