Jetting DellOrto Carburetors Below is a screenshot of my spreadsheet for DellOrto carbs. Near the bottom of this page 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 the spreadsheet: 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. Some Dellorto's have an idle mixture limiter screw adjuster instead of an air screw adjuster. The A paragraph is for carbs with an air screw, and B is for mixture limiters: A) 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. B) Turn in the slide stop screw till you have a slightly fast idle. Then slowly turn the mixture 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 counter-clockwise the mixture 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). ******* Install the correct idle jet if necessary and repeat procedure, adjusting the idle mixture 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 mixture 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.
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 of the jetting sheet. If you aren't sure of the transfers duration then enter 125-130 for race engines or 115-120 for street/trail engines.  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 C41 of the Dellorto 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.   4) At C26 to H26 enter the data for your needle and the program calculates the needle diameter for every 1/8th distance of the carb bore.  5) The needle jet (which Dellorto calls an atomizer) hole diameter is indicated in its ID. Take the first 3 numbers and put a decimal point after the first one to get its diameter in millimeters. If you aren't sure of its ID then you may have to measure the hole size.  Enter the height of the half circle needle shroud over the top of needle jet at E9. (It's called an atomizer bush and is pictured below.) If there is no half circle needle shroud then enter 0. Enter the height of the needle jet over the venturi base at A11.  
If your carb has a mixture limiter screw instead of an air limiter screw then put 1.5 at C7 and if the beginning of the graph is off from reality then change the # at A7 to make it represent what you are feeling when reving and riding it.
Multi-tapered needles can be good like this 1.1*/3* one, or they can be terrible like this 2*/4* one. It's just such a huge advantage to test them virtually using this software first. 
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.xlsm DellOrto DellOrto calls their needle jet an 'atomizer' and you need to make sure your DellOrto carb has a 2 stroke atomizer because the 4 stroke atomizers have more air holes in their sides which changes the flow which makes my calculator unsuitable for selecting a good needle. (This holds true for all carburetors.) 2 stroke atomizers are labeled AQ, AU, AV, AS, AB, AF, AR. The hole diameter is known by dividing the atomizer # by 100. This site lists the atomizers and their carburetors. This site also sells atomizers without listing any spec but their hole diameter. Treatland sells genuine Dellorto jets. Here's what DellOrto carbs are often equiped with:   Following is the Dell'Orto data on needles for carbs PHBN, PHBG, PHBL, PHBH, PHBE.      The commas represent decimal points in tthe chart below. It looks like some needles have 3 tapers but actually the last one is just a straight section.    If you have any questions then just email me at a57ngel@yahoo.com
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 - PHBN, PHVA, PHBL, PHBE, PHF, PHM, VHSB, VHST, VHSH, PHVB, VHSA, PHSB, PHBH, PHBG. Model letters ending in SB, SC, VB, BN, or VA will be calculated as having an oblong venturi so if yours has one of these ending letters but it has a circular venturi then change its end letters or just leave this entry blank. 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 C4 - Because most dellorto's have additional adjustments for the idle mixture that aren't taken into account here, you need to change the idle jet # until the graph shows the same low and mid throttle richness or leanness that you experience when riding. Usually it needs to be increased. D4 - This is optional information. The needle specs have to looked up and entered at row 26. see http://www.dragonfly75.com/moto/jettingDellOrto.html 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 you'll need to enter the # of A1 into A5. B6 - This is from velocity sheet, F7 for 2 stroke or F34 for 4 stroke. C6 - For PHBG the more turns, the richer the idle mixture. That is because the screw affects the flow area between venturi and idle jet, not the idle air passage. With all others the outcome is typical- more turns out causes less richness. D6 - Needle shaft width at straight portion E6 - Needle jet hole diameter 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. Carbs beginning with "VH" don't need to enter anything here. B8 - End of pipe poweband RPM C8 - The cutaway height in the middle in millimeters. My PHBN has 4mm for a #40 slide. D8 - the area between the needle shaft and the needle jet E8 - Enter the millimeters that the half circle needle shroud extends above the top of the needle jet (if there is a half circle shroud. If not then enter 0. See http://www.dragonfly75.com/moto/images/34slide2.gif Refer to I51 below. 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 - Enter 1 if there is only one idle mixture exit hole under or to the rear of the front of the slide (when slide is closed), or 2 if there is an additional hole at the base of the venturi in front of the slide. 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 shroud (or just the upraised needle jet if there is no half circle shroud and the needle jet extends up into the venturi). H10 - This is an estimate of idle RPM just based on engine size. 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 O22 - # of complete turns out of the slide stop screw to acheive the slide height you will enter at P23. O23 - Measured slide height with this many screw turns out. B26 - Enter the needle specs here that are available from http://www.dragonfly75.com/moto/jettingDellOrto.html (click on "instructions" at E2) D27 - Slide open mm @ taper start calculated from data you enter at A32 to B34. Use this at C28 or just measure by the method listed on my web site. The message "increase slide open" will appear when the needle is too short for the # entered at C28 and the needle will pull out of its jet completely at WOT causing a sudden richness. B28 - Use the # calculated at C32 after entering data at A32, B32, A34, B34. Or just measure the distance using my method outlined on my web site. O29 - This value is the idle slide stop screw turns out that produces a slide height too dificult to measure which is why you're using this calculator. A31 - Height from bottom of slide to where the needle clip rests on. It is dimension C in the graphic below starting at A48. B31 - #1 is the highest position and #5 is the lowest. After changing it be sure to enter the value from D28 into C28. If you put washers under the clip then add their thickness here. O31 - This is the calculated slide height at idle which you need to enter at C41. I32 - 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. A33 - Millimeters that the narrow part of the needle jet is below the level of the bottom of the venturi. A positive # is for the distance below the venturi base, and a negative # for when it's above the venturi base. B33 - Length from top to bottom, not including the little end cones. A needles have 10-14mm + taper length as is listed on the chart, W has 44.5mm, D has 51.5mm, X has 54.5mm, U has 67.5mm, K has 73.5mm, V has 74.5mm C40 - This measurement lost importance for version 12 on up. Just enter 1. H40 - % large fuel droplet amount of total fuel at WOT. These contribute to cooling and lubrication. A85 - RPM extended to 0/8 K106 - 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. A108 - Distance from top of gasoline level (usually at top of float bowl) to the top of the needle jet. Enter this value if you know it, otherwise leave C109 blank so the computer can use my chart to the right which are derived from photos but aren't precise. D109 - mm from top gasoline level in float to top of needle jet B110 - 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. H140 - height of slide with throttle closed for idle E142 - value for cell I9 which should be I15 if the main jet is sized correctly J142 - height over venturi base of the top of the skinny part of the needle jet opening A144 - Enter the needle specs here that are available from http://www.dragonfly75.com/moto/jettingDellOrto.html (click on "instructions" at E2) A151 - Enter the needle specs here that are available from http://www.dragonfly75.com/moto/jettingDellOrto.html (click on "instructions" at E2) A163 - Enter the needle specs here that are available from http://www.dragonfly75.com/moto/jettingDellOrto.html (click on "instructions" at E2) A172 - raw Ndl Flow Area - zone A348 - Modified S+M drops at idle. A349 - extended to 0/8 value based on straight line from idle to WOT A360 - fuel/air at idle A362 - extended to 0/8 A364 - extended to 0/8 |
