TUNING (KZ400 - 2-1 VM34)

Many of you have helped in my tuning process already, and I thank you for that.

Now that I have the right size carb, the Mikuni VM34, I am getting REALLY close. But I could still use a little input.

3 areas of adjustment on this carb are the pilot jet, the needle, and the main jet. I started out really rich so I kept going down on everything until it was lean. Then wen't one up. Here's where things get weird...

Right now, at low-mid RPM's it is acting lean at time. Not all the time, but most of the time. Leading me towards the pilot jet and/or the needle. More than likely the needle because it's not lean right off of idle, but slightly above.

The strange thing is that my plugs are still pretty black. But I can't really feel anywhere in the power band where it is rich.

I guess if I narrow it down to which jets are lean (like the pilot and/or needle) then by the process of elimination I could assume that the main is rich?

Thoughts?

That is not an uncommon problem. What I find is, there is a short section where the needle is not tapered (or in the 550's, tapered too little). When you are just off idle, you will be on that section of the needle. Since it is not adding fuel as you just start to open the throttle, it goes drastically lean for that tiny bit. However, your idle and any position beyond that can still be very rich.

The reason it is like this, is because you will do all of your light cruising at the small throttle opening, and there will be a benefit in fuel economy. It won't affect power, since you won't be in that region while riding hard. The problem is, there is a very tricky balance you have to achieve, and your setup is just a little bit too lean in that region. The solution is to raise the needle slightly (a full step will usually be too far, but you can try that first), and compensate down on the main jet and pilot jet and mixture screw.

I'm not familiar with how you raise the needle on the VM34, but if you put a shim under the needle clip to raise it, make sure the needle is not mechanically bound tight. That can wear the orifice the needle goes into.

Using a wideband O2 sensor makes it easy. It will take more time if you are just test riding and looking at plugs etc. Also pay attention to the color of the tailpipe.

loudhvx wrote: That is not an uncommon problem. What I find is, there is a short section where the needle is not tapered (or in the 550's, tapered too little). When you are just off idle, you will be on that section of the needle. Since it is not adding fuel as you just start to open the throttle, it goes drastically lean for that tiny bit. However, your idle and any position beyond that can still be very rich.

The reason it is like this, is because you will do all of your light cruising at the small throttle opening, and there will be a benefit in fuel economy. It won't affect power, since you won't be in that region while riding hard. The problem is, there is a very tricky balance you have to achieve, and your setup is just a little bit too lean in that region. The solution is to raise the needle slightly (a full step will usually be too far, but you can try that first), and compensate down on the main jet and pilot jet and mixture screw.

I'm not familiar with how you raise the needle on the VM34, but if you put a shim under the needle clip to raise it, make sure the needle is not mechanically bound tight. That can wear the orifice the needle goes into.

Using a wideband O2 sensor makes it easy. It will take more time if you are just test riding and looking at plugs etc. Also pay attention to the color of the tailpipe.

The needle in the VM is actually adjustable. It has a clip that you can move up and down. It is currently on the middle of 5 settings.

You're thinking that I should go richer (up), like one clip, on the needle, but leaner on both the main and pilot?

Also, what do you mean by "wideband o2 sensor?" Is this a tool that I could rent from O'Reilly or something that will make my life much easier?

Yes, raise the needle one clip, but leaner on pilot and main and mixture screw. Usually one clip will be too much. They are usually 1 mm increments. So you might have to use a half-step shim washer, or whatever it takes to get the needle to move a half of a clip.

But realize, jetting is not really easy to diagnose over the internet. I can only make guesses to what you need.


A wideband O2 sensor is usually not a rentable tool. You have to weld a bung into the exhaust for the o2 sensor, and hook it up to a micro-controller with a gauge or logging system.

kzrider.com/forum/3-carburetor/307483-in...-sensor?limitstart=0

The second page of that link shows some AFR results for steady throttle positions. Notice it starts relatively rich at idle, then goes very lean, briefly, as the throttle position is off idle (cruising), then goes back to slightly rich for all higher throttle positions.

CV carbs are a little trickier because the slide position is a big unknown for any given throttle position.

Another side note, in case you didn't know... when you open the throttle, the mixture goes rich until the engine catches up to the throttle position, and as you close the throttle, the mixture goes lean until the engine slows down to the throttle position. That's why if you ride the bike like a bat out of hell, the small lean-hesitation disappears. It's only when you do some slow cruising that you notice the hesitation.

Thanks for all of the great information. I will see if I can find a o2 sensor that will work for me.

Also, the VM34 is not a CV carb. It is a round slide carb.

I understand what you're saying about how you ride it. When riding like a "bat out of hell" I hardly notice any lean hesitation at all. When cruising at a few thousand RPM, and then letting off the throttle is when I notice it. Or when revving it slowing (from a stop behind a slow ass car) it feels lean if I shift at low RPM, but not at higher RPM's.

treitz wrote: Thanks for all of the great information. I will see if I can find a o2 sensor that will work for me.

Also, the VM34 is not a CV carb. It is a round slide carb.

I understand what you're saying about how you ride it. When riding like a "bat out of hell" I hardly notice any lean hesitation at all. When cruising at a few thousand RPM, and then letting off the throttle is when I notice it. Or when revving it slowing (from a stop behind a slow ass car) it feels lean if I shift at low RPM, but not at higher RPM's.

Yeah, I was just mentioning the CV to support manual-slide combo being way easier to tune.

With a manual slide carb, throttle control is more important when the mixture is correct. When it's lean, you can usually crank them open as fast as you want. As the jetting is set richer in the lower areas, you may have to control how fast you twist the grip... which is normal (and basically what the CV does automatically).

No eBay APP ID and/or Cert ID defined in Kunena configurationThis is the setup I use. The analog gauge is better, in my opinion, since you don't have to stare at it to see what the AFR is doing. When you are doing maximum acceleration tests, you only need to see the gauge out of the corner of your eye, so you can watch where you are going. It also shows a better representation of how fast the AFR changes, where the digital ones just jump numbers.

Way cheaper now. "Buy it now" $208, I paid closer to $350, I think.

Thanks again for all the input. This looks like a pricey and fairly permanent solution. I will look into it and if it get's too complicated, I'll consider it.

Update: Went one size down on my pilot, and went up a notch on the needle. Noticed a slight improvement while riding. Less of a stutter at low throttle/slow acceleration.

Haven't had a chance to check the plugs yet. Will do that tonight.

Brand new plugs. Hoping they are black as hell like the rest of the plugs I've used in it!

Only issue I am still having is that it runs pretty lean when it's cold out. Between 40 and 60 degree's in the morning it run leans. Mid day to evening when it's 80+ it runs 10 times better.

Update. Went back up on pilot to where I was. It was too lean. Middle setting on the needle appears to give the best results.

Still occasionally lean at low RPM's, but my plugs are still really black. Tempted to lean out the main one to see what happens.

Why the hell does it feel and act like I'm running lean, but my plugs are still black?

I am running stock Sportster mufflers which are pretty restricting. Do you think opening up my pipes would offer a improvement?

Current specs (as much for my own documentation as for you):
Carb: Single VM34
Pilot: 25
Needle: Middle clip
Main: 145
Air filter: POD
Exhaust: 2-2 sportster slip-on's w/ header wrap