Jetting DellOrto Carbs (2023)

Jetting DellOrto Carbs (1)
Click here for my page verifying the jetting calculator for DellOrto carbs (except SHA).

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.

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Here's the sequence to follow:
0) 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. 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 correct idle jet if necessary and repeat procedure. Then turn the slide stop screw till you have a normal idle speed. The idle jet that gives the best starting and idling is the right one for use with this program and you can change it back later. Normally if you have to make the idle mixture too rich for 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 or you need a skinnier needle or fatter needle jet (the brass hole the needle slides into).

1) Make sure your main jet is correct. Click here to find out the best way to size the main jet. 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.

2) Determining mid throttle needle-dependent jetting - Mark your throttle body and throttle grip so you can see where 1/2 throttle open is. 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 entr a jetting richness/leanness # at A8. To lean out the mid throttle jetting a bit you can lower the needle by repositioning the clip higher, or do the opposite to richen the mid throttle jetting. But be aware that the needle position also affects the transition of power off idle. If you have to lower it so much that it causes too much of a weak zone off idle then you should just buy a needle with less taper angle.

Jetting DellOrto Carbs (3)Reading the graph:
The numbers indicate rich if over 1.0 and lean if under 1.0. Racers need mid range richness but not over 1.07, and street/trail bikes need less than 1.04 max. Ignore the 1/4 reading since that often is incurably lean, and it is just off closed throttle which you blast through.
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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.
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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. The calculated value of 11% of the carb bore is at C14 of the Air Velocity sheet which is acceptable with reed valves. If you absolutely don't want to take the carb off to measure the slide opening then use this value.

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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.
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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.
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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.
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7) Adjusting the end of 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 under load then enter the value from I15 into I9. I9 sets the height of the end of the grey line (which represents perfect jetting which your calculated jetting is compared to). In this example .53 needs to be entered at I9. Jetting DellOrto Carbs (14)
8) Now set the beginning of the
"ideal jetting" gray graph. If your
idle air screw is correctly
adjusted and you have measured the
idle slide height and entered it at
C45, then enter the value of A1 at
A7 (5.12 in this example). This
will set the beginning of the grey
graph in case you virtually make
any changes that affect idle
jetting (idle jet, air screw,
shroud height, cutaway) so the
graph will show how far off it
is from ideal. The beginning and
end of the gray graph are the most
important to set.
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9) Now you can set the 5/8 slide position ideal graph value according to your own evaluation of mid throttle jetting. This completes the skeleton of the ideal graph which the program will fill in the remaining points for a complete graph for your jetting to be compared to. Hover your mouse pointer over A8 to read the lengthy comment stored there. It basically says that you need to enter 1.0 if the jetting there is perfect, or a higher # if jetting there is too rich, or less if the jetting there is too lean. Then enter the # at E28 at A9 which will set the gray graph mid point that all future changes will be compared to. After setting the three main points on the grey graph you need to leave them alone. All virtual changes will be to try to get the blue graph to be equal to the gray graph. To be the most precise in this step it iis best to remove the carb and open the throttle till the slide is 5/8 (62%) open and then mark the throttle and throttle body. (You can use White-Out)
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Virtually Finding The Best Jetting
10) If you want to lower the blue jetting graph from 1/4 to 5/8 slide open then you'll need to 1) raise the needle clip # at B32 for a lower # at D28 to enter at C28, or 2) reduce the clearance between needle and needle jet by selecting a needle with fatter diameter or a needle jet with smaller diameter. If the graph there is too lean then you need the opposite. That change also affects idle and main jetting and so the number at A1 will change. When that happens then change the air screw turns or idle jet # till A1 returns to what you recorded before at A7. You can also lower the needle shroud height (and increase the main jet) to lessen the graph from 1/4 to 7/8.
Also less slide cutaway makes the jetting there richer. Additionally you can lower the slide opening at C28 (by lowering the needle clip) 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 C28.

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11) 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. Jetting DellOrto Carbs (18)

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.
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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.xlsm

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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:
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Following is the Dell'Orto data on needles for carbs PHBN, PHBG, PHBL, PHBH, PHBE.

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Jetting DellOrto Carbs (24) Jetting DellOrto Carbs (25)

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K needles are for DellOrto PHBE carburetors (30-38mm):
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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.Jetting DellOrto Carbs (28)

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Jetting DellOrto Carbs (30)

If you have any questions then just email me at


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