Jetting Mikuni VM/TM carburetors
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 and the program will graph the jetting. If you hover the mouse pointer over a cell with a red corner then a message will pop up telling you about that cell. 3/4 down this page 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 on the spreadsheet are all the VM and TM needles and their data.
The 2nd graph to the right is an easier/quicker way to try to change the jetting if you have a good idea of how the jetting is at low throttle and 5/8 slide open (a little more than half throttle). Both methods here first require that you follow my advice for finding the right idle jetting and WOT jetting. Click here to read about this method.
Here's the sequence to follow:
1) I think it's best to choose an idle jet size that gives a good idle running and decent off-idle running. If above-idle is too rich (runs rough when slowly opening it up) then you need a smaller idle jet if you don't want to buy a fatter needle. If above-idle is too lean (doesn't want to accelerate till half throttle) then you need a larger idle jet if you don't want to buy a skinnier needle.
What is happening is that part of the gas for idle actually comes up thru the main jet and thru the needle jet. Yes, the old jetting charts were over-simplified and didn't show the whole truth. Here's one way to tell if the clearance between the needle jet and needle is too much: the idle slide height will be below 1/10th the millimeters of the carb size. And if the idle slide opening is more than 1/8th the carb size then the clearance is too little.
2) Adjust the needle height (via clip position) to be a good compromise between good jetting at 1/4 throttle and 1/2 throttle. The higher the needle is, the farther you have to open the throttle before the neeedle taper becomes active (as it rises above the needle jet) to richen the mix.
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. The most correct method is testing acceleration times to find the jet size that allows the best acceleration. Click here to read about getting the main jet right.
If after steps 1-3 you think the jetting isn't right between idle and WOT then you can use the program to help you find a better needle.
|
Reading the graph: If this program was perfect then the gray line would represent the perfect reference point to compare the calculated jetting to. But it isn't perfect so you need to know beforehand how you want to change the jetting. You should note the #s of the blue line at the different slide openings and then make the changes to the needle and main jet to make the blue graph vary from the previous graph the way you are intending. In this example maybe the engine really is too rich at 1/2 open for trail riding and so the virtual changes would be trying to bring down the 1/2 open # from 1.08 to at least 1.06. |
3) Go to the last sheet (by clicking on the velocity tab at the bottom left of the screen) to find out the carb air velocity you'll need to enter at B7 at the Mikuni VM sheet you are using. If you aren't sure of the transfers duration then enter 125-130 for race engines or 115-120 for street/trail engines. The carb air velocity is at F7 on the Velocity sheet which is 52.5m/s in this example.
4) Enter either the 4/042 main jet # (@ F3) or the N100.64 main jet # (@ F5), 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 ID (code such as 4F15) so the program can find the needle in the listing farther down the same sheet and use its data for the calculations. The needle ID is imprinted near the top of each needle. Data for custom needles or ones not listed on this sheet can be entered at rows 178 to 184. Give any custom needle a unique ID.
6) Enter at C28 the millimeters the slide is open when the top of the needle taper is at the top of the narrow section of its jet (the hole it slides into). You can use the manual method described on the previous page or use the value from D29 (5.8 in this example) after you enter the needle ID at D5 as well as the data for A35, B35, and A37.
|
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 was selected for best power (not plug color) 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 77.4 needs to be entered at I9. The beginning and end of the gray graph are important to set because between those two points the graph follows a predetermined curved line which won't be correct if the start or end isn't correct. Adjust the beginning of the grey graph to show how the off-idle jetting really is. If it is too rich then the engine never sounds/feels right off idle till you crack the throttle more open. If it is too lean then slowly opening the throttle gives a weak response and is even worse going slightly uphill. Change the # at A7 from 1.0 till the graph represents what you feel when riding, showing the low throttle jetting to be too rich, just right, or too lean. |
|
|
8) Reducing Too Rich Jetting
If you want to lower the blue jetting graph from 3/8 to 3/4 slide open then you'll have the options to 1) raise the needle clip # at B35 for a higher # at D29 to enter at C28, or 2) get a different needle with a smaller taper angle. In this case the needle was already lowered the maximum amount and couldn't be lowered any more and so the only recourse was to get a needle with less taper angle, in this case from 2.5 to 1.5 degrees.
When you use the needle height to change the mid throttle jetting but the needle is too far off with its taper angle (too much or too little angle) then you'll ruin the off idle jetting which can be annoying. The correct change is having the proper needle in it.
9) Correcting Too Lean Jetting
If you want to raise the middle of the blue graph then you have the option to 1) lower the needle clip # at B35 for a lower # at D29 to enter at C28, or 2) get a different needle with a larger taper angle, in this case from 1.5 to 2 degrees.
Keep in mind that with any of these changes they can throw off the WOT jetting which you'll then need to correct with a different main jet.
10) 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
You can email this to me with any questions you may have at 19jaguar75@gmail.com
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 more vacuum at the needle jet which pulls more fuel up. Usually the VM18/22/24 have a needle shroud. All the TMS/TMX's have one. 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: NicheCycle.
Click here to download an Excel sheet which gives dimensions on absolutely all the Mikuni needles ever made.
(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:
Series 4 for VM18-24 carbs
Series 6 for VM30-38 spigot, TM38 carbs
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.
NEEDLE JETS
Here's the chart showing the inner diameters of the jet holes:
This shows what needle jet series is used in each model carb and which ones are available from SUDCO. Ones for carbs under 24mm aren't available.
continued...
Below are all the hidden comments for you to see if your computer won't let you see all the comments in the spreadsheet:
If you have any questions then just email me at a57ngel@yahoo.com
Cell Comment
F1 Added 4 stroke air velocity calculator on Velocity sheet. Added section for Keihin PWK29 needles. Changed piston port correction factor on last sheet. Made graph start at 1/8 slide opening. Added power jet ability for 1st three sheets and simplified jetting calculation. Expanded programs ability to handle idle slide heights over 14% of the carb size. Adjusted vacuum effect of air velocity to be less and less as velocity increases. Dellorto sheet has updated formula for idle air screw effects and all sheets have updated formula for effects of intake tract length.
A2 Enter VM or TM
E2 The Mikuni Power Jet uses the same #/diameter system as the N100/604 Main Jets. If you don't have a power jet then leave this entry as 0. Don't forget entering the needed data at B256 if you have a power jet. The hole area of the power jet is listed at J1. The normal range for it is between 1.5x and 2x the area difference between the before and after main jets [area listed at F9].
F2 All VM's and most TM's have the 4/042 main jets. TM's with N100.64 jets: 33/36/40mm (HS40)
G2 feet elevation from sea level of riding area
H2 This is optional for you to be able to convert meters elevation to feet.
I2 This is optional for you to be able to convert Celsius to Farenheit.
B4 Throat diameter in millimeters
D4 The program uses this ID to look up the specs which you can see below row 53.
E4 Enter the code stamped on the jet (all of them are listed at http://www.dragonfly75.com/moto/jettingMikuni.html)
F4 All VM's and most TM's have the 4/042 main jets. TM's with N100.64 jets: 33/36/40mm (HS40)
G4 riding area temperature in farenheit
A6 Enter 1.0 if the 1st half of throttle opening causes clean running with good acceleration when slowly opening the throttle. Enter higher than 1.0 for richer jetting (sputtering a bit at steady low throttle), or lower for leaner jetting (tendency to bog when quickly opening the throttle). This depends on your riding experience. Set this number to show on the jetting graph that the 2nd to 4th graph numbers represent what you feel when testing the low throttle jetting. Ignore the 1st number because that is just off idle and you blow right past that, even when slowly opening the throttle. After changing this you'll need to enter the # of A1 into A5.
B6 This is from velocity sheet, F7 for 2 stroke or F34 for 4 stroke.
C6 1 to 2 turns OUT is the premium effective range.
H6 This is the diameter at the intake bell with the air intake hole (that leads to the air screw) middle being at the outer diameter. The cross sectional area there is more than inside the carb venturi which makes the air velocity there less which makes the suction at that hole less.
I6 Measure the height of the slide bottom in the center. Measure from the bottom of the front of the slide. See drawing to the right of here. This is only necessary with VM carbs.
F7 Carbs other than Mikuni and Keihin have diameters the # of the jet divided by 100
A8 Most carbs have one idle mixture outlet hole under the front slide, and one in front of the slide. 2 holes is a racing design.
B8 RPM at end of pipe powerband.
C8 The cutaway height in the middle in millimeters
D8 the area between needle shaft and needle jet. This should be close to 1/3rd that of the value at I14.
E8 This is a half circle extension going up from the needle jet into the venturi. All TM's have one and VM 18/22/24/30/32/34/36 have one, usually 4 - 6mm tall. VM16/20/26/44 don't have one.
G8 % ethanol in gasoline. Use a $12 ethanol % tester if you don't know. Jetting needs change with different % of ethanol.
H8 If you don't know your idle RPM just enter 900.
I8 Enter the # from I15 here if the WOT jetting is good. Do this before entering anything different from your current "best WOT jetting" setup.
C10 Air screw factor
D10 This is a rating of the needle jet diameter and needle shroud based on their relation to the idle jet and main jet. Less than .8 means the needle jet is too small or the needle shroud is too low (or both). More than 1.3 means the needle jet is too large or the shroud is too high (or both).
E10 Diameter of needle shroud.
H10 This is an estimate of idle RPM just based on engine size.
C11 This is where the beginning needle taper usually is at the top of the needle jet
A12 Needle diameter at its jet at said slide opening
A14 Composite main and needle flow area
A17 The cross sectional area at the exit side of the slide
N17 slide stop screw turns CCW out from fully screwed in.
N18 measured lower slide height. I use mini allen wrenches for this.
A19 The open cross sectional area at the slide intake
N21 slide stop screw turns CCW out from fully screwed in.
S21 total gas divided by RPM
N22 measured higher slide height. I use mini allen wrenches for this.
A25 Measurement starts at the top center tip of the needle
N25 This value is the idle slide stop screw turns out that produces a slide height too dificult to measure which is why you're using this calculator.
B27 Degrees of each 10mm section
N27 This is the value you need to enter at C41.
B28 You can enter the # from D29 or measure this yourself directly as is outlined on my web site. Click "instructions" at E1.
D28 This is the estimated slide open distance from the data entered at A35, B35, A37. Use this at C28 or just measure the distance going by the method outlined on my site. "Increase C28" is the message that appears when the needle is too short for the carb and the mm of slide open you entered. With that error the needle will pull out of its jet completely at WOT causing a sudden richness.
B29 Needle diameter @ 1/8 slide.
J31 Closed throttle needle jet area divided by combined raw flow area at WOT of needle jet and main jet. Larger than 0.8 means low throttle jetting will be too rich. Too large means the needle to needle jet clearance is too big and you need a fatter needle or skinnier needle jet.
A32 Get this # from D column in charts below.
B32 If dual taper then put 2nd taper from D column in charts below.
E32 Idle jet area
A34 Height from bottom of slide to where the needle clip rests on. It is dimension C in the graphic below starting at A211.
B34 Position 1 is the highest and 5 is the lowest. After changing it be sure to enter the value from D29 into C28. If you put washers under the clip then add the measurement of them. If you put washers under the clip then add their depth to 5. Example: adding a 1.2mm washer requires you to enter 6.2
A36 Millimeters that the narrow part of the needle jet is below the level of the bottom of the venturi. On my VM18 it is 1.8mm. On VM38 it is 2.3mm.
B36 Needle diameter reduction every 1/8 slide using 1st taper angle.
B37 Needle diameter reduction every 1/8 slide using 2nd taper angle.
C40 Very Important! If this is wrong then it screws up the rest of the graph. I use small allen wrenches to measure the closed throttle slide opening. This opening has to be so that the engine is smoothly idling. For engine braking you might want this opening to be smaller but for this program you need to find the opening that allows smooth idle. After finding that then you can lower it back to where it was. The accurate range for this divided by carb size is .0305 to .13 (0.13 x carb size).
T42 fraction of the large drops at idle that will be added to the small and medium drops.
H43 % large fuel droplet amount of total fuel at WOT. These contribute to cooling and lubrication.
M51 length of 1st taper
O51 Length of 2nd taper
B53 Measured from its top center tip.
C53 Measured from its top center tip.
D53 This is the full angle, not from the centerline.
A55 measured needle
A61 measured needle
A63 Measured needle
A65 measured needle
A69 measured needle
A210 unmodified velocity at needle
A211 idle RPM
A214 Front slide air velocity
A215 Bell suction factor
A223 pre idle RPM for graphing purpose
J253 squaring factor for the air velocity at the power jet. The range is .7 to 2, with 2 being ideal when the jet outlet is right at the center of the flow area. The closer the outlet is to the outer edge of the flow area, the closer it should be to .7 which is because the speed and suction is lowest there.
A255 Distance from top of gasoline in the bowl (which is normally close to the top of the bowl) to the top of the needle jet. Enter this value if you know it, otherwise leave A256 blank so the computer can use 17mm for TM, or 10mm for VM.
B255 For power valves this is the distance in mm from the bottom of the venturi to the top middle of the highest location of the supply tube. Look at the picture to the right.
B262 idle jetting
A272 L/Vol extended to 0/8
A474 Modified S+M drops at idle
A475 Extended to 0/8
A486 Idle gas/vol
A488 extended to 0/8
A491 Ideal idle mix