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:
1) First make sure your real life idle jet is correct. 1/4 turn inwards on the air screw from the position that causes peak idle RPM should result to be within 1+1/4 to 1+3/4 turns out. If it is less than that then you need a bigger idle jet. If it is more than that then you need a smaller idle jet.
2) 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. And the top of the piston should be black except close to the transfers and boost port. If it is all black then the main jet is too small, and if it is too clean then 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 lean jet and then lower the shroud 1.5mm (.06") at a time till the jetting is right. [Plug Reading]
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.
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.
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.