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:

1) 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. (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.

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. [Plug Reading]

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 1.03) so they can crack the throttle open without bogging. You can usually ignore the 1/8 + 1/4 slide jetting since that often is too rich or lean, and it is just off closed throttle which you blast through unless it is a big bore street or trail bike.


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.

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. 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 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. Then leave A7 and
I9 the same while you virtually try
different needles, needle heights,
needle jets, and slide cutaways.

If the beginning graph is off from reality (blue too high or too low) then change the # at A7 to make it represent what you are feeling when reving and riding it. This will compensate for any bad measurements you've made or the idle circuit design varying from what is expected by the formulas.

Virtually Finding The Best Jetting
9) 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.




10) 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.


11) 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


HOME