Mikuni VM-22 main jet thread size.

#1
I recently ordered a 22mm Mikuni (this one) and a couple of diffrent sizes main jets (these) and a Pilot jet (here).
Package arrived this week in good order, and the extra ordered main jets, don't seem to fit this carburator (slightly different thread-pitch)
When I measure the main jet thread size that came in the carburator, it is M5x0,8 thread, with a 8mm round head.
The extra ordered main jets have a 6mm round head and their thread size is M5x0,75.
The pilot jet does fit and this is M4x0,7 thread.
Is this a common problem? Or does anyone just screw in the main jets? And screw up the thread...
According to the jet description, the jets should fit.
Main jet used in Mikuni RS and HS carburetors, and small body Mikunis.

Fits Mikuni Small Body 22mm thru 26mm Round Slide Carbs. Also used in 33mm Pumper Carbs
Small Body Carbs are used in OEM applications. Also referred to "Chinese" Mikunis (though not always true) or new style Round Slides.
These are also what is typically used in Mini-Bike & karts due to the lower cost

The carburator looks very nice.
There's a very little hole on the intake manifold site, under (and outside) the rubber O-ring gasket. What's it's use? If bolted to a manifold this small hole will be blocked. If I would make a manifold where this hole enters, then it might leak, because the hole is outside the 0-ring gasket.
 
#2
If I spent money at a vendor and their stuff didn't fit, first thing I'd do is call them. The jets should fit correctly. I note that your vendor has a highlighted bit that says, "make sure picture matches your jets." Well, the main jet photos on your vendor's site don't show what I need to see to be 100% on the answer. This is the jet that should fit your carb. Note that area with no threads. I assume the pilot jet is fitting. I buy all of my jets from that Ebay vendor, and have had good service from them.
The small hole is for the air bleed. It allows you to introduce air into the front to adjust idle mixture. Don't cover. O ring should prevent it.
 
#3
Hello,
Thanks for the answer.
First to clear something about the air bleed; If I don't block it, with the manifold, isn't it going to collect dust, because it's outside the manifold (unfiltered air)? If the airbleed hole should be open inside the manifold, then the O-ring should be around the airbleed hole, too. Right?

"make sure picture matches your jets." ... Well, as I didn't have the carburator yet, when I ordered the parts, I couldn't make sure the picture would match my jets. Therefor I read the description carefully and my conclusion was that the jets should fit the ordered carburator. Perhaps my mistake... And as I didn't receive an answer to my send email to them, I asked the question here.

In the link you provided it doesn't tell which thread pitch it is. It says Main Jet N102.221/120 and according to that info, the pitch is M5x0,75. Picture
These are exactly the main jets that I received and don't fit. They have the Mikuni stamp (the 2 squares) in it, too.
The main jet that came in the carburator has 8mm round head and M5x0,8 pitch (not M5x0,75 pitch). It's a small difference, but enough to make them stuck after I turn them around 1-2 times.
The Pilot jet fits and looks and measures the same. (thread is M4x0,7 pitch)
 
#4
Edit: I could not find ANY Mikuni jets M5 X .8 thread.

I do see a VM11/22 jet with a M6 X .8 It almost seems like you got a different carburetor, or that they sent you Keihn jets.

The jets you ordered appear to be the same one's I referenced, and the same ones I used. M5 X .75 (Chinese Mikuni VM22)

@ole4 can you measure the thread and pitch for this guy from the Netherlands on your Chikuni jets? I only have SAE.

As far as bleed hole, it does not seal with O ring, and it is open. It it used to meter fuel in the pilot jet.
 
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#5
Hello,
I just received a message from the vendor and they never had issues before and they did pull a couple of carbs and did find the mismatch...same jets as they always used.....carbs seemed to have changed....

So the problem is not the jets, but the "needle jet thread" in the carb. It is M5x0,8 pitch, while Mikuni jets are M5x0,75 (the jets I ordered).
I think and I'm quit sure that the jets you reffered to on ebay are exactly the same as the main jets as I received them.
 
#6
Hello,
I just received a message from the vendor and they never had issues before and they did pull a couple of carbs and did find the mismatch...same jets as they always used.....carbs seemed to have changed....

So the problem is not the jets, but the "needle jet thread" in the carb. It is M5x0,8 pitch, while Mikuni jets are M5x0,75 (the jets I ordered).
I think and I'm quit sure that the jets you reffered to on ebay are exactly the same as the main jets as I received them.
HAHA! Well, there you go! Will they supply the replacement jet holder so you can use the correct jets? I think you have a Keihn jet holder in there, which was used on the Chinese motorcycles. That explains how this part got into the supply chain.

Good job getting it figured out! Note my O ring answer above.
 
#7
Here are the jets for the majority of the chikuni carbs



The following pics were from a member who was asking about the jets not fitting his carb and the pilot was different as you can see in these pics. IS

1547678033559.png
Here are some of mine both large head and small head and I interchange them without issue both have same thread pitch however, I did not measure the pitch. Next trip to shop I will check pitch.

DSC01406.JPG DSC01407.JPG DSC01407.JPG

Dave is correct little hole below rubber o ring and next to air bleed adjustment is how air gets to idle adjustment. Most of the manifolds made for the chikuni do not cover the hole.
 
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Littlebear

Active Member
#8
I'm not of much assistance, but off to drawer to find that bag of jets I had the same issue with. Since I had more I never looked into it. Has me curious now......:rolleyes:
 
#9
Ole4,
Thanks!
I'll be making my own manifold for the carb so that's why I was curious as how to make it. I'll make the manifold in such a way that the small hole will be located outside/below the flange.

That pilot jet (2nd pic) does look very different...
The pilot jet that came in my carb looks like the ones in your first pic. And the extra ordered pilot jet fits perfect. No problem there.

The extra ordered main jets have the smaller head (6mm diam) and have M5x0,75 thread (Mikuni). And they have the Mikuni stamp (square in square) stamp on them. Length is same as the large head jet that came in the carb.

Picture shows carb, the needle jet/jet holder taken out, main jet and in the caliper the pilot jet (has Keihin star stamped on it, with number 38).
20190116_232829.jpg
Most likely the jet holder has wrong thread or jet holder itself is incorrect for the carb. Some Chinese might have made a mixup, while assembling it.
 
#10
@ole4 he has a carburetor with the "jet holder" (they call it a needle jet) that is set up for the Keihin carb which is used throughout China on their domestic cycles. It's easy to see how a Chinese Mikuni could have ended up with these "holders" and got shipped to NR for US distribution.

Note that Needle Jet VM15H1/17-O-0 is cited as for use in Mikuni Clamp On carbs, but it looks exactly like what is in the Chikuni.
Link

Thanks for the great info as always Ole!
 
#11
As the jets are okay(M5x0,75). But the jet holder in the carb has wrong thread (M5x0,8) in it, I will modify the jet holder to accept Mikuni jets.

20190117_191133.jpg
This is from the carb. Large round head jet, jet holder, needle jet, needle. Also a Mikuni jet I ordered with the smaller round head.
Someone that can confirm that this jet holder is the right one for the carb VM-22 3847? Except for the threads...
 
#13
The large head jet is the one that came in the carb. That is why it is different. When the jet holder is modified i will not be using that jet.
Thanks for the pics.
 
#16
You can drill original jet to .050 to start That is around a 120 mikuni jet size, I run 125 on a tec 5hp flathead and 125 or 130 on my GX200 clones. It should run OK with the 120 but may be on the lean side, you could then go up if its lean to .0520
 
#17
You can drill original jet to .050 to start That is around a 120 mikuni jet size, I run 125 on a tec 5hp flathead and 125 or 130 on my GX200 clones. It should run OK with the 120 but may be on the lean side, you could then go up if its lean to .0520
Ahh... When is Trump converting to the metric system? Kidding aside; I can convert the numbers to mm.
0.050=1.27mm
0.052=1.32mm
I will be having some fun getting the carb tuned in (when i get to it), correctly. I did found these articles;
http://www.iwt.com.au/mikunicarb.htm
http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html
https://www.mikunioz.com/tuning-tips-and-manual-downloads/?v=796834e7a283
 
#18
Hello,
I just wanted to let you know that NR racing came back to me on the issue and they are sending me some new jets. Nice.
In the meantime 2 more carburators arrived, which I've ordered in Europe. These carbs look the same and the Mikuni jets fit. There even were Mikuni marked jets installed #95 and #15 pilot. On the intake flange is also a stamp saying: 22 3847. The other carb has 5VLB 32 marked on there.
Case closed, thanks for any help.
 

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