Ninja 250 Carburetor Needle Information
The needle in a carburetor is the main method of adjusting the mixture at partial throttle openings. It blocks some of the passageway to the main jet (the main jet size determines the carb's overall mixture), reducing the amount of fuel allowed to flow at part throttle. This keeps the mixture consistent through a range of throttle openings: as the throttle opens, raising the slide, the needle also raises (it's attached to the slide). The needle has a tapered profile, so that as it pulls out of the main jet port, it blocks less and less of the port, allowing more fuel to flow, ideally matching the amount of air flowing through the carburetor. By the time the carburetor is at full/wide-open throttle, the needle is completely clear of the main jet port, allowing it to flow without restriction.
The rate and positions at which the needle tapers, along with the size of the main jet, determine the exact mixture at a given throttle position. Ideally, the actual air:fuel mixture stays about constant at any given throttle opening, so that as more air is drawn into the engine, correspondingly more fuel is drawn in as well.
A given needle only really has one adjustment: height. As you raise a needle (by moving the clip lower on the needle, or adding washers), you enrich the mixture. As you lower a needle, you lean the mixture out, all at partial throttle.
You can also "adjust" the needle by substituting different-profile needles for the stock units. There are practically two ways to do this: you can purchase a jet kit, or you can try needles from other carburetors. Enterprising riders with machining skills may also manufacture their own needles on a lathe, but this is beyond the scope of this article.
The Factory Pro and DynoJet "jet kits" provide clip-adjustable needles, and are misnamed: they're really a set of custom-made needles (into which FP or DJ has put a lot of work and research), with some jets thrown in very nearly as an afterthought. They should be called needle kits.
You also have the option to try needles from other carburetors, but this can be very hit-or-miss. If you decide to try out other needles, use the following dimensions as a starting-point.
If you find closely matching needles from other carburetors, please post as many details to the forum as you can, and we'll add the information here.
The stock needle in a pre-08 Ninja 250 (this article doesn't specifically pertain to 08+ bikes) is known to have a very lean midrange, and a relatively rich top end. This is because the EPA performs its emmission testing at midrange, and the manufacturer wants to have the mixture as lean as possible there, to produce fewest emmissions. This is one of the reasons to replace the stock needles: midrange performance can be noticeably increased by enrichening the mixture, which can only be done with a new (or reprofiled stock) needle. Reprofiling a stock needle is a job for a machinist, and is beyond the capabilities of most Ninja 250 owners.
The Ninja 250's stock carburetor is a Keihin CVK30.
The stock needle is marked N16I at the top, and is Kawasaki part number 16009-1461. It's made of brass.
The stock needle has the following dimensions, measured from the bottom surface of the top lip:
Factory Pro claims that its custom-profiled needle for the Ninja 250 increases power when compared to the stock needle. Factory Pro will only sell needles as part of a jet kit.
The Factory Pro needle is not marked, but is made of steel, unlike the stock needle.
The Factory Pro needle has the following dimensions, measured from the top end of the needle (see below for clip positions):
The Factory Pro needle has 5 clip positions, which would result in the following needle lengths, measured from the bottom surface of the clip:
In addition, the Factory Pro kit includes washers which are about 0.54 mm thick, so that the needle can be adjusted in about half-millimeter increments.
BrianM submitted this information about an aftermarket jet needle. He was unsure which needle it was, but as the measurements don't match those of the Factory Pro needle above, it's likely that this is the DynoJet needle.