So, you just plunked down $350 on a slip-on or full system and are wondering if you will need to rejet eh? Well, [again] I'm no mechanic, but here's what CycleWorld's mechanics say on the subject:
November CycleWorld, Page 142 has a guy that wants to change the exhaust on his Katana and wants to know if he will have to rejet. The response is as follows:
"With rare exception, installing an aftermarket exhaust requires jetting changes of one kind or another, especially if you hope to realize the full potential of the new system. The stock exhaust systems on modern high-performance motorcycles aren't designed just to reduce noise; they're an integral, elaborately engineered part of the engine's overall tuning package. Similarly, the stock jetting is intended to maintain a delicate balance between optimum performance and the manufacturers' need to comply with federal exahust-emissions regulations. When you replace the stock exhaust with something that is different in any way, you alter the tuning of the exhaust system. It doesn't matter if you go from 4-into-2 to a 4-into-1 or a 4-into-2-into-1 or whatever; you're changing the way in which the exhaust gets from the combustion chambers to the atmosphere. And any change that affects the way in which the burned fuel mixture gets out of the engine also affects the way in which the new mixture gets into the engine. As a result, some jetting adjustments usually are necessary to compensate for a change in intake tuning brought about by a change in the exhaust system's tuning.
If you decide to make this modification and rejet the carbs, the best and easiest way to do the jetting is on a rear-wheel dynamometer. These days, many bike shops across the country have Dynojet dynos, which are excellent tools for tuning work such as carb jetting. It might cost a few bucks to have a shop do this for you, but it's the best option for someone who isn't an experienced mechanic. And rejetting is the only way you'll get maximum benefit from your investment in a new exhaust system."
(Just saved you $3.50 off the cover price of $3.50)