Mikuni VM34 jettingquestion, again....

[the Norwegian]

This winter, waiting for next summer i have decided to take a closer look at the carbs on my xs. The reason for this is that it runs pretty rich on idle and low rpm's and feels a little lean on higher rpm's.
When i dismanteled them i got confused regarding the needle and jets. They are nothing near what i expected them to be, the needle is also in the top position.

The bike is a 78' SE with a 447 engine.

VM 34
Slide: 2.5
Mainjet: 220
Needle: 6F5
Needle jet: 159 P3
Pilot jet: 25
Air jet: 2.0
Riding mainly at 700-1000 feet.

Is it possible that there is a system in this madness, regarding the mainjet-needlejet-needle size?
Or should i change the mainjet-needlejet-needle to 180-190 and 6F9 and 159/P6 and take it from there?

And it would be great if you post what's work for you.

 

[grizld1]

A lot of combinations will work for you. One thing you need is a 10X loupe so that you can read the numbers; Mikuni does not produce a P3 needle jet. As to posting what's worked, for myself and others, that's been done many times, here and elsewhere. But I'm sure you'll get plenty of responses from guys who understand that the information really doesn't apply unless someone writes it out just for you.

 

[XS Cole]

I have VM 34's also and live in the same elevation range you are. I agree with grizld1 you really need to know what you have in the carb. My carbs were supposed to be jetted correctly from supplier however they were way too rich. One of the best guides I found was http://www.amckayltd.com/VM34-36.pdf , the mikuni manual http://www.mikuni.com/pdf/vmmanual.pdf and the sudco tuning manual.

Take a look at this thread: http://http://www.xs650.com/forum/showthread.php?t=21087&highlight=Mikuni+Jetting. XS1961 had some suggestions for jetting which worked out very close to what I ended up with.

Take your time, only do one change at a time and record it. I would put notes in my smartphone. Know what range (throttle opening) you are in such as idle, 1/8, 1/4, etc. Use some kind of tape on the throttle housing and a mark or pin in the throttle grip. Expect take the carbs apart many times.

What I ended up with:
Air Jet: Took it out
Pilot: 25
Air Screw: 1 1/4 turns out
Throttle Valve: 2.5
Needle jet: 159 P6
Needle: 6F9, clip in the #3 position
Main Jet: 180

My engine responds sharply. It is in a good state of tune with just under 2,000 miles on it. Both cylinders have equal compression. The exhaust sound is clean. Have not taken it to redline yet. At night when it is noticeably cooler it may be bit lean just off idle. There is a lot of good information out there and on this forum. Take the time to check it out. I spent the better part of two weeks on mine. Expect to be purchase some brass. While frustrating at times, when it was done I felt it was well worth it and learned a lot.

 

[the Norwegian]

Thanks for the reply's and the links.
I really appreciate all input regarding this in order to find the best starterpoint for further adjustment.

Here is the picture of the "mystery" needlejet: Maybe it calls for further investigation.

 

[XS Cole]

A mystery for sure. It is not listed in the Sudco catalog. I can tell you according to the numbering convention Mikuni uses it would be 3 steps leaner @ .005mm for each step compared to the 159 P6 I am using, the needle while slightly leaner in the beginning seems very rich afterwards and the main jet seems to high.

Have a look at the downloadable Sudco catalog. It has a listing and explanation on the sizing of needle jets and jet needles. http://www.sudco.com/CatalogPDF.html

 

[grizld1]

That's interesting, Norwegian. The only odd-numbered NJ of any series listed in the US catalogs is the 5; but many Mikuni items available in Europe (TM series 34 mm. 4-stroke flat slide pumpers, many replacement parts for the RS series, etc.) aren't offered over here. You might have a look at the offerings at the German distributor, www.mikuni-topham.de . Bear in mind that fuels vary from place to place and North American jetting may not be a very accurate guide for you, so you might want to give Topham's a call and see what they think of the brass you have; they might be better informed of your local conditions. Remember--trying to jet by recipe is largely a waste of time, you need to hear what your engine is telling you and give it what it asks for.

Using a smaller NJ has its advantages in improved fuel economy in the cruising range, and within limits a smaller NJ can be compensated for with a larger main; but selecting a viable needle can be tricky.

There are strong interactions between needle position and the pilot circuit. With the needle raised to #5 it's not at all surprising that the mixture is rich at idle and low throttle.

 

[the Norwegian]

Regarding the fuelquality, the only option here is 98 octane unleaded. I dont know which quality you have in the US?
The temperature are usually between 45 and 70°F during the summer.

I also want to change the xs-pods to something better, what should i look for?

Great links.

 

[XS Cole]

I use premium fuel and in my area and it is generally 91 to 92 octane using the r+m/2 method. Then there mandatory summer blend with 10% ethanol if I remember correctly. Have not had any issues so far.

I am still in the process of working with my jetting. With what I have been experiencing when temps get into the 40° to 60°F range it feels lean in the idle to 1/8 range. Tried tweaking the air screw, did not change much. Above 70°F everything is good. Not sure if the octane rating would mean a changing jetting.

I am running K&N pods with my VM's and have not felt any performance problems. Mounted the filters 2" back using 2 1/4 fuel hose and an adapter I made. Have heard very good responses with the UNI filters. Mostly with the standard CV carburetors.