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I still can't believe the '03 ZX-9's are carbureted. Re-jetting isn't quite necessary when just installing a slip on. However, the bikes are set up a tad on the lean side from the factory just to combat emissions, so re-jetting is always a good idea. Now if you have plans on putting a KN or BMC or any gauze air filter in, rejetting will be required.
Think of it this way, your engine is really just one large air compressor. The better it flows the exhaust gases out, the more air it can flow in. When your engine is flowing more air, and the fuel/air ratio is not changed by rejetting, you create a LEANER condition. Leaner is a greater air to fuel ratio. It would be a good idea to buy a jet kit and have the bike rejetted regardless. It will definately open up a bit more power and cause the motorcycle to run a bit cooler.
Think of it this way, your engine is really just one large air compressor. The better it flows the exhaust gases out, the more air it can flow in. When your engine is flowing more air, and the fuel/air ratio is not changed by rejetting, you create a LEANER condition. Leaner is a greater air to fuel ratio. It would be a good idea to buy a jet kit and have the bike rejetted regardless. It will definately open up a bit more power and cause the motorcycle to run a bit cooler.
