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Understanding Jet Needles

Dec 31, 1969
Dallas, Texas
Updated June 9, 2018 - Conversion Tables added.

Keihin Jet Needles

By: CanadianDave

Breaking the Code– OK put away your Dick Tracy decoderrings and hang that trench coat back in the closet because breaking the KDX’s jet needle code is going to be simple. Check the chart below and follow through this example and you’ll have it figured out in not time.

Lets use the stock 1995 to 2001 KDX200 jet needle as an example. Kawasaki identifies the needle as an R1174N. Have a look at the image below and you can see how the part number is broken down into 4 sections as well as the parts of the needle. Each section of the code identifies a difference part of the jet needle's profile. I've color coded the the tables and diagrams to make it easier to identify the different parts and arrange the code in the proper order as you work through the example.

R= the Keihin part number/ series N427-48??? jet needle, 11 = Taper of 1 degree 15 minutes 74 = straight section diameter 74 x .01 + 2.005 = 2.745mm N = L1 measurement

Using the Kawasaki to Keihin Conversion Table and plugging those values into the Keihin's Profile Table we can determine we are looking at a needle with a 1 degree 15 minute taper, a 2.745 mm straight section diameter and a 39.95mm L1 measurement. If we look at the conversion chart you can see the equivalent Keihin part number is N427-48BGQ. Remember the Kawasaki code and Keihin's part number are not in the same order.

This chart will work for 1988 to 2001 KDXs equipped with a Keihin PWK Carburetor. For those who own KDXs built before 1988 the parts and function of your jet needles will be the same but the conversion chart isn’t going to work for you. Sorry


Here’s where things can get tricky . . . Kawasaki uses proprietary needle profiles. Keihin won’t be able to supply a direct replacement for most Kawasaki needles. In this case they do but if you were looking for an R1173L ( 97 to 01 KDX220 ) you’d see there is no direct replacement for a needle with an " L" L1 measurement on the conversion table. Not every needle is available from Kawasaki or Keihin so you’ll need to reference both charts and make you decision based on what is available.


Applicable Keihin jet needles are listed here. Needles with diameters less than 2.665 or greater than 2.745 were omitted and so were needles with a taper greater than 2° . These profiles aren't applicable to KDX owners.

Keihin N427-48??? Series Jet Needles


Understanding Jet Needles:

Remember that no throttle range is independent of the others. When adjusting specific throttle ranges its important to remember that there are a number of parts working together to deliver a measured amount of fuel. It’s therefore important to make one change at a time and test the result when ever possible.

The straight section – has the most effect between 1/8 and 1/4 throttle where the transition is made between the pilot/slow circuit to the main circuit. If this range is too rich a needle with a leaner/ larger straight section diameter may be required. If the range is too lean a needle with a richer/smaller straight section diameter may be required.

L1 length – The L1 measurement is normally changed by increasing or decreasing the length of the straight section, I’ve never seen it done any other way, but then again I certainly haven’t seen every available needle either. Changing to a needle with a longer or shorter L1 measurement effectively lowers or raises the jet needle in the needle jet. If your jetting is rich from 1/4 to 3/4 throttle you can either lower the needle byraising the clip position or go to a needle with a longer L1 measurement. The opposite is true for a lean condition.

You would normally go to a needle with a different L1 measurement for a couple reasons:

  1. Your present needle doesn’t have enough adjustment, i.e. you’re in the highest or lowest clip position to properly calibrate your jetting.
  2. When changing to a needle with a different L1 measurement remember there is 0.9mm between each clip position and that the L1 measurement is taken from the top of the first (#1) clip position. If you are in the #5 (richest) clip position of a needle with a 39.95 mm L1 measurement (G) the effective L1 length of your needle is the original L1 length minus the distance from the top of the #1 clip position to the #5 clip position or.

    39.95mm – 0.9mm x 4 = 36.35mm

    As a general rule of thumb you should attempt to select a needle that allows you maximum adjustment.

    A) If at 1/2 throttle the jetting was close in the #5 clip position and you’d like to be in the mid clip position when properly calibrated then . . . . knowing the effective L1 measurement of your needle in the #5 clip position was 36.35mm and you need to richen the circuit further you would likely go to an L1 = E or 38.15mm (BEQ). Why? In the #5 position the original needle has an effective L1 measurement of 36.35mm. If you need to change needle and want to maximize adjustment, i.e. start in the mid clip position, then 36.35mm + 0.9mm x 2 (two clip positions to #1) is 38.15mm

    B) If at 1/2 throttle the jetting was excessively rich in the #5 clip position then . . . . knowing the effective L1 measurement in the #5 position was 36.35mm and you expect you’ll need to adjust the needle more than 0.9mm, one clip position, you’d likely go with a L1 of C (36.35mm). That will allow you to start in the #1 position (equal to #5 with an L1= C or a BCQ) and continue to lean the needle from there.
    • Fine Tuning -Your BGQ is too rich at 1/2 throttle in its present position (#4) but leaning the needle one position (to #3) causes the engine to run lean. In this case you can choose a needle with an L1 measurement between the two. Adding half a clip position length or 0.45mm to your BGH results in a GHH (39.95mm + 0.45mm = 40.40mm).
Just a note here you can place a 0.4 to 0.5 mm spacer under the needles clip to make fine adjustments rather that going through the expense of buying a new needle.

Jet Needle Taper – Taper changes are made to " rebalance" fuel delivery between 1/4 and 3/4 throttle.

This is an area a lot of mistakes are made. It’s important to remember we are talking about a taper (angle). Changing tapers doesn’t result in a linear change. Changing to a needle with a smaller taper, say a B taper (1 degree 15 minutes), to a smaller taper A (1 degree), richens the jetting at lower throttle setting (1/4 throttle) and leans the jetting at higher throttle settings (3/4 throttle). Going to a larger taper leans the mixture at lower throttle setting and richens higher settings.

Changes in taper are not normally required unless major physical changes have been made to the carburetor or to the engine.

Shopping - Popular carburetor resellers include Carburetor Parts Warehouse andSudco Racing . Both supply factory Keihin parts. Sudco also supplies a line of less expensive brass jet needles. Either is likely a less expensive alternative to dealer parts and have a wide variety of needle profiles available.
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Jun 13, 2018
This is absolutely remarkable! Thank you for taking out some of your scarce time to share this informative and educative article here. I am more enlightened now.
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