Mikuni VM Needle Nomenclature

Because many forum members employ Mikuni carbs on their Commandos, and because this forum membership is extremely well informed, I was hoping to get an answer to a Mikuni question that I can find no answer to. Try as I may, I cannot find a complete description of the nomenclature employed by Mikuni to describe their jet needles. For example, below is a table of their “6” series needles that provides the following information.

• Needle series
• Needle diameters measured at 10 mm increments
• A and B lengths (A overall length, B where 1st taper starts)
• Taper angles (each alpha letter increases angle by 0.25 deg, e.g., D is a 1 deg taper and H is a 2nd taper of 2 deg)
• AND THE MYSTERIOUS FINAL NUMERIC CODE THAT I HAVE BEEN UNABLE TO DECIPHER

Does anybody know what the final numeric code, e.g., the "17" in 6DP17, signifies? I"ve plotted the needle dia vs needle lengfth data for the five DH series needles and try as I may can see no rhyme, reason or trend in the data that jibes with the numeric code that ends the needle description. Likewise I have done the same thing for the 3 DP series needles in hopes some trend my emerge, but to no avail.

I figure if someone here doesn’t know what the ending numeric code of the needle descriptor means it is likely unknown to all. Thanks for any help you may be able to provide.

This question has gone unanswered with me for decades as well.
maybe like airline fares...

The terms and descriptions I find vary:
- factory reference
- serial number
- reference to the first time a particular needle was produced for a certain manufacturer/model of a bike/engine (OEM)

I think it as posted above.
There is also this post by MattR that shines some light on the subject.

WZ507 said:

Because many forum members employ Mikuni carbs on their Commandos, and because this forum membership is extremely well informed, I was hoping to get an answer to a Mikuni question that I can find no answer to. Try as I may, I cannot find a complete description of the nomenclature employed by Mikuni to describe their jet needles. For example, below is a table of their “6” series needles that provides the following information.

View attachment 19748

• Needle series
• Needle diameters measured at 10 mm increments
• A and B lengths (A overall length, B where 1st taper starts)
• Taper angles (each alpha letter increases angle by 0.25 deg, e.g., D is a 1 deg taper and H is a 2nd taper of 2 deg)
• AND THE MYSTERIOUS FINAL NUMERIC CODE THAT I HAVE BEEN UNABLE TO DECIPHER

Does anybody know what the final numeric code, e.g., the "17" in 6DP17, signifies? I"ve plotted the needle dia vs needle lengfth data for the five DH series needles and try as I may can see no rhyme, reason or trend in the data that jibes with the numeric code that ends the needle description. Likewise I have done the same thing for the 3 DP series needles in hopes some trend my emerge, but to no avail.

I figure if someone here doesn’t know what the ending numeric code of the needle descriptor means it is likely unknown to all. Thanks for any help you may be able to provide.

Click to expand...

17 denotes the length from 1st taper to 2nd taper?????

Bernhard said:

17 denotes the length from 1st taper to 2nd taper?????

Click to expand...

Probably not. Here are plots of the needle diameters vs needle length (from the Mikuni specification) for a few select series of needle having the same taper angles. The final number in each plot legend entry is the “B” value that indicates where the 1st taper starts.

I can’t see any way the final numeric value of the full needle descriptor has anything to do with needle dimensions. Perhaps we’ll have to accept that in fact the “final numeric” has nothing whatsoever to do with needle dimensions and indicates something entirely different as others have suggested above.

I’m officially ending my hunt for information on the significance of the final numeric code of Mikuni jet needle descriptors. A friend mentioned today that he had a 6DP5 needle that he could not find in his current Mikuni needle literature so he called Mikuni and inquired about it. He was informed that it was now called a 6DP17. Sure enough, I looked in some old literature to find the 6DP5 and some more recent literature to find the 6DP17 and in fact the exact same needle diameter profiles are given for both needles. This suggests strongly to me that the final numeric component of the needle descriptor has nothing to do with any critical needle dimensions, so with that the hunt is over.

They never made a range of needles to pick from, it's more like a range of needles that were developed for specific motors. There's just as many that aren't on any list because they came in a Honda or Yamaha or something.

WZ507 said:

Because many forum members employ Mikuni carbs on their Commandos, and because this forum membership is extremely well informed, I was hoping to get an answer to a Mikuni question that I can find no answer to. Try as I may, I cannot find a complete description of the nomenclature employed by Mikuni to describe their jet needles. For example, below is a table of their “6” series needles that provides the following information.

View attachment 19748

• Needle series
• Needle diameters measured at 10 mm increments
• A and B lengths (A overall length, B where 1st taper starts)
• Taper angles (each alpha letter increases angle by 0.25 deg, e.g., D is a 1 deg taper and H is a 2nd taper of 2 deg)
• AND THE MYSTERIOUS FINAL NUMERIC CODE THAT I HAVE BEEN UNABLE TO DECIPHER

Does anybody know what the final numeric code, e.g., the "17" in 6DP17, signifies? I"ve plotted the needle dia vs needle lengfth data for the five DH series needles and try as I may can see no rhyme, reason or trend in the data that jibes with the numeric code that ends the needle description. Likewise I have done the same thing for the 3 DP series needles in hopes some trend my emerge, but to no avail.

I figure if someone here doesn’t know what the ending numeric code of the needle descriptor means it is likely unknown to all. Thanks for any help you may be able to provide.

Click to expand...

6DP17 - the 6 refers to the carb diameter to which the needle is suited. The DP refers to the taper on the needle - D is the leanest rate of taper, F is the richest. The 17 refers to the finish in the needle - some are brass and some are hard anodised aluminium. However it is a long time since I changed motorcycle, so I might not be really up to date. The Mikuni needles in my 34mm Mk2 are 6D - which is the leanest, but I ron methanol at 9 to 1 comp. with a 0.116 inch needle jet. I can afford to have the carburation slightly rich - but I don't - always as lean as reasonably possible. If you need quick throttle response, you might use F needles. With my bike, I feed the throttle on in one controlled confident squirt. I never whack it open.
If you look at the specifications for 1970s Japanese bikes - every one of them uses different taper needles because of their different configurations. If I was using petrol, for racing, I'd experiment with different taper needles.
All of the needles in most common carbs, are usually the same diameter on their parallel part, they are only different below the shoulder. I don't believe in using Amal needles, they have been too wrong to often. It is what works best in your particular motor, which is important, and nobody can tell you that.

It might be important if you are leaning right off on the needle, to make sure you are not metering off the end of it at full throttle. If you do plug chops before and after changing the main jets,you should see a darker or wider ring on the porcelain, down inside the plug. If it doesn't happen, you need to recess the needle jet a couple of millimetres, and meter further down. The end of the needle must remain trapped at all times. Otherwise you might take a quick trip off onto the wilderness with the throttle wide open.