Why would I want to change my jetting?

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From the factory, carbed bikes tend to be set up in the following way to make it through the EPA hoops. See the other parts of the Jetting section for an explanation of terms.

  • Jetting is about right for the pilot at sea level and is too rich at altitude, hence the smaller jet (usually a #35 for the Ninja above roughly 4000 feet).
  • Jetting is lean at sea level for the mid-range, which is why you put shims under the needles to raise them up.
  • Jetting is rich on the top end for sea level, and worse at altitude. This is to keep from burning up the pistons at high rpm. This can be corrected by adding a smaller main jet.

You will need to rejet if you make any changes to your exhaust or intake. If you add a new pipe and/or filters, your carbs will run lean, so you'll need to richen things up. A carb kit will do this by giving you larger main jets and adjustable jet needles. The jet needles also have a slightly different taper than stock, which will smooth out throttle response through the whole rpm range and add quite a bit of power in the 3,000-6,000 rpm range.

If you don't rejet and run your bike lean, it will a) not run without the choke being partially on, and b) may burn (melt) your exhaust valves if you run it hard lean.

If you are going to spend the $400+ to add the pipe and intakes, go ahead and spend the money for the jets and do it right.

One thing to consider

Jetting is really more art than science. Please read ALL of the other articles in this section before you start changing your intake and exhaust systems. It takes quite a bit of trial and error.

Also, please heed this note from one of our resident mechanics:

  • Jetting is 100% bike specific.
Each bike will need something slightly different, even if they're in the exact same location. Location REALLY means major differences in what works best.
The best solution is taking the bike to a professional with a Dyno and an EGA (electronic gas analyzer, sometimes called a "Sniffer" or "4 gas analyzer") to see what kind of exhaust the bike is putting out at different rpm while under appropriate load. The next best solution is to dive head and heels into the massive amount of jetting information that's been published. There are dozens of different books dedicated to the subject. There's absolutely no way to impart that knowledge in the space of the FAQ.