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 how to use it: Enter data into all the light blue cells of the spreadsheet. If you hover the mouse pointer over a cell with a red corner then a message will pop up telling you about that cell. Here's the sequence to follow:

0) First make sure your real life idle jet is correct. Turn in the slide stop screw till you have a fast idle. Then turn the air screw till you find where it idles the fastest, then readjust the slide stop for the same fast idle. Then turn clockwaise the air screw till the idle speed starts to drop off rapidly and return the screw position to just before that. This should result to be within 1 to 2 turns out. If it is less than 1 turn then you need a bigger idle jet. If it is more than 2 turns then you need a smaller idle jet.

1) Make sure your main jet is correct. The spark plug color should be light chocolate brown or medium grey. Too dark means the jet is too big. For small carbs you may need to lower the needle shroud if one main jet size is too rich and the next smaller size is too lean. In that case put in the rich jet and then lower the shroud .5mm (.02") at a time till the jetting is right. [Plug Reading]

2) Mark your throttle body and throttle grip so you can see when riding what the correct throttle opening is for 1/2 throttle and 3/4 throttle. Ride at a steady speed on a flat road, or on an inclined road if the bike wants to accelerate much at those settings. Then listen to the exhaust note. If it is irregular and/or sputters occassionally then the needle is too rich at that setting. If it runs fine but drops power when you quickly open the throttle then it is too lean there. Something between those two extremes is desirable. You need this info to be able to verify if the programs jetting graph is showing you the same thing. If not then you need to adjust the beginning of the ideal jetting graph (the grey line) so that what it shows is what you experienced. My video about that is on Youtube.


3) Go to the last sheet (click on the velocity tab at the bottom left of the screen) to find out the maximum air velocity to enter at B7. If you aren't sure of the transfers duration then enter 125 for race engines or 120 for street/trail engines. It is best to measure the idle slide height but for "close" results you can just use the calculated value.


4) Look at the 3 needle types at section C23 to C27 and if you want to graph a needle that is within that group then enter its code and then enter the same needle code at D5. Or ignore the section from C23 to C27 and just enter at D5 a needle code for one of the needles listed from B56 to B105. Or enter the data for a custom needle starting at B107 and enter at D5 whatever you named it. 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. At the right side of the sheet you can see the code breakdown starting at the J column. For OEM needles (example: R1172J) the 1st pair of numbers is the taper angle, the 2nd pair of numbers the diameter, and the last letter the L1 length.


5) 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 the appropriate diameter at E7.


   
6) 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. At D32 the slide
height is calculated so you can use that.
But the most accurate way is measuring
it if you have a digital caliper. One of my
videos shows me measuring it.


7) Adjusting the "ideal" jetting grey
graph. Look at the graph results
to see where the blue graph is above
or below the grey line at full slide
opening (far right). If you know the
main jet size is clean running with
good power and it gives a chocolate
brown colored plug then enter the
value from I15 into I9. I9 sets
the height of the end of the grey
line (which represents perfect
jetting). In this example .39 needs
to be entered at I9. If you know
the bikes jetting is a tad rich then
subtract .04 from I15 and enter
that at I9. Vice reversa
if it's running a tad lean.


8) If you want to make any virtual changes to the jetting that affect the beginning of the graph (idle jet, cutaway, air screw, needle width) then you'll first have to set the beginning of the gray graph so we have a reference point of comparison. If your idle air screw is correctly adjusted and you have measured the idle slide height and entered it, then enter the value of C16 at A9 (2.36 in this example) and put "Y" at A7.


9) If you want to change the blue jetting graph from 1/4 to 3/8 slide open then first make sure the % at D10 is close to 30%. That % is the needle jet flow area at closed throttle (compared to what it is at WOT) that adds to the fuel flow at all throttle settings. The bigger that percentage, the richer the mixture at the lower throttle positions. If it's less than 30% then use a skinner needle or get a needle jet with a bigger hole. Also less slide cutaway makes the jetting there richer. First make sure the slide cutaway is close to the carb size divided by 10. Additionally you can lower the slide opening at C45 to raise the beginning section of the blue graph to be closer to the ideal grey graph. If that exceeds the physical limitations of the slide and needle clip then you can put home-made washers under the needle clip to reduce the slide opening at taper start. Then measure the new distance and enter that value at C45.



10) Selecting a different needle taper. Notice the blue graph at 1/2 slide opening and if it is above the grey graph by very much then you need a different needle with a smaller taper angle. If the blue graph is below the grey one then the needle needs a bigger taper angle.


Multi-tapered needles are often too rich at 1/2 or 5/8 slide opening.
If you feel that is the case then find a single taper needle to use.

11) Re-record this spreadsheet with all of your data in it. For example, if saving data for a YZ250 then save it as JettingCalcYZ250.xlsx 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.

Here's the needle code list for the rare needles for the PWK29:


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


JD Needle Blues
Supposedly the experts at JD Jetting sell a custom made needle that is supposed to be ideal for the PWK carbs. But a person just consulted with me for better jetting and he measured it all and I figured out its angles and when I plugged its specs into my jetting calculator the result was really lean up till 3/4 slide opening because the length of it till the taper was about 6mm too long. So I found a needle that SUDCO sells that will give him perfect jetting according to my calculator. What is ironic about this bitter tale is that the spreadsheet by JD Jetting is what I used many years ago to find a better needle for my '89 Honda CR250. It was a good program but it only compared needles so you had to be experienced enough to know what part of the needle needs to be fatter or skinnier. About a year ago I asked if he could share his needle data with me since he no longer sells his spreadsheet and he said no and then broke off communication with me. I thought that was pretty stinky but by using his calculator I was able to logically derive the needed specs from a slew of needles he had programmed into his spreadsheet. He could of been nice and just gave them to me but instead he made me work for hours to get them. So now I have the full story. Any way here are the before and after jetting graphs.


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


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