Jetting Mikuni VM/TM carburetors




Click here for my page verifying the Jetting Calculator for Mikuni VM/TM carbs.

Below is a screenshot of my spreadsheets page for Mikuni VM and TM carbs. 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 the screenshot are the links to the needle and needle jet info that you will need to know, such as the needle jet ID of your carb. Starting at row 53 are all the VM 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. It is best to measure the idle slide height but for "close" results you can just use the calculated value.


4) Enter either the 4/042 main jet # or the N100.64 main jet #, depending on which your carb has. VM carbs come with 4/042 main jets but some TM's have the N100/604 jet. These series of TM carbs come with N100/604 main jets: TM33-8012, TM36-68, TM40-6. Put 0 in the cell not used because the graph will crash if both jet #'s are there.

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 needles or ones not listed on this sheet can be entered at rows 178 to 184.

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 .39 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 (.98 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 single 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 are more lean at the beginning than single or double taper needles so if you are riding street or trail then you definitely need to get rid of that needle if the graph verifies that its jetting is lean at the first half.


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


Mikuni Carburetor Jetting Calculator



These graphics will show you what I mean by "needle jet shroud" which on the spreadsheet you have to enter its height at E9. It causes less vaccuum at the needle jet which pulls less fuel up. This can bring the vacuum at the needle jet into the "sweet zone" if the carb is undersized and has a high air velocity. It also helps with jetting a small VM because the available jets don't have sizes close enough together for the small sizes. Usually the VM18/22/24 have a needle shroud. All the TMS/TMX's have one so that question isn't asked on the TMX sheet. It is basically a little more than a half circle extending up from the needle jet on the air filter side.




Listings of all the Mikuni VM needles: Sudco  NicheCycle.
(The Mikuni needle data on my spreadsheet is even better than what they list because it has corrections for obvious errors they made in copying.)



Here's what Mikuni VM's typically are equipped with.


VM needles:




Mikuni TM carbs :


TM NEEDLES
TM24, TM28, TM32, TM34: 5D120 5FL14 5DP7 5F3 5L1 5F12 (lean --> rich) $7.04
TM24, TM28, TM32, TM34: 5DL31 5DP10 5DP39 5E75 5EP6 5F21 5FP17 5FP96 5J11 5N13 $7.04
TM33-8012 PUMPER CARB: J8-5FP96-3 $7.04
TM38 FLATSLIDE: 6DH2 6F9 6DH3 6l1 6DP1 6DH4 6DH7 6DH8 6FL14 6F15 (lean --> rich) $7.04
TM38 FLATSLIDE: 6DJ30 6DP4 6DP17 6F4 6F5 6F8 6F16 6F21 6FJ6 6FJ40 6FJ41 6FL25 6J1 6FM46 6N1 $7.04
TM36 TM38, 39 + 41 PRO SERIES: 6FJ41, 6FM46, 6FJ40, 6DP4 $7.04


Here's what Mikuni TM's typically are equipped with.


The normal #5 series of standard Mikuni needles are suitable for the TM34, and the #6 series is good for the TM36 to TM38 non-pumper carbs.
Listings of all the standard Mikuni needles: Jets R Us (but theirs aren't all genuine Mikuni needles).  NicheCycle and Sudco also have needles.
SUDCO: VM carb parts, Mikuni needles, Mikuni jets, VM slides, TM carb parts.
(The #5 and #6 data on my spreadsheet is even better because it has corrections for obvious errors in copying.)

Each MIKUNI NEEDLE is identified by the letters and numbers stamped on them. Example: 6FH7
6 is the length, which in this case is more than 60mm but less than 70mm
F is the top taper, A-E having less taper
H is the bottom taper (sometimes the NEEDLE has only one letter, in that case it refers to a single taper)
7 is a mfg code not normally used in tuning.

If you have any questions then just email me at 19jaguar75@gmail.com


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