Jetting Keihin carburetors
Below is a screenshot of my spreadsheet for Keihin carbs (PWK, PJ, PE). Below the instructions here are 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 and the program will graph the jetting. 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: 1) First make sure your real life idle jet size is correct. Turn in the slide stop screw till you have a slightly fast idle. Then slowly turn the air screw till you find where it idles the fastest, then readjust the slide stop for the same fast idle as before if it has changed. Then slowly turn clockwise the air screw till the idle speed starts to drop off due to starting to be too rich (but which is good for starting w/o the choke in warm weather). This should result to be within 1 to 2 turns out. (Forget the untrue 1.5 turn "rule" which never came from the carb manufacturers.) 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. Install the correct idle jet if necessary and repeat procedure, adjusting the idle air screw. Then turn the slide stop screw till you have a normal idle speed. The right idle jet is what gives these two things: 1) the most consistent idling when the engine is hot, 2) the best off-idle power as you open the throttle slowly. Later you can change the slide stop position, idle jet, and air screw setting to your own preference, the best example being that racers let the slide low enough for engine braking that is too low to allow the engine to idle. Normally if you have to make the idle mixture too rich for smooth idling in order to help compensate for weak throttle response as you crack the throttle open then you either have the needle clip in too high a position (lowered needle), the needle is too fat, or the needle jet (the brass hole the needle slides into) is too narrow. 2) Make sure your main jet is correct. The simplest method is to try a few jets and pick the richest one that allows clean WOT running. Click here to find out the best way to size the main jet. For small carbs you may need to cut down 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]
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 at the Keihin sheet. If you aren't sure of the transfers duration then enter 125-130 for race engines or 115-120 for street/trail engines.  4) You'll need to set the slide stop screw for an even engine idle and then take the carb off and measure the idle slide height for C45 of the jetting sheet. That is hard to do and so I made a mini calculator on the 1st Mikuni sheet, N15 to O27, that allows you to use two allen wrenches, one of a size smaller than the slide open mm, and one larger than that to calculate what the slide opening is according to the turns out of the slide stop screw.   5) 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.  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. 
If the beginning graph is off from reality (blue too high or too low) then change the # at A7 to make it represent what you are feeling when reving and riding it. This will compensate for any bad measurements you've made or the idle circuit design varying from what is expected by the formulas.
 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) After everthing is done, and the graph shows too lean or rich mid throttle than what you experience, then you'll need to adjust the graph with an adjustment factor at A9. 1 doesn't change it at all but higher than that (up till the max listed at A10) will bow upward the graph before WOT. This page tells the correct procedure for evaluating the jetting. 12) 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 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. This shows that you just use the last 3 letters of this code.  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.  I just lately came across the N3C data and don't conveniently have the room to squeeze it into the program so I put it at the bottom of the spreadsheet (starting at row 183 although the calculator doesn't access that data) and you'll have to enter any of their data in as a custom needle at one of the rows from 107 to 111. For all other 65mm needles you can follow instructions at my video to find their specs to enter into this spreadsheet. 
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. Just look at the main menu and then type in the page # you want in the upper left hand corner. These needles are for carbs PWK33-39, PJ34-38, PE36, PWM38 :  Slide Cutaway Info: 
JD Needles  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 of its 4 tapers and when I plugged its specs into my jetting calculator the result was really rich at 3/4 slide open. There are better standard needles that work better such as the lowest graph here. 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 jetting graphs. On them the grey line represents perfect jetting and the blue graph is the jetting the needle delivers.  Jetting graph of a commonly available needle for the PWK: 
Below are all the hidden comments for you to see if your computer won't let you see all of them in the spreadsheet: A2 - PWK, PJ, PE, PWM E2 - Jets available from SUDCO are from 35 to 140. If you don't have a Pwr Jet then leave this as 0. Don't forget entering the needed data at B135 if you have a power jet. The hole area of the power jet is listed at A1. The normal range for it is between 1.5x and 2x the area difference between the before and after main jets [area listed at F9]. F2 - TPS is "throttle position sensor". With it the power jet only turns on above 1/2 slide open which usually leaves 3/8 and 1/2 positions lean. G2 - feet elevation from sea level of riding area H2 - This is optional for you to be able to convert meters elevation to feet. I2 - This is optional for you to be able to convert Celsius to Farenheit. C3 - idle jet area B4 - Throat diameter in millimeters D4 - Enter the needle ID that you want to graph. If it isn't one of those listed below B51 then you also have to enter it at the appropriate one of the 3 cells from C23 to C27. E4 - This is optional information G4 - riding temperature in farenheit A6 - Enter 1.0 if the 1st half of throttle opening causes clean running with good acceleration when slowly opening the throttle. Enter higher than 1.0 for richer jetting (sputtering a bit at steady low throttle), or lower for leaner jetting (tendency to bog when quickly opening the throttle). This depends on your riding experience. Set this number to show on the jetting graph that the 2nd to 4th graph numbers represent what you feel when testing the low throttle jetting. Ignore the 1st number because that is just off idle and you blow right past that, even when slowly opening the throttle. After changing this number you'll need to re-enter the # from A1 into A5. B6 - This is from velocity sheet, F7 for 2 stroke or F34 for 4 stroke. C6 - 2 turns has the maximum effect D6 - Needle shaft width at straight portion E6 - Needle jet hole diameter. 2.9mm for PWK33-39, PWM38, PJ34-38, PE36, and FCR28-41. 2.6mm for PE26-28, PWK26-28, PE20-24 G6 - the relative humidity of your riding area H6 - This is the diameter at the intake bell with the air intake hole (that leads to the air screw) middle being at the outer diameter. The cross sectional area there is more than inside the carb venturi which makes the air velocity there less which makes the suction at that hole less. I6 - Measure the height of the slide bottom in the center. Measure from the bottom of the front of the slide. See drawing to the right of here. This should be close to 1/2 of the cutaway millimeters. A8 - Entering a number higher than 1 will show the jetting between 1/8 slide and WOT to be richer. So you can adjust the graph to show what you feel it is doing when you ride. For a full explanation of how to judge your jetting just go to dragonfly75.com/moto/carbtuning.html Number 1 keeps the graph unchanged. B8 - End of pipe powerband RPM C8 - The cutaway height in the middle of its arc in millimeters. When this is changed then the # at I7 also needs to be changed. D8 - the area between the needle shaft and the needle jet E8 - The PE and PWK28 doesn’t have a needle shroud. See www.dragonfly75.com/moto/images/34carb2.gif G8 - % ethanol in gasoline. Use a $12 ethanol % tester if you don't know. Jetting needs change with different % of ethanol. H8 - Use the # at C12 on the velocity sheet if you don't know your idle RPM. I8 - Enter the # from I15 here if the WOT jetting is good. Do this before entering anything different from your current "best WOT jetting" setup. A10 - Idle jetting richness. D10 - This is a rating of the needle jet diameter and needle shroud based on their relation to the idle jet and main jet. Less than .8 means the needle jet is too small or the needle shroud is too low (or both). More than 1.3 means the needle jet is too large or the shroud is too high (or both). E10 - Diameter of needle shroud. H10 - This is an estimate of idle RPM just based on engine size. A12 - Needle diameter at its jet at said slide opening A14 - Composite main and needle flow area A17 - The cross sectional area at the exit side of the slide A19 - The open cross sectional area at the slide intake C22 - 3 letter ID. Look at J1 to P19 for codes and meanings. B24 - Millimeters that the narrow part of the needle jet is below the level of the bottom of the venturi. C24 - This has to be 3 letters. Look at the codes from J21 to P34. B26 - Dimension C in graphic below starting at A138. In the PWK35 slide it is 17.4mm. This is needed to calculate the slide open mm at C31 which is the slide open mm with needle clip at mid position and taper at top of needle jet. C26 - This has to be 6 characters, the middle four are numbers. Look at the codes from J36 to P60. B28 - 1 is the top position and 5 is the bottom position. After changing it be sure to enter the value from D32 into C32. If you put washers under the clip then add their thickness here. D31 - Slide open mm @ taper start according to the needle data and data at B25, B27, B29. Enter that # at C32 or just measure the distance for C32 according to my site instructions. The message "Increase slide open" at C31 appears when the needle is too short for the # entered at C32 and the needle will pull out of its jet completely at WOT causing a sudden richness. B32 - Use the # calculated at D32 after entering data at B25 and B27. Or just measure the distance using my method outlined on my web site. Click on "instructions" at E1. I34 - Closed throttle needle jet area divided by combined raw flow area at WOT of needle jet and main jet. Larger than 0.8 means low throttle jetting will be too rich. Too large means the needle to needle jet clearance is too big and you need a fatter needle or skinnier needle jet. C35 - The carb size divided by 8 C44 - Very Important! If this is wrong then it screws up the rest of the graph. I use small allen wrenches to measure the closed throttle slide opening. This opening has to be so that the engine is smoothly idling. For engine braking you might want this opening to be smaller but for this program you need to find the opening that allows smooth idle. After finding that then you can lower it back to where it was. Normal range of height is 8 to 12% of carb size. If it's so big that it causes B15 to be less than zero then it screws up the graph. H46 - % large fuel droplet amount of total fuel at WOT. These contribute to cooling and lubrication. Q50 - # of turns out for good idle R51 - calculated slide height at idle A57 - This is the fraction considered in the formula for air velocity at the needle jet, the fraction of the additional cross sectional air space there. More space equals less air velocity which affects the richness. I'm using 0.5 for old style slides like the Mikuni VM's and most Dellorto's have, and 0.25 for flat slides that don't allow much empty space there. G106 - Just enter the 2nd taper length if there is no 3rd taper. H106 - Leave this blank if there is no 3rd taper. I106 - Leave this blank if there is no 3rd taper. J106 - Measured from top tip of needle to the bottom of the taper. M106 - Angle of 3rd taper if there is a 3rd taper. B107 - Store data here for custom needles or any needle that isn't listed above. A134 - Distance from top of gasoline in the bowl (usually equal to top of bowl) to the top of the needle jet. Enter this value if you know it, otherwise leave C130 blank so the computer can use 15mm which is a typical value. B134 - For power valves this is the distance in mm from the bottom of the venturi to the top middle of the highest location of the supply tube. Look at the picture here to the right. K159 - squaring factor for the air velocity at the power jet. The range is .7 to 2, with 2 being ideal when the jet outlet is right at the center of the flow area. The closer the outlet is to the outer edge of the flow area, the closer it should be to .7 which is because the speed and suction is lowest there. J234 - old formula for suction from velocty J235 - new formula A288 - extended to 0/8 A328 - Modified S+M drops at idle A329 - extended to 0/8 A331 - Gas/Air at idle A333 - extended to 0/8 A336 - ideal mix at idle |
