Noob vs. Mikuni VM carbs: Questions

Could be the juice I suppose...:umm:
The settings Ive used on several of my XS's over the years are the same, I cant seem to (or need to ) improve/alter, are thus ;
2.5 slide
185 main jet
25 pilot
6DH4 (or occasionally 6F9)
159 P6 needle
2.0 air jet
I get full redline & 45 MPG .

I might try removing the air jet & see what happens:umm:
 
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Sorry for the steady stream of questions in this thread but with each day I feel I'm getting closer to getting these problems worked out. It seems with every victory I have a loss.

So I went to the garage this morning and I found a pull of gas under my bike. It seems that the bowl is filling up and overflowing. I have my gas off, I did take the carb apart again to make sure the floats were free but same problem. What would cause this. My other carb is drippy to but not as bad. This problem isn't related to jets is it?
 
XS1961, that jetting (and performance) is just about identical to the street baseline for lower elevations on this side of the pond. I doubt you'll see much difference if any with the 2.0 AJ's removed.

vdubplate, you have two problems. First, your petcocks are leaking; if they weren't, fuel wouldn't keep flowing into the carburetors in the first place. Second, either your float valves aren't working or your float level is too low (fuel level too high); if you didn't have one or both of those problems, fuel wouldn't overflow the float chamber vents and wind up on the floor. If this is a new and sudden problem, you could simply be looking at a bit of debris in the float valve seat that's keeping the needle from sealing. Sniff the dipstick; if the oil smells of gas, change it before you start the engine again.
 
Your totally right. I took both carbs apart and thought I might have to adjust that little clip in the middle of the float but thought I should check the fuel line. After prying it off I noticed that the petcock was leaking. Its brand new to. I've ridden this bike 6 time I think. But your right, it was the petcock. My bike has always been leaking oil. I figured if this was an ongoing problem and fuel was leaking into the engine that might be thinning the oil and letting it leak out easier. I smelled the oil but that didn't smell like fuel. Might just have to change my brand of oil to fix that problem. Thanks for the advice. It all helps.
 
XS1961, that jetting (and performance) is just about identical to the street baseline for lower elevations on this side of the pond. I doubt you'll see much difference if any with the 2.0 AJ's removed.
Click to expand...
Thanks Grizld, thats probably saved me a bit more messing about :thumbsup:

Hope you didnt mind my input vdubplate, thought that any extra jetting info might add to your knowledge:wink2:
 
Not at all XS1961, these are the first carbs I've laid my hands on so any extra information is just a help to me. I'm just trying to absorb all I can and get these VMs working correctly. Thanks everyone for all the help!
 
I'm back at it again, my bike is not running at all. Fouling my spark plugs no matter what I do.

@5twins: I am using a 6f9 needle. I'm not sure how to check if I'm using a p series set
 
The "P" refers to the needle jet size, or part of it anyway. The higher the letter in the alphabet, the larger the jet. These sizes are broken down even finer by a number after the letter (anything from 0 through 9), like the P-6 XS1961 mentioned he's running. For use on the 650, guys usually run a P-4, P-5, or P-6. To find out what yours is, you'll need to remove it so you can read the number stamped on the side. You probably won't have much luck tuning these carbs unless one of those 3 needle jets is in there so you better check it. Needle jets cost more than the mains and pilots so many times, guys setting up the generic VMs don't change them out.
 
Ok, I really go into my carbs this morning. Found that I have a 175 main, a 25 Pilot, 6F9 Needle but I cant figure out how to free the Needle Jet. The Needle jet is the long tube inside the carb right? Im looking at the tuning manual and its a little misleading. I'm pretty sure it is. Here are some pics. FYI The smaller tube right of the middle is where the pilot is.

The underside of the needle jet:
photollw.jpg


Top of the needle jet inside the carb:
imageaid.jpg


Here's another side view:


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Needle jet is held in at the top of the slide. Take the cap off of the carb and there will be a spring, remove the spring and there should be a small washer-ish piece that traps the needle in place. Remove that washer-ish piece and take the needle out.
 
The main jet holds the needle jet into the carb body. Once you remove the main and it's washer, you gently tap the needle jet up into the carb body and out. You need to have the slide and needle removed. There is a groove down the side of the needle jet and a peg sticking out the side of the hole. Make sure you align them when re-installing. The jet is just a push-in fit, it's not threaded in. I find a small screw or bolt that will thread into the main jet hole and tap on that. I think it's an M5 size.
 
Cool, I'll give that a try tonight. Where is a good place to pick up some pilot jets for these? I looked at MikeXS and didn't see anything for VM34s. The shipping there is crazy high to.
 
The VMs use the very common Mikuni VM22/210 pilot. You should be able to get them at just about any cycle shop or numerous places on-line. The last time I ordered jets, I went here .....

http://www.amazon.com/l/A3CI6RH32DCFNW?keywords=mikuni jets&qid=1350508343&rh=k:mikuni jets

They have free shipping on $40 or more orders and their jets are genuine Mikuni. I mentioned the needle jets cost more than the other jets. If you need them, a pair of those along with some mains and pilots will put you in the free shipping zone.

Your jetting is very much "in the ball park". I would check your float levels, air screw settings, and needle clip position. It shouldn't be super rich with those jets.
 
I think the problems I am having are with the Pilot Jets. When the bike runs it generally runs great at half throttle and beyond. Its usually when I start the bike and at around 1/4 throttle.
My needle clip position is in the middle so I think that might be fine. I should take a look at my floats and verify that they are where they should be.

They have VM22 jets on MikeXS site. 6 small jets = 10.00 in shipping. Is mike in Japan lol?

I found this great video on Youtube. It gives a quick run through of how the general Mikuni VM series carb functions.
 
The needle jet is an extremely important component in the low-throttle cruising range. If you have the generic Q2 NJ's in place you'll wind up dropping all the way to 17.5 PJ's, and things still won't clean up. You can keep thrashing around and confusing yourself or you can pay attention--your choice.
 
I thought pilot was for idle to 1/4 throttle, needle jet and needle for 1/4 to almost full and Pilot is full. My problems seem to be in the idle to 1/4 throttle so thats why I suggested the pilots need to be changed out.

I got the needle jet out this morning. I have a 159 P6 which is also in the range.
Kind of disheartening as I was hoping something would be way off so I would have a
idea of whats going on.
 
I thought you read the Carb Guide. I harped on this point, but here it is again: THERE ARE NO HARD COMPONENT BOUNDARIES!

Trouble shooting is a process of elimination, so believe it or not, you're making progress--you now know for a fact that your needles and needle jets are correct. Now-- make sure your needle clips are in the middle groove, install fresh plugs, fire up, and check if your engine responds to the mixture screws at low idle. Remember, on these carbs you turn the screws out to lean the mixture. If you have to go out as far as 3 turns for highest idle, drop to 22.5 PJ's. If the motor does not respond, you probably have float valve issues. Remember, I told you that both petcocks AND float valves could be responsible for your overflow problem.
 
Ha I've read the Carb Guide and Mikuni Tuning manual many times, and now have a pretty extensive notebook on points to cover every time I mess w the carbs not to mention a bag or bad spark plugs but like you also said I need to reread probably 100 more times.

My needle clip is in the middle. I replaced the petcock(fixed the overflow problem), checked that gas runs free through the hoses so that shouldn't be an issue. I might try opening the airscrew all the way open and try adjusting backwards from out to in instead of in to out.

I should also note that I have aftermarket long pipes and what others have called cheap pod filters so that could be hampering things. That is most likely not making things better but thats how I got the bike. How could I account for these mods do you think jet wise?

I'm also going to take a new compression test and then redo the valves and cam chain to see if I notice anything thats off.

Also, how can I troubleshoot the floats to make sure they are where they should be? The bowls aren't overflowing anymore so maybe there is no need to check them.

Thanks for al the help. Really appreciate it.



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