TOPIC:

The needle jet is marked but you have to remove it to see the number as it's on the widest part which is hidden by the carb body.
It taps out toward the float chamber after you remove the needle jet tube etc.
Sizes listed are 0.4 - 0.5 and 0.8 depending on country and model.
Unless the needle jet is damaged you are most likely wasting your money on new ones
If your original Mikuni needles ( 5dl31 = Keyster Y63 ) are not damaged i would re-fit them.
You need to go back to stock and only change one thing at a time otherwise you won't find the cause of your problem.

Ok so you have changed carb bodies, sets and may have interchanged some parts? There should be an ID stamped on the flange below the choke lever or on the same flange on # 4. See attached pic.

If you post the ID from both sets I may be able to tell you more about the jet needles and the needle jets that were stock in the different sets. As to the needle jet that is an oh not a zero. Some where I have a chart of them, when I find it will post.

Here is the info I have on needle jets and jet needles.

The needle jet is the main fuel passage to the main bore (venturi) of the carburetor. Depending on the inside diameter of a specific needle jet, this will also effect the function of the needle. therefore, needle jets and jet needles act together as the main system in controlling the amount and mixture of the fuel which is drawn in mid-range (1/4 - 3/4) throttle operation.

Needle jets are available not only in types, but by series and size. Mikuni series numbers for style of the needle jet. This series includes not only the physical fitment dimensions but also the nozzle profile, and the atomization pattern. The sizes (inside diameter size) within each series are listed according to a letter-number combination. The letter shows the inside diameter size in increments of .50mm. See the Mikuni Needle Jet Sizing Chart below for a better understanding


Mikuni Needle Jet Orifice Diameters in mm
-0 -1 -2 -3 -4 -5 -6 -7 -8 -9
N 2.550 2.555 2.560 2.565 2.570 2.575 2.580 2.585 2.590 2.595
O 2.600 2.605 2.610 2.615 2.620 2.625 2.630 2.635 2.640 2.645
P 2.650 2.655 2.660 2.665 2.670 2.675 2.680 2.685 2.690 2.695
Q 2.700 2.705 2.710 2.715 2.720 2.725 2.730 2.735 2.740 2.745
R 2.750 2.755 2.760 2.765 2.770 2.775 2.780 2.785 2.790 2.795
S 2.800 2.805 2.810 2.815 2.820 2.825 2.830 2.835 2.840 2.845
Y 3.100 3.105 3.110 3.115 3.120 3.125 3.130 3.135 3.140 3.145
Z 3.150 3.155 3.160 3.165 3.170 3.175 3.180 3.185 3.190 3.195
AA 3.200 3.205 3.210 3.215 3.220 3.225 3.230 3.235 3.240 3.245
BB 3.250 3.255 3.260 3.265 3.270 3.275 3.280 3.285 3.290 3.295
CC 3.300 3.305 3.310 3.315 3.320 3.325 3.330 3.335 3.340 3.345



Jet needle Selection Guide:

Often we are asked what needle to use when an OEM needle is no longer available. The way Mikuni Needle numberings work is:

The first number is the length series. Lets take the series 4 needle. It could be 40.00-49.99mm in length. In turn a 5 could be 50.00-59.99mm in length.

The second and sometimes third characters being letters describe the taper from the clip end to the tip in 15minute (1/4th of a degree) increments. For example "B" is 30 seconds, and "E" is 1 degree, 15 seconds. Therefore a "BE" would be a taper starting out at 30 seconds (1/2 degree), followed by a second taper of 1min 15sec.

The last digit be it one or two characters is the suffix or revision number. Example. 4D3, and 4D8 are the same style needle, similar in length and taper. However the taper may begin higher up on the needle than the other. This would make a smaller tip diameter, or a shorter needle. If shims, or clip positions were incorporated on one needle it is possible that it may effectively perform like the other. For this reason we sometimes suggest ordering a 4D3, or 4D8 for example if an OEM application calls for a 4D5 which is not available, instead of a 4E?

Lastly there is sometimes a number designated by OEM's and this designates the clip position. Example: 5DP3-4. However this "-#" designation is not part of the needed part number.
One last note: A suffix number has the following unknown variables between one needle and another. Exact length, Major OD, Point of begining of taper in reference to clip position. Without physically measuring them there is no way to determine these characteristics just by looking at the part number.

a b D-1 D-2 D-3 D-4 D-5
4D3 50.3 25.3 2.511 2.511 2.421 2.253 2.100
4D8 50.3 22.0 2.519 2.519 2.381 2.211 2.000
4D10 Not available

4D20 Not available

4DG6 50.3 24.0 2.518 2.518 2.405 2.118 1.850
4DG8 50.3 24.0 2.518 2.518 2.405 2.118 1.850
4DH7 50.3 23.0 2.518 2.518 2.386 2.090 1.790
4DH11 Not available

4DH22 Not available

4E1 50.3 28.0 2.515 2.515 2.345 2.127 1.924
4EJ4 Not available

4F6 50.5 25.3 2.514 2.514 2.400 2.145 1.876
4F10 50.2 24.5 2.513 2.513 2.385 2.1?5 1.?7?
4F15 50.3 26.5 2.512 2.512 2.400 2.120 1.881
4J6 50.3 24.2 2.513 2.513 2.233 1.827 1.472
4J11 41.5 21.3 2.512 2.506 2.188 1.776
4J13 50.2 24.0 2.513 2.513 2.230 1.800 1.400
4L6 50.3 24.5 2.515 2.515 2.178 1.660 1.190
4L13 45.1 25.0 2.51? 2.51? 2.339 1.842
4P3 50.5 25.0 2.510 2.506 2.436 2.284 2.122

Series 5

a b D-1 D-2 D-3 D-4 D-5 D-6
5C4
5D1
5D5
5D6 59.3 27.5 2.515 2.515 2.460 ? 2.290 2.120
5D120 59.1 ? 28.2 2.520 2.520 2.429 ? 2.311 1.???
5DL13
5DP2
5DP7
5DP22
5E3 58.0 27.4 2.518 2.518 2.419 2.??? 1.???
5E13
5EH7
5EJ8
5EJ11
5EJ13
5EL8
5F12
5F15
5F18
5F21
5FJ9 59.2 35.0 2.517 2.517 2.517 2.364 2.021
5FJ10
5FL7
5FL11
5FL14
5FP8
5FP17
5J6
5J9 N/A
5L1
5I4

Series 6

a b c D-1 D-2 D-3 D-4 D-5 D-6
5DH21 N/A
6CF1 N/A
6D16
6DH2
6DH3
6DH4 62.3 25.5 2.520 2.520 2.440 2.258 1.915 1.575
6DH7
6DH8
6DJ30
6DP1
6DP4
6DP5
6DP10
6F3
6F4
6F5
6F8
6F9
6F13
6F15
6F22
6F16 69.1 30.0 18.7 2.518 2.485 2.485 2.372 2.104 N/A
6FJ6
6FJ11 N/A
6FJ40
6FJ41
6FL14
6FM46
6H1
6J1
6J3
6L1
6N1

Series 7

a b D-1 D-2 D-3 D-4 D-5 D-6 D-7
7DH2
7DH3
7DH4
7DH5
7F6
7F7

I'm currently fighting through the same issue on my 78 650.
With 110 jets and no air filters on, it ran pretty darn good.
Once the air filters get put on (uni filters), I get the same sputter in the mid to upper range.
I'm currently waiting for another set of main jets to arrive.

Dear Gents
Yesterday i have dismantled my complete carbs again. Primary, to send Plungers for overworking of the seals to Zed and secondary, to see which Jet Needle is integrated.

Carbs which i have used and which ones creating that WOT Problem:
Serial: 106 9A
Y63 Keyster Needle
Needle Jets: 258-O4 (Looks not nice - seems, that someboday has also dismantled them using something strange....)

Carbs which Comes original with that bike
Serial: 102 2A
5CN15 Needle (?)
Needle Jet: 258-O8 (looks used, but not damaged)

So i think i have found the problem:
Y63 Needle (which should be same as Mikuni 5DL31) in combination with Needle Jet (little damaged) 258-O4.

So i will put that original carbs (102 2A) with original needle jet 258-O8 using all other parts (102.5 Main and 15 Pilot which should be complete stock) and Y63 Needle at second richest groove) together and test it with the new K&N Pods.

Question: Or should i start with 17.5 Pilot and 110 Main)?

The only reason why i have not used These original carbs was that the throttle slide in Carb3 stucks (corrosion). After removing and using another slide there was a Little Play (slide and carbwall) because i had to polish away a Little the pitting. Is Play in the slides leading to a Problem?
Many thanks and greetings!

Leave the pilots stock and go 107 to 110 on the main.
Fit air jets.
The loose slide should not cause a major problem unless it is excessive low down at the carb bore/venturi allowing air to draw around the slide rather than under and not pick up fuel.
If that were the case you may show lean on that cylinder but the slide would have to be VERY loose..
Play further up in the slide bore would not be such an issue and with good throttle shaft seals and carb top gasket no additional air should be able to enter but with the reduced restriction of the pods that is even more unlikely to happen than with the airbox.

zed1015 wrote: Leave the pilots stock and go 107 to 110 on the main.
Fit air jets.
The loose slide should not cause a major problem unless it is excessive low down at the carb bore/venturi allowing air to draw around the slide rather than under and not pick up fuel.
If that were the case you may show lean on that cylinder but the slide would have to be VERY loose..
Play further up in the slide bore would not be such an issue and with good throttle shaft seals and carb top gasket no additional air should be able to enter but with the reduced restriction of the pods that is even more unlikely to happen than with the airbox.

Ok, i will start with 15 Pilot and 110 Main bcs i have ALL main jets up to 127.5 EXCEPT 105 and 107.5
Y63 Needle with Clip on second leanest Position.
Sure, i will add your air jets.
Remark regarding installing air jets: Do not know why but i have used screw lock medium during Installation. Was not nice to remove them. Two jets are now on the head Little damaged. But afaik will that not affect the function...

Keep you updated if the bike now RRRROOOAAARRRSSS ;-o
Cheers

KZer78 wrote: I'm currently fighting through the same issue on my 78 650.
With 110 jets and no air filters on, it ran pretty darn good.
Once the air filters get put on (uni filters), I get the same sputter in the mid to upper range.
I'm currently waiting for another set of main jets to arrive.

I´ll keep you updated if the K&N are worth their money and if my new setup will work.

Womtong wrote: Remark regarding installing air jets: Do not know why but i have used screw lock medium during Installation. Was not nice to remove them. Two jets are now on the head Little damaged. But afaik will that not affect the function...
Cheers

The damaged head will not affect their function but you should not use any thread lock.
They only need screwing in and lightly nipping up.
They will not unscrew by themselves.
You are safe to deepen the screw slot with a small junior hacksaw blade or similar by a few millimeters.
Just be wary not to block the jet orifice with swarf.

Both of those set of carbs are rather alike. The only difference I am aware of is the jet needle and the needle jet. Both have a 1.5 slide cutaway with a notch on the engine side of the slide. Both of the stock needles were set the 4th slot (from the top).

If the aftermarket needle you have is supposed to be equal to the 5DL31 then the use of the O-8 needle jet would be closer to stock than the O-4 would. If on the other hand you make use of the 5CN15 needle the O-4 needle jet would be closer to the stock setting.

Regarding correct way of insert/extract the Needle Jet - any recomendations how to do that professionally to avoid damages on the Jet/Carb Body?

Thanks and Cheers
Chris

The needle jet taps out toward the float chamber from the venturi side.
Remove the needle jet (emulsion) tube and slide etc and use a suitable size punch that will fit inside the needle shroud that protudes into the venturi.
I use a piece of aluminium bar turned down at the end .. The aluminium is softer than the brass so won't damage it.
To refit you can either reverse the process and use a punch to tap it back in from the under side or use the action of screwing in the needle jet tube to press it home.
A wooden tooth pick or similar slipped in the needle jet is a good way to guide the jet straight.