Lets talk some carb theory here for a sec. CV carbs....constant velocity carbs....the carbs that are on the Vmax, arent like the carbs on muscle cars and the like. Usually when you put headers or a better exhaust on a car, you MUST richen up the carb......IE go up a couple of jet sizes in the carb...etc.
With CV carbs, when you put on a free flowing exhaust.....a 4-1 and the like, Your making the carb TOO rich. Look at it this way, with more air flowing out of the bike, you will have more air flowing INTO the bike, and with that, the extra air will PULL more fuel into the carb, making you rich EVERYWHERE.
For generalities sake, you are dealing with 3 different fuel circuits in the carbs. The pilot screws.....A/F screws...cover from 0-4000 rpm (roughly) and 0 to 1/8th....maybe 1/4 throttle. From 4000 rpm to roughly 7000 rpm, this area is governed (the mid range) by the needles, and 7000 and above its the main jets (which also control fuel EVERYWHERE), so if you go leaner on the main jets, it will make you leaner EVERYWHERE.
So, when you mention a free flowing exhaust..is it a 4-1..or atleast a 4-2 (a full header and baffle)? The generic recipe is dropping the main jets from the stock 152.5's to 147.5's and putting a shim under the needle (to pull the needle open sooner.....and thus allow more fuel. Remember, by dropping 2 main jet sizes you have leaned it out everywhere, so you have to richen up the midrange...or else you will be too lean, using the stock needles.)
If you just have slip-ons (which arent considered free flowing....because the main restrictive area in the exhaust are the stock head pipes), typically you drop 1 main jet size...because the stock Vmax jetting is just a bit rich from the factory, which even a stock Vmax will typically benefit from as well.
Make sense?
