Jetting Mikuni TMS/TMX carburetors

Below is a screenshot of my spreadsheet page for Mikuni TMS and TMX carbs. Unfortunately it doesn't work with the late style carbs with a powerjet. Here's how to use it. Enter all the data in the light blue data boxes. If you hover the mouse pointer over a cell with a red corner then a message will pop up telling you about that cell. 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. Starting at row 51 of sheet 1 are all the needles and their data. Here's the sequence you need 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. Also get the idle slide area to put at C50.

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

5) At D5 you enter the needle jet 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 to 91.

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. Tht 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 "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 and it gives a chocolate brown colored plug then enter the value from I15 into I9. I9 sets the height of the end of the grey line (which represents perfect jetting). In this example .67 needs to be entered at I9. If you know the bikes jetting is a tad rich then subtract .04 from I15 and enter that at I9. Vice reversa if it's running a tad lean.

8) If you want to make any virtual changes to the jetting that affect the beginning of the graph (idle jet, cutaway, air screw, needle width) then you'll first have to set the beginning of the gray graph so we have a reference point of comparison. If your idle air screw is correctly adjusted and you have measured the idle slide height and entered it, then enter the value of C16 at A9 (1.17 in this example) and put "Y" at A7.

9) If you want to change the blue jetting graph from 1/4 to 3/8 slide open then first make sure the % at D10 is close to 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 30% 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 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 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.

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

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.

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=6.25 degree. A=0.25 degrees and increase by .25 degrees per letter increase. One letter indicates a single taper needle, and two letters indicates two 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.

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