This info is from this website which contains a lot of good info
http://www.motorcycleproject.com/index.html Motorcycling's Sacred Cows: #8, Pipe Jetting
One of our industry's most insidious misconceptions, and one that is promoted constantly by advertisements, as well as by discussions over accessories counters and Internet threads worldwide, probably tens of thousands of times each day, is the idea that installing an aftermarket exhaust system necessarily requires the changing of carburetor jetting. It just isn't so. Not every aftermarket exhaust affects carburetion. A number of complex physical things are going on in an exhaust system, most of which we'll touch on near the end of this article. First however, let's start by defining the three kinds of aftermarket exhausts.
Different Exhaust Types
First are those exhausts made solely for looks. No one claims anything but style regarding these systems. Think of the cruiser crowd and "shotgun" pipes. Second, there are those exhaust systems about which are implied (if not actually claimed) performance advantages. This is the largest category. The simple fact is, even these "implied performers" are, as with most aftermarket exhausts, like the first group marketed for asthetics more than for performance. Terry Vance (of Vance and Hines) and Dennis Manning (Bub Enterprises) have both publicly acknowledged this, and it's far from a secret within the powersports industry that style and sound are where it's at with these systems, not performance. Most of what little performance is possible with these exhausts comes from a very different source than from the exhaust itself. We'll explore that in detail in just a bit. Finally, there is the third category, the actual tuned exhaust system, a very rare bird indeed and one usually found only in limited, hand-made quantities. We'll deal with that category separately. We'll also take a quick look at collector exhausts, probably the most popular design of all.
We already know what the "for looks only" exhaust is for, so let's examine the latter two exhaust types, the implied performer and the tuned exhaust, one at a time. The implied performer gets any performance it produces in some surprising ways. First, part of the increased vehicle performance, if any, comes from a significant weight savings. Though not measurable on a dynamometer, any weight reduction on such a light vehicle as a motorcycle or ATV offers impressive results. A 450 lb motorcycle for example that produces 100 hp has a power-to-weight ratio of 4.5:1. That is, the vehicle weighs 4.5 lbs relative to each horsepower. Installing an exhaust that is 10 lbs lighter changes this ratio to 4.4:1, which affects performance (mainly acceleration) in a way equal to increasing the engine's power by over 2 hp. There isn't actually 2 hp more, it's just that the vehicle performs as if there were, due to the comparable weight saving. Nearly every aftermarket exhaust offers this potential increase in performance. Even the first, for looks only, variety.
What Pipe Rejetting is Really All About
But there is another, more interesting source of potential increased performance that the implied performer exhaust offers. This requires some explanation. Carbureted powersports vehicle manufacturers do not jet their carburetors perfectly, for two reasons. First, there are emissions considerations, mostly affecting the idle and midrange carburetor circuits. Consequently these circuits are jetted leaner than normal, and should be richened slightly for better performance. However, this rarely requires more than an adjustment to the idle mixture screw, and in some cases an adjusted slide needle height. Second, manufacturers ship their product to many different places across the globe. They purposely jet too rich on the main jet, the carburetor circuit that presents the most liability in terms of engine overheating. Therefore, all road-going powersports vehicles are jetted "fat" on the main jet. Contrary to what many people believe, but true nonetheless. Manufacturers do this because they cannot jet individually for each market, so they simply err on the rich side rather than the lean side, because the latter would present more problems. The fact is, engines are much more forgiving of a 3% rich condition than an equal lean condition, and just as importantly, few customers will notice a rich main jet, while nearly all will complain about a lean one. Make sense? (Incidentally, fuel injection has brought increased peformance to our market for several reasons, not the least of which is the fact that manufacturers no longer need to provide this rich margin. These fully electronic systems adjust themselves to the destination conditions automatically.)
A certain amount of low speed driveability, and even some high speed engine performance, is sacrificed by the manufacturer's stock carburetor settings. In the case of the idle to midrange leanness, careful recalibration can wake up the engine's throttle response and acceleration. As for the main jet, its richness reduces to a very small degree potential full-throttle performance, performance that is waiting to be tapped by the individual willing to take the time to carefully dial in the carburetion to the "nth" degree. Performance shops all over the world "dyno tune" to do this very thing. However, and the main point here, is that this dyno tuning is in many cases being done irrespective of whether an aftermarket exhaust is installed or not. That is, a considerable amount of dyno-based carb work is done on vehicles having stock exhausts. For various box stock racing classes, for example. The fact is, much of the performance increase resulting from replacing the exhaust and the subsequent carburetor rejetting is due simply to the rejetting, not the exhaust. The same or nearly the same result can be achieved by merely jetting out the lean and rich settings left by the manufacturer, without ever intalling the exhaust. In other words, you can rejet without the aftermarket exhaust and get virtually the same benefit as you would by jetting "to" the exhaust. In many cases, exactly the same benefit.
Remember two things about jetting. First, unless you're converting to alcohol or some other exotic fuel, never replace the idle jet. (At least on inline fours -- big bore singles are another matter.) When tweaking your carburetor, simply adjust the idle mixture screw for slightly more output. This is preferably done using an exhaust gas analyzer to dial in the mixture by the percent of CO (carbon monoxide) emitted by the exhaust (about 3 percent on 1984 and earlier models, half that on subsequent models). However, if skilled you can do it without one. Second, as far as the main jet is concerned, it is obvious that during box stock dyno tuning the main jet is decreased in size, not increased. The goal remember is to remove the rich margin. This doesn't fit what most of us are doing, and of course, adding an exhaust changes things, theoretically at least. With the increased flow capability of some aftermarket exhausts (many are actually the same or more restrictive than stock), some richness may be required. However, because that need is offset by the leanness that the engine actually wants, the net jetting result of intalling most aftermarket exhausts is the stock main jet. That's right, the run-of-the-mill "implied performance" type exhaust frequently requires no jetting change at all! In many cases, the stock jet will do just fine.
Aftermarket Air Filters
At least, that is, if no changes are made to the air filter. The air filter is what really makes rejetting necessary, not the exhaust. In fact the air filter affects the carburetor fully five times as much as does any exhaust system, even a good one. The ever-popular K&N brand air filter (and its many clones) is extremely freer flowing than any stock unit. So if you install one of these, count on going up two to three sizes on the main jet.
"Placebo Jetting"
Interestingly, some customers just won't accept that their vehicle doesn't need rejetting. They insist that we (the shop) do it, even though they know it isn't needed. The misinformation machine that is so strong in our industry has persuaded them, and they're worried that if we don't change the jetting for them, their engine will burn up or be damaged in some way. Or more likely, they cannot reconcile in their minds the expense of the aftermarket exhaust on the one hand, and little to show for it in terms of added power in the other hand (which even the beginner knows accompanies a legitimate jetting change). This is an odd situation, and has resulted in a huge number of rejetting scenarios that just are not necessary. Call them "placebo" rejets, if you like. Just like the traditional sugar pill the doctors used to give out to hypochondriacs, folks who go around thinking they're sick all the time. And just as in the medical profession, in which it is documented that takers of those pills do get better, our customers do likewise. They go off with their newly-rejetted motorcycle or ATV very happily knowing that they "did the right thing," completely oblivious to the fact that it was unnecessary.