Just read on a website, that after putting on a slip-on exhaust (just bought a scorpion for my '03 ZX9) you should re-jet the carbs. If not the engine will run lean, which results in a ruined engine...... Any truth to that???
I mean I would imagine the lean condition would have to be severe enough to encounter certain symptoms. For instance my bike was running extremely rich due to a clogged air filter so I put in a hotter set of plugs and a K&N air filter. It doesn't seem to exhibit any severe symptoms except for an occasional hesitation in the upper rev range. If your bike starts backfiring, your plugs look white or melted, or your bike starts running hot you have a problem and need to take care of it soon. If not, like my set up, I think you'll be ok.......
In theory it's correct because a slip on "will" reduce the scavining (sp) effect lowering the low end TQ and leaningout the mixture but in real life that is going to be few and far between. Factory pipes are preety much the same as slip ons only they weigh 50 lbs more lol. So unless the mixture is way off from the factory (seen it a handfull of times) then you shouldn't have to worry about it. Just look for melted/discolored electrodes on the plugs and adjust accordingly.
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.
Most exhausts will actually make the bike run rich, and not lean. Jetting can make the power delivery smoother by removing holes and bumps in the curve, but is not normally necessary for just a slip-on.
Running a bike dangerously lean can result in burnt-out engine components, or even holed pistons.