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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 09:21 #835635

  • Toecutter
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Greetings.... I'm just about going "batshit" crazy on this issue. I have stock Mikuni VM26SS carbs on my 1977 KZ 1000 LTD. The fuel/air mixture screw is located on the bottom of the carb just before the intake manifolds. One article says if THAT screw is located where I mentioned, it is a FUEL screw. If THAT screw is located farthest AWAY from the carb and closest to the air box, it is a AIR screw. Can some tech wizard CONFIRM this? The reason is: if it's closest to the carb,and you "open" the screw up, you let more gas in. If it's located by the air box and you "open" the screw up, you let more air in. So, basically, I cannot figure if I am making the carb richer or leaner if the screw is located closest to the carb! THANKS!
Just acquired a 1977 KZ 1000 LTD after riding Harleys for 30 years. I remembered these bikes back in high school with the Kerker pipe and figured what the heck? A very fun bike!

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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 09:37 #835636

  • kevski
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On the underneath it is the pilot circuit fuel metering screw for low end running, at the airbox side the screw meters air going into the pilot circuit, they both do the same thing, adjusting fuel mixture for low end running, so the one underneath richness mixture the more you undo it, the airbox side screw weakens the mixture the more you undo it.
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 09:46 #835638

  • Mikaw
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The pilot screws under the bowl near the manifold boots are FUEL adjustment. turn in to lean out to richen. Adversely pilots near the air box are AIR adjustment In will richen, out will lean. One you adding fuel the other adds air. They do get adjusted the same as for best idle, but from then after reading plugs at low speed testing you need to know which way to adjust to get mixture correct.

So with yours you are adding fuel (out) or subtracting fuel (in). Tune for best idle and read plugs.
1976 KZ 900 A4
1976 KZ 900 B1 LTD
1978 KZ 1000 B2 LTD
Kowledge Speaks, But Wisdom Listens.
Jimi Hendrix.
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 09:48 #835639

  • zed1015
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On your VM26's the screw is a fuel screw
Turn out to richen and in to weaken..
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 13:26 #835650

  • gd4now
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Everything others have mentioned I agree with, but think there is an important thing to add. To my way of thinking when one talks about rich or lean, it is in relation to the air to fuel ratio. A pilot screw, unlike an air screw, does not have a direct effect on the air to fuel ratio in the pilot circuit. The air to fuel ratio is controlled by the size of the pilot jet and the pilot air jet. By turning the pilot screw out you are not changing the air to fuel ratio, rather you are increasing the amount, of what ever the air to fuel ratio is, to enter the carb throat on the engine side of the slide. Likewise turning the screw in will not change the air to fuel ratio, but will decrease the amount of the mix to enter the carb throat.

On the other hand an air screw does have a direct effect on the air to fuel ratio in the pilot circuit. It controls how much air is allowed to mix with fuel pulled up by the pilot jet. Turn this type of screw in the decrease the air and richen the mix in the pilot circuit, or turn it out to increase the air and lean the mix in the pilot circuit.
1977 KZ650 B1
Pods and Denco header


This is my Z

OLD KAW OWNERS SMILE ALOT
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 15:55 #835657

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gd4now wrote: Everything others have mentioned I agree with, but think there is an important thing to add. To my way of thinking when one talks about rich or lean, it is in relation to the air to fuel ratio. A pilot screw, unlike an air screw, does not have a direct effect on the air to fuel ratio in the pilot circuit. The air to fuel ratio is controlled by the size of the pilot jet and the pilot air jet. By turning the pilot screw out you are not changing the air to fuel ratio, rather you are increasing the amount, of what ever the air to fuel ratio is, to enter the carb throat on the engine side of the slide. Likewise turning the screw in will not change the air to fuel ratio, but will decrease the amount of the mix to enter the carb throat.

On the other hand an air screw does have a direct effect on the air to fuel ratio in the pilot circuit. It controls how much air is allowed to mix with fuel pulled up by the pilot jet. Turn this type of screw in the decrease the air and richen the mix in the pilot circuit, or turn it out to increase the air and lean the mix in the pilot circuit.


Great post. This is exactly correct.

Suzuki GS bikes with VM carbs use both a pilot fuel screw and a pilot air screw. Isn't there some KZ bike that's same? Anyway, VM's are a lot easier to tune than CV type carbs. Gotta love them for that.
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 16:12 #835659

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gd4now wrote: Everything others have mentioned I agree with, but think there is an important thing to add. To my way of thinking when one talks about rich or lean, it is in relation to the air to fuel ratio. A pilot screw, unlike an air screw, does not have a direct effect on the air to fuel ratio in the pilot circuit. The air to fuel ratio is controlled by the size of the pilot jet and the pilot air jet. By turning the pilot screw out you are not changing the air to fuel ratio, rather you are increasing the amount, of what ever the air to fuel ratio is, to enter the carb throat on the engine side of the slide. Likewise turning the screw in will not change the air to fuel ratio, but will decrease the amount of the mix to enter the carb throat.

On the other hand an air screw does have a direct effect on the air to fuel ratio in the pilot circuit. It controls how much air is allowed to mix with fuel pulled up by the pilot jet. Turn this type of screw in the decrease the air and richen the mix in the pilot circuit, or turn it out to increase the air and lean the mix in the pilot circuit.


Make complete sense.
1976 KZ 900 A4
1976 KZ 900 B1 LTD
1978 KZ 1000 B2 LTD
Kowledge Speaks, But Wisdom Listens.
Jimi Hendrix.
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 25 Sep 2020 17:02 #835661

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Here are the differences between and air screw and pilot screw round slide carbs. (see attached)

To the best of my knowledge none of the larger KZs (900,1000) came stock with carbs that have both an air and pilot screw. But yes the Suzuki's (750,850,1000) did come with vm26ss carbs with both screws. Some of the KZ650s did come with VM24ss carbs with both types of screws.
1977 KZ650 B1
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This is my Z

OLD KAW OWNERS SMILE ALOT
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 26 Sep 2020 01:50 #835673

  • zed1015
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All VM26 and 28's on Suzuki fours have both air and fuel screws as stock.
All VM26 and 28's on Kawasaki fours have either one or the other but never both as stock.
Any VM26 or 28 on a Kawasaki four having both screws are originally off a Suzuki.
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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 26 Sep 2020 02:51 #835674

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If he wasn't bat sh*t crazy, he is now.:laugh:

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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 02 Oct 2020 12:11 #836041

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I know i am sorry i even looked at this post for help now...

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Mikuni VM26SS fuel mixture or air mixture screw? Which is it? 03 Oct 2020 10:27 #836104

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A fuel mixture screw affects the overall fuel air ratio at idle because there is a certain amount of air coming past the throttle slide or plate. This air has little fuel in it. This air mixes with the fuel and air from the pilot circuit, which has much more fuel to air than that passing through the throttle opening.

So even if the fuel screw doesnt change the mix ratio in the pilot circuit, by mixing it with clean air from the main carb opening, you can control the overall idle mixture.

Carbs often have more than one way to control the mix at a particular throttle position and this is one example. You can effect idle mix with the screw, or air jet, or pilot jet, and on some carbs, even adjusting the throttle position will affect mix. This happens especially on simple carbs like the tk22. Its easy to see using a wideband.

EDIT: BTW, Mikaw's photo shows clearly how the mixture screw affects overall mix ratio at idle, even if the mix ratio in the pilot system doesn't change.

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