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.  The 2nd graph to the right is an easier/quicker way to try to change the jetting if you have a good idea of how the jetting is at low throttle and 5/8 slide open (a little more than half throttle). Both methods here first require that you follow my advice for finding the right idle jetting and WOT jetting. Here's the sequence to follow: 1) I think it's best to choose an idle jet size that gives a good idle running and decent off-idle running. If above-idle is too rich (runs rough when slowly opening it up) then you need a smaller idle jet if you don't want to buy a fatter needle. If above-idle is too lean (doesn't want to accelerate till half throttle) then you need a larger idle jet if you don't want to buy a skinnier needle. What is happening is that part of the gas for idle actually comes up thru the main jet and thru the needle jet. Yes, the old jetting charts were over-simplified and didn't show the whole truth. Here's one way to tell if the clearance between the needle jet and needle is too much: the idle slide height will be below 1/10th the millimeters of the carb size. And if the idle slide opening is more than 1/8th the carb size then the clearance is too little. 2) Adjust the needle height (via clip position) to be a good compromise between good jetting at 1/4 throttle and 1/2 throttle. The higher the needle is, the farther you have to open the throttle before the neeedle taper becomes active (as it rises above the needle jet) to richen the mix. 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. The most correct method is testing acceleration times to find the jet size that allows the best acceleration. Click here to read about getting the main jet right. If after steps 1-3 you think the jetting isn't right between idle and WOT then you can use the program to help you find a better needle.
3) Go to the last sheet (click on the velocity tab at the bottom left of the screen) to find out the carb air velocity you'll need to enter at B7 of the jetting sheet. If you aren't sure of the transfers duration for B7 then enter 125-130 for race engines or 115-120 for street/trail engines. F7 shows the average air velocity the Dellorto sheet needs at B7 which is 78.2 in this example. Also enter the bore, stroke, and transfer duration at L3, L4, and L5. These are referenced by the jetting sheet.  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. In this sample take 270 which becomes 2.7mm.  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 it's too lean mid throttle and tends to bog if you crack the throttle open too quick then you need a needle with more taper angle if raising the needle doesn't solve the problem. 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. 
10) Re-record this spreadsheet with all of your data in it. For example, if saving data for a YZ250 then save it as JettingCalc-YZ250.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 |
