VM30 tuning...

Hi guys, I'm getting a bit frustrated so any ideas would be appreciated. It take 10 days to get different jets and things so if I can get a good idea on what to try next that'd be great.
So I have VM30s on the kz400, k&n air pods, and new mufflers, and it's still super rich at all throttle positions. Plugs soot up after a minute or two, wether idling, cruising, or WOT. It idles good when fresh plugs, but after a little time it soots up enough that it doesn't start or idle as well.
Carb sets now
main jet 145
jet needle pf4 leanest(top) clip
needle jet q2
slide cutaway 2.0
air screw is out two now, doesn't seem to make a difference where I go though.
What am I forgetting...
My hunch is the slide cutaway is too rich, I've found some combos on the net with the 3.0 cutaway, and with it I'm thinking I'll have to richen up the main, the clip on the jet needle, and the pilot. The last two changes I've tried is the q2 needle jet, and the main to 145. They helped slow the sooting, two minutes instead of 15 seconds, but it's still there.
I have new boots from the carb to the motor, they seem to seal up well, and if they were leaking would that be a lean situation anyway...sorry for the long post but I'm running out of summer riding time!

Pilot jet?
Have you tried a smaller cut away?

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Oh Yeah... Pilot Jet Is At 20. I Havent Would That Be What I Should Try Next?

Have you asked Sudco what their recommendations are? Send an email to the general mailbox telling them you just purchased the carbs and need a baseline. On paper, it looks like you are wayyy lean, at least on the mains. Typically the mains will not foul plugs unless you are full throttle all the time...and really rich. It doesn't even seem like the pilots would be too rich....20's are pretty darn small to begin with. Are you sure your ignition system is up to snuff?

Pilot jet should be 15, which is stock for most of the smaller VM series carbs.
Don't quote me, but I thought the higher the number for the slide means more of a cut away,
which draws more fuel into the engine.
I would opt for a smaller cut away slide.
Your 400 has 200cc per cylinder, that is smaller than the original Kawasaki Z1 at 903cc.
That is 226cc per cyl., with a 28mm VM series carb.
They were jetted around 102 to 106.
The factory vm28 jetting on my KZ1000st is 15 pilot and 110 main.
I would go for something a little richer than that in 115/120 range.
And you might get away without changing your slide.



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Thanks. Never Thought About Trying Sudco. The Only Thing That Isnt Set Lean, On Paper Anyways Is The Slide. Double Checked The Manual And The Higher The Number The Leaner The Mix. It Lets More Air In. I Found One Combo On Kz400 And Hes Running A 25 Pilot, 3.0 Slide, And A 230 Main! I Was Figuring With A Richer Slide I Could Balance The Mix With Leaner Jets, But It Isnt Working.Im Wondering Now If Jumping To A 3.0 WillbBe Too Much. Or Going To A 15 Pilot Would Be betteR And Lean It Out Enogh. r. Ignition System Is Good. I Had To Adjust The Left Exhaust Valve, It Was Too Tight. Thanks For The Ideas.

Very first thing to change is the pilot jet. Don't change a thing until idle is correct.

Second thing to change is your slide cutaway. That is only after you get the pilot where it is supposed to be.

Then start playing with your needle positions to determine what the needle jet should be. Change the needle jet as necessary; don't change the needle just yet.

After all that good stuff is figured out, you can start playing with your main jet.

Once all those are figured out, you can start playing with the different jet needles, air jets, ect.

If you notice, those are progressively in order from closed throttle to wide open throttle. That's how Sudco recommends tuning and it works pretty good, I'd say. Also, if you don't have it yet, you need:

thovda said:

Thanks. Never Thought About Trying Sudco. The Only Thing That Isnt Set Lean, On Paper Anyways Is The Slide. Double Checked The Manual And The Higher The Number The Leaner The Mix. It Lets More Air In. I Found One Combo On Kz400 And Hes Running A 25 Pilot, 3.0 Slide, And A 230 Main! I Was Figuring With A Richer Slide I Could Balance The Mix With Leaner Jets, But It Isnt Working.Im Wondering Now If Jumping To A 3.0 WillbBe Too Much. Or Going To A 15 Pilot Would Be betteR And Lean It Out Enogh. r. Ignition System Is Good. I Had To Adjust The Left Exhaust Valve, It Was Too Tight. Thanks For The Ideas.

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How many turn out are you on the idle mixture screw?
That will tell you up or down.
As for recommendations from others you have to take altitude into concederation.

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thovda said:

Thanks. Never Thought About Trying Sudco. The Only Thing That Isnt Set Lean, On Paper Anyways Is The Slide. Double Checked The Manual And The Higher The Number The Leaner The Mix. It Lets More Air In. I Found One Combo On Kz400 And Hes Running A 25 Pilot, 3.0 Slide, And A 230 Main! I Was Figuring With A Richer Slide I Could Balance The Mix With Leaner Jets, But It Isnt Working.Im Wondering Now If Jumping To A 3.0 WillbBe Too Much. Or Going To A 15 Pilot Would Be betteR And Lean It Out Enogh. r. Ignition System Is Good. I Had To Adjust The Left Exhaust Valve, It Was Too Tight. Thanks For The Ideas.

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I have seen similar recommendations on the KZ400 forum for VM30's and bet that is about what Sudco will recommend. I have VM32's (I know, probably a bit big) that were on my KZ400 when I bought it, and they are jetted at 35 pilots (too rich, but don't foul plugs), 6DH7 needles (maybe a touch rich still) and 180 mains (seem about right).

It might go without saying, but make SURE you are using genuine Mikuni jets. My bike was rejetted when I got it, and the Keihin jets were used (the longer hex head jets). They physically bolt in, but the jetting scale is a bit different and caused me to chase my tail for a while.

There's some good advice there. Start with the idle and get that right first. Q-2 is a very rich needle jet and it's probable that you will end up with something much smaller. See what others are using but it really needs a Bleed type. Yours is probably a 182 series and should be replaced with a 176series. I'd start with a P-4 unless the needle has a very fat root diameter.

PF4 is not a Mikuni needle number. Either it was hard to read an incorrect or it's a Repair Kit part and should be replaced with genuine Mikuni parts. The needle should be something like 5DL31 or 5F3. something like that starting with a number.

I like to get the Main Jet right as the second step, but in reality most bikes will run up to 1/2 throttle with the main jet sitting on the bench because the needle is restricting flow to some degree.

Are the carbs new or were they "pre-loved" and if so what were they used on?

PF4 sounds like it should be 6F4. My VM32 is running that for a needle.

Possible, but that's a long needle for a 30mm small body carb. 6 Series are common on 32/34mm large body carbs. That would be even more curious.

Thanks fellas. Going off of memory trying to remember the jet numbers, and yes it's a 6F4 jet needle. The combo that I found on KZ400 is he's running a P-5 needle jet and a 6F5 jet needle. My whole combo after the pilot jet is way too rich. I'm think I'm going to pretty much order the setup the fella has on KZ400 to try and get this thing close, and start tweaking from there. These are new Mikunis, Mike @ Power Barn has been helping.
pilot jet mine is 20 his is 27.5, I have a 25 so i'm not going to order anything there.
needle jet mine is Q2 his is P-5
jet needle mine is 6F4 his is 6F5
throttle cutaway mine is 2.0 his is 3.0
I know it's best to just change one thing at a time, but I can't even take it for a ride to warm it up after idle and see how it idles with the pilot jet. The only thing I can do now is let it idle with absolutely no throttle for a good 15 minutes, or until I hear it start to run bad and chech the plugs to see how the pilot is affecting the mixture. I'm hoping that if I don't touch the throttle at all the pilot alone should be good and i don't have to try the 15 or 17.5?

Are you floats and needle valve assembly set to the correct heights and springing back like they should be?

And it's just a suggestion (which you already know), but I wouldn't go changing a bunch of stuff at once. You need change one thing at a time or you won't know what caused the different changes.

Also, if you think you can throw in the same brass someone else has and expect your bike to run like it should without putting in the wrench time, you are going to be disappointed with those carbs.

I think a lot of people buy Mikunis and expect their bike to have a perfect dyno curve right out of the box. Even with Sudco "custom" jetting that is simply never going to be the case. Carb tuning requires meticulous tuning and an understanding that each change to a particular setting with have a ripple affect elsewhere in the system. You can use what others have done as a guide, but don't expect that to get you anywhere other than ballpark.

I had a similar problem, all the setting i tried were super roch, it turned out i was running it with choke on thinking it was off.... Then i ended up needing a bigger main jet

Right, one thing at a time. The Q-2 needle jet and the 2.0 slide, I think, are probably causing the super rich problem. Just don't know until you try. Definitely didn't think the new carbs were going to be a slam dunk, but I was looking for hunches from the pros here because it takes ten days to get the new parts. I'm going to get 3.0 slides ordered up today and get a needle jet in the P area. I was just thinking if a P-5 worked for another KZ400D it'd be the best place to start, it came with a Q-8, the Q-2 i put in a week ago is still too rich. Thanks again for the ideas.

I know its frustrating, but hang in there. Not sure if you have read through this, but it's worth your time. http://www.mikuni.com/pdf/vmmanual.pdf

The tuning manual I posted has more information, but that should be you started.

Thanks fresh, I have the manual and it's definitely helpful.
I got to check the idle at lunch. 20 pilots, air screw 1.5 out, and fresh plugs.
It started right away when choked, 3000 rpm idle, and after it warmed for 15 seconds it dropped to 1500 rpm, sounded real good, and then after a minute it started to sound pretty rough, and died after a total of 4 minutes. Plugs were very sooty and exhaust was pretty black.
Changed the air screw to 2.5 turns out, fresh plugs, again started right away to 1500 rpm idle, and same thing, died after 4 minutes, sooty plugs, and black exhause. So I'm going to try the 15 pilot, it just seems to go against what is working for everbody else, but each bike to it's own.
Thanks for getting me to simplify it, one step at a time.

thovda said:

Right, one thing at a time. The Q-2 needle jet and the 2.0 slide, I think, are probably causing the super rich problem. Just don't know until you try. Definitely didn't think the new carbs were going to be a slam dunk, but I was looking for hunches from the pros here because it takes ten days to get the new parts. I'm going to get 3.0 slides ordered up today and get a needle jet in the P area. I was just thinking if a P-5 worked for another KZ400D it'd be the best place to start, it came with a Q-8, the Q-2 i put in a week ago is still too rich. Thanks again for the ideas.

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Not sure what has been done to your bike all around but I have a 74 KZ400 with a 2-1 manifold running a single VM32 carb. I am running the 3.0 slide and just ordered a 6F4 needle and a P5 jet. I also sitting about 3500 ft above sea level