Shop talk... is there a "best" name for this thing? 3 friends all call it 3 different things... frustrating for a noob.

"So you mean the fuel screw"?
No no, the air screw.
No no, the mixture screw.

"OK, F it"

#2 - How do you go about setting yours? Start out? Start in? What's the standard for where you ultimately set it?

#3 - I noticed Kouba sez their screw is only spec'd for a max 2.5 turns out - any more and you should go with a bigger pilot. Out of curiosity, is there any benefit to going with a larger pilot and having your air/fuel mix screw in further (other than it not rattling out so easy)?

 

#1, on different carbs it is different things, and discussion is with people that don't know what they are talking about, or do.
#2 turn it in and out till it runs at the highest idle rpm, then turn it in a little till it is just above slowing rpm.
#3 No, only if you run out of adjustment, why would it be otherwise?

 

#1 http://www.keihincarbs.com/tips/gate.html - basically, if it's 'upstream' (before) the throttle valve/slide, it meters air (and is therefore an air adjustment screw). If it's downstream (after) of the throttle valve/slide, it meters fuel (and is therefore a fuel adjustment screw).

#3 I believe it's more of a 'guide' that if you have to go more than 2.5 turns out that typically means that you probably should be running a bigger/richer pilot jet. My opinion is that if you had it at 3 turns out, I wouldn't bother with the bigger pilot and leave the Kouba at 3 out. If, however, you needed to go, say 4.5 turns out to get it set correctly, then you'd need a larger pilot (or more likely some other problem going on.) I have mine at 3 turns out (snorkel removed, FMF pipe, jet kit) to get the fueling set right.

 

Don't forget the danger of the idle mix screw rattling out on its own if it's set too many turns out..

 

Hey, thanks all. Makes some sense now :-)

 

I never have posted this, but I have a fix for the loose Kouba screw. If I can find the pics I'll post.
Basically I used the spring out of a pen, cut about 1/8th inch off one end and inserted the Kouba screw through it then into the bottom of the carb....keeps mine from rattling out while allowing me to screw it in all the way as well to set it correctly at 3 turns.

 

1. is pretty much spot on from what I've learned over the years. You want richer with the adjustment before slide/butterfly you turn it IN. You want richer with the adjustment after the slide/butterfly you turn it OUT.
   
2. If I remember right, the trick is to turn the screw in until the idle starts to drop, turn it out until the idle starts to drop again, then make the setting half way between. This maximizes the range over which the mix will be "in the ball park". The guys at the shop used to do this idle drop method with a tachometer that read to a much finer digital definition, I don't remember the increments, but definitely accurately within at least 100 rpm or less and for some reason I'm thinking less. It's been over 20 years so I don't remember the exact accuracy. They could do an ultra-fine tune using idle drop and synchronizing carbs.
   
3. Kouba is basing their recommendation on exactly what Richard pointed out. They are not depending on the rider to make any mechanical modifications to the spring. There is another trick though - a blob of silicone seal to hold the needle in adjustment, as I once did on my trials bike's Mikuni.

 

The issue is not with what you want to do, it is trying to figure out what you have!
The screw is coming out of the carb, but is it an air screw or a fuel screw?
It is different on different carbs. Is it labeled.. NO. And the passages inside the carb can go anywhere. Even within the same manf, some screws are air, others are fuel.
One has to know exactly which carb your talking about, what the model # is, and the manf schematic or workup on that specific carb to know for sure what exactly that screw does.

 

Obviously you don't believe we've told you the truth. So here it is from the sources.

In the VM Mikuni and with the TMX it is at the mouth before the slide relative to the direction of flow and referred to as an air screw - it adjusts the air intake for the pilot - open to make leaner. It is also in the similar position on the RS Mikuni and also referred to as an air screw, metering air in. These are the ones I have worked with the most, knowing I needed to turn the screw out to lean out the mix.

But then on the TM pumper it is on the intake side of the slide, the intake port side, and is referred to as the pilot screw, it meters the gas at the pilot jet, open to make richer. Same with all the CVs, like the stock KLX 34mm CVK. I learned the screws were turned out to richen the mix. Quite the opposite of the ones located at the mouth of the carburetor.

Still a non-believer, here is another source that confirms the information given:

Ian Williams tuning. The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

Need a visual? Here's a great video, since both Mikuni and Kiehin are similar:


This information is a mouse click away with a bit of a search.



For what it is worth from MikuniOz:

Here’s our simple outline for basic tuning to get you started.

1) Idle Circuit:


With your idle circuit, start at 1 1/2 to 2 turns out on the mixture screw and then wind your idle screw back to give a very slow idle whilst at operating engine temp with the bike upright. Follow this with 1/2 turn checks winding in one direction every 10 seconds till the engine starts to stumble, at this point then wind back in the opposite direction counting your half turns till it stumbles again. Wind back ½ the number of counted turns to the centre point. Then count the turns from this point to bottoming out, if you take less than 3 and more than 1 complete turns to achieve bottom, the jet is correct and you simply wind back to the centre point. Job done.

If the jet is wrong, either having no stumble in one direction or the centre point being out of the 1-3 range this will let you know that a jet change is required and weather you need to order richer or leaner jets.

Nothing else other than the pilot jet has an effect at idle. Remember if you don’t wait 10 seconds for the engine to catch up every half turn at a very slow idle this tuning process will not work. Some engines will actually have two size jets that may fall within the 1-3 spec in which case see which feels better when riding, generally the richer one.

 

Not trying to be argumentative, but I don't have a VM or a TMX.
And to add to confusion, your comment speaks of "back" & "front" of the carb. Don't really know what part is the "back". Seems like the front should be the direction all the screws are facing.
Hopefully you are understanding the issues of which I speak, I don't think half the people discussing these screws know of what they are talking about and take it for granted they everyone understands the manf & model of the carb of which they are discussing, as generally they really don't mention this information.
As example the OP KLN-VN didn't mention any of this info.

And another confusion, your reference to Mikuni states the air screw is near the intake & Ian Williams comment states it is at the 'back' (where ever that is) and the fuel screw is at the 'front'. Maybe he is talking about the 'back' & 'front' of the moto, but what if you have the carb on a midget or gokart, then it would be very confusing.