Jetting Old Carburetors




Below is a screenshot of my spreadsheet for jetting old carbs. 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 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. [Plug Reading]

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.

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.


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 125-130 for race engines or 115-120 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 125-130 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. 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.


4) At C26 to G26 enter the data for your needle (obtained by measuring the needle with a digital caliper) and the program calculates the needle diameter for every 1/8th distance of the carb bore. If it is a single tapered needle then the dimensions are just A, B, and C.


5) If you aren't sure of the needle jet (atomizer) hole diameter then you may have to measure the hole size.


6) Enter at C28 the millimeters the slide is open
when the start of the needle taper is at the top
of the narrow section of its jet. You can use the
manual method described on the previous page.
At D28 the slide height is calculated so you can
use that. That depends on the needle specs as
well as the data you enter at A32, B32, A34, B34.
But the most accurate way is measuring it if you
have a digital caliper. One of my videos shows me
measuring it.




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.


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.

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)


Virtually Finding The Best Jetting
10) If you want to raise the blue jetting graph from 1/4 to 3/8 slide open then make sure the % at D10 is between 20% and 30%. That % is the needle jet flow area at closed throttle (compared to what it is at WOT) that adds to the fuel flow at all throttle settings. The bigger that percentage, the richer the mixture at the lower throttle positions. If it's less than 20% then use a skinner needle or get a needle jet with a bigger hole.



Also less slide cutaway makes the jetting there richer. First make sure the slide cutaway is close to the carb size divided by 10. Additionally you can lower the slide opening (by lowering the needle clip) at C28 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.




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.

Multi-tapered needles can be good or terrible. Use this calculator to "try them out" before investing any money in them.


12) Re-record this spreadsheet with all of your data in it. For example, if saving data for a CZ250 then save it as JettingCalcCZ250.xlsm

Old Carburetor Jetting Calculator




If you have any questions then just email me at a57ngel@yahoo.com


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