How do I install a jet kit?

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The instructions that come with the package should be easy to understand, although they assume you have some technical ability; if you have a service manual it will clarify a few things.

There is a bit of an art to jetting. This will become clear after putting the carbs back in after the first rejet session. You will most likely have to pull the carbs out several more times before getting results you really like.

There are three main steps to this process:

  • Replace the stock needles with new clip-retained needles.
  • Replace the stock jets with the recommended replacement (this may not be the jet you end up keeping).
  • Remove the idle mixture screw covers, close each screw, then open each one 2.5 turns.

It is important to be careful with these parts; they are manufactured to very specific tolerances and are easily damaged. Also, recognize that most of the retaining screws are made of brass, and again are easily damaged. Use no more pressure than necessary when adjusting them, and do not use the screwdriver included with the bike, or a cheap Wal-Mart screwdriver; use a hardened screwdriver with well-cut blades ($8 at a hardware store should get you something acceptable).

Don't drill the slides

If you have a Dynojet kit, it will tell you to drill the vacuum hole ("the slides") of each needle-holder diaphragm with their included drill bit. However, it is HIGHLY RECOMMENDED that you don't do this. This is another reason why Factory Pro kits are recommended.

Dynojet kits are made for race applications. That larger hole they want you to drill means there needs to be a larger pressure difference between the top of the diaphragms and the carb body before the slide raises. That higher pressure difference means the slides don't start to move until there's more air passing through the carbs (this contributes to a 'bog' feeling at low rpm) and when they do go up they tend to snap up. You loose the CV part (Constant Velocity) that makes this kind of carb so desirable for street bikes. On the other hand, it's ideal for race bikes, where you're wide open most of the time and you just want the slide to be there to make it 'stupid-proof', so you don't bog down the motor when you snap the throttle open.


There are two companies that make jet kits for the Ninja 250:

Generally, Factory Pro is recommended over Dynojet. They have more experience with EX250s and better customer service. See the notes above about Dynojet kits being for race bikes.

Do yourself four favors:

1. Read all of the articles in the jetting section of the FAQ. An informed tuner is a happy tuner.

2. Don't drill the slides, as Dynojet would have you do.

3. Even if you're not using their kit, take a look at the pdf file at the Dynojet site. It will give you a good idea of what's involved in rejetting your carbs.

4. Read the Factory Pro tuning guidelines, so you'll know what you're getting into before you start.