Jetting DellOrto Carburetors
Below is a screenshot of my spreadsheet for DellOrto carbs. 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 the spreadsheet: 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.
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 on the velocity 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.
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 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. Treatland sells genuine Dellorto jets.
Here's what DellOrto carbs are often equiped with.
To get the needle jet (atomizer) diameter just divide its number by 100. So the 209GA atomizer has an inner hole diameter of 2.09mm.
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.
Here's more Dellorto needle charts that may be of use to you:
If you have any questions then just email me at email@example.com