Jetting Mikuni TMS/TMX carburetors

Below is a screenshot of my spreadsheet page for Mikuni TMS and TMX carbs. Here's how to use it. Enter all the data in the light blue data boxes for the program to automatically display the jetting results in a graph. If you hover the mouse pointer over a cell with a red corner then a message will pop up telling you about that cell.

At 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. Starting at row 51 of the TMX sheet are all the needles and their data. Here's the sequence you need 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.

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 of the TMX sheet. If you aren't sure of the transfers duration then enter 125-130 for race engines or 115-120 for street/trail engines.

4) 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 C40 of the jetting 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.

5) Enter the 4/042 main jet # at F5 on the TMX sheet.

6) At D5 you enter the needle ID (code) so the program can find the needle in the listing farther down the same sheet and use its data for the calculations. The ID is imprinted near the top of each needle. Data for a custom needle can be entered at rows 88- 91 if its needle data is missing. Give any custom needle a unique ID.

7) 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. Normally the
needles are 34mm plus the taper length but you
need to measure it from top to bottom, excluding
the little end cones.

8) 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 .67 needs to be entered at I9. If you know the bikes jetting is a tad rich then subtract 4% from I15 and enter that at I9. Vice reversa if it's running a tad lean. The beginning and end of the gray graph are important to set because between those two points the graph follows a predetermined line which won't be correct if the start or end isn't correct.

9) 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 C40, 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.

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
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.

11) Selecting a different needle taper. Notice the blue graph at 1/2 to 5/8 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 larger taper angle.

Triple-tapered needles typically are too lean till WOT.

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

Here's what the carbs usually come equipped with:

TMX + TMS needles:

If you aren't sure what needle jet is in your carb then you'll have to get an idea what size opening the needle jet has by inserting different size drill bits into it unless of course you can unscrew it from the carb. What SUDCO stocks is series 633 S-4/7/8/9, and series 914 S-9.

This is from the Jets R Us website concerning needle jets for the TMX. S-7 has an inner diameter of 2.835mm so if your needle jet is not removable and is impossible to measure then just use that diameter by entering S-7 at E5. That should be close enough to reality.

TMS/TMX Needles from Jets-R-Us:
EARLY STYLE TMX35 FLATSLIDE: 6EN11-(52/53/54/55/56/57/58) $7.04
EARLY STYLE TMX38 FLATSLIDE: 6EJ12-(53/54/55/56/57/58/59/60) $7.04
TMS38-77, 78 125cc: 6GDY12-(56/57/59) $7.04
TMS38-77, 78 250cc: 6DGY04-(54/55/56/57/58/59/60) $7.04
TMS 38-78: 6CEM(01/02/03/04/05) $12.04

Example Mikuni TMS needle: 6DEY26-66
The first 6 is the length, which in this case is more than 60mm but less than 70mm

The -66 translates to the straight part of the needle. Divide it by 100 and add to 2 so that 66 = 2.66mm in diameter. And 6DEY26-67 = 2.67mm dia.

Other options:
6DEY27-66 is a 1/2 clip richer than a 6DEY26-66

Letters DEY indicate triple taper needle. D=1 degree taper, E=1.25 degree, Y=3.1 degree. A=0.25 degrees and increase by .25 degrees per letter increase till G, thereafter the letter increase is .075mm each. Two letters indicates dual taper, and three letters indicates three tapers.

SUDCO has a complete listing of parts, jets, and needles for the TMS and TMX carbs. Their needles are probably machined more precisely than those from Jets-R-Us since all the needles I have bought from Jets-R-Us aren't to spec.

I can be contacted at a57ngel@yahoo.com

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