Jetting Keihin carburetors




Click here for my page verifying the jetting calculator for Keihin PWK/PJ/PE carbs.

Below is a screenshot of my spreadsheet for Keihin carbs (PWK, PJ, PE). Below this screenshot are the links to the needle and needle jet info that you will need to know such as the inner diameter of the needle jet of your carb. Here's the sequence to follow:
1) Enter all the data into the light blue cells to the left of the graph. If you hover the mouse pointer over a cell with a red corner then a message will pop up telling you about that cell.
2) Enter the type of exhaust pipe at A7 (1 for non-expansion chamber, 2 for street/trail expansion chamber, 3 for racing expansion chamber) which will determine the WOT jetting setpoint if you enter N at A9 (indicating you don't know if the main jet is the best size).
3) Look at the 3 needle types at section B22 to C27 and if you want to graph a needle that is within that group then enter its code and then enter an X at H23 or 25 or 27 to designate to the program which of those 3 needles you want to graph. Then enter the same code at D5, or enter a code for one of the needles listed from B54 to 94.
For the 3 letter needles (ie: DEK) the 1st letter indicates the taper angle, the 2nd letter the L1 length, and the 3rd letter the maximum diameter.
For OEM needles (ei: R1172J) the 1st pair of numbers is the taper angle, the 2nd pair of numbers the diameter, and the last letter the L1 length. (see the codes chart to the right of the jetting graph.)
4) The needle jet (the hole that the needle fits in) diameter for E7 is 2.9mm for PWK33-39mm, PJ34-38mm, PE36, PWM38. And it's 2.6mm for PWK26-28mm, PE26-28, PE20-24. Enter it at E7.
5) Enter at C32 the millimeters the slide is open when the start of the needle taper is at the top of the narrow section of its jet. You can use the manual method described on the previous page. The data in the spreadsheets lower list of needles (B54 on) also includes the mid clip slide height (listed @ C31) when the beginning taper is even with the top of the needle jet. If the cell at C32 is left blank then the program will read that stored value. If there is no stored value and C32 is blank then a warning message will tell you to enter a value at C32 (best is usually 1 or 2mm less than 1/4 of the carb bore). If there is a stored value then you can add or subtract 1 or 2mm from it and type it into C32. Watch the red graph as you change its value.
6) Look at the red graph results to see if it is above or below the 1.00 line at WOT slide opening. If the correct main jet size for this carb is unknown then you change the main jet size till the red graph is at 1.0 or really close. If you know the currently installed main jet gives good clean power and a chocolate brown colored plug then enter Y at A9.
7) If you want to raise or lower the beginning of the graph then change the idle jet size at C5. To raise/lower the graph up to 1/2 slide opening then either 1) change the slide cutaway millimeters (G5) to raise or lower the beginning section of the red graph to come closer to the 1.0 level, 2) change the value of slide height at C32.
8) Then notice the graph at 1/2 slide opening and if it is above 1.00 then you need a needle with a shallower 1st taper angle, or if it is below 1.00 then look for a larger angled needle. Above 1.0 is too rich, and below it is too lean.
To store the results of any needle just click onto any of the "Record Results" macro buttons before entering data for a different needle. The graph for the recorded needle will also show. So you can see the graph for 4 needles at once after recording for 4 different needles. To clear out all previous records just clisk onto the ERASE RECORDS macro button.
9) Re-record this spreadsheet with all of your data in it. For example, if saving data for a YZ250 then save it as JettingCalcYZ250.xlsm

The red graph at 1/4 slide can also be varied by changing the difference in diameters between the needle and the needle jet by selecting a different needle jet at E5. The clearance area is listed at D9 and the percent of it compared to the flow area at I14 is listed at D10. The higher the percentage, the richer the jetting at 1/4.
Keihin Carburetor Jetting Calculator


Keihin needle jet inner hole diameters
2.9mm for PWK33-39, PWM38, PJ34-38, PE36, FCR28-41
2.6mm for PE26-28, PWK26-28, PE20-24

Heres what PWKs are often equipped with:


Needle Data for PJ34-38 PE35-38 PWK33-39 PWM38 from Jets R Us:
The 48 series are for the large PWK carbs. The 46 series are for the smaller ones.



As an example look at the top left needle which is AEF. The first letter refers to the needle taper degrees/minutes, the second letter to the L1 length, the third letter to the diameter. So AEF has 1 degree 0 minutes (1.0 degrees), 38.15mm L1 distance, and 2.655mm needle diameter (at the straight section). CEJ has the degrees increased by 34 minutes (34/60=.57 degrees), same length, and diameter increased by .02mm for a leaner mixture at low throttle settings.




 

Here's Keihin's engineering drawing for the N3 series needles for the PWK33-39. They all have the same dual taper but vary in upper width.


Analyzing the needle diameter graphs on the JD Jetting Spreadsheet I came up with these specs on the listed needles which are now programmed into the Keihin sheet on my jetting calculator.



Needle Data for PE26-28 PWK26-28 from Jets R Us :





Needle Data for PE20-24 from Jets R Us :


Keep in mind that the L1 length is not the length to the beginning of the taper but is the distance to where the taper will have a 2.515mm diameter. With needles that have a max diameter less than 2.515 then that L1 spot is actually imaginary, as if the taper kept extending upwards past the taper start. Needles with less than 2.515mm diameter: PE26-28, PWK26-28, and most needles for PE20-24mm. This was the explanation given to me by Jets-R-Us.

Main jets and needles are also available from Sudco which is a preferred source for genuine Keihin parts.

If you have any questions then just email me at 19jaguar75@gmail.com


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