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I have a 2 stroke motorcycle, it's Kawasaki KR150. When I try to do some jetting on its carburetor (it's Keihin PWL26), I saw that there's an air screw.

So now I've been wondering about this air screw on 2 stroke motorcycle carburetor, since I'm a newbie to this things I tried to browse on so many sites and asked people. Some of them said that this air screw only affects the pilot jet which is the jet from 0-1/4 throttle, but there are also people and sites that said this air screw affects the jet from 1/4-full throttle.

Which one is true about these theories?

Then if it's true that the air screw only affects pilot jet, how about main jet?

Is there any kind of something like "air screw" that affects for main jet?

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    You'll probably need to add information such as what make of motorbike etc. Otherwise this is unlikely to be answerable
    – Rory Alsop
    Jan 13, 2015 at 8:55
  • @RoryAlsop thanks for your information. I'll add some more information about this question.
    – edwin
    Jan 14, 2015 at 2:43

1 Answer 1

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An air screw on a Keihin PWL26 only effects the idle circuit of the carburetor.

The idle circuit pulls through the pilot jet exclusively. The pilot jet typically can deliver no more than 15% of the overall fuel when the throttle is wide open.

The claim of 15% is based upon the inside diameter of the pilot jet and the inside diameter of the main jet.

The air screw, as you probably know, will effect mixture at idle primarily. If you are having rich conditions higher up or off idle then you need to take a look at your needle and main jet.

3 on the diagram below is your tapered needle, it sits in the throttle valve(#2) or slide, depending on who you talk to. The jet needle is known to primarily effect mid range mixture while the main jet on wide open effects mixture under those circumstances.

There are clips on older models where you can raise or lower the tapered needle. Raising the tapered needle allows for more fuel to enter the venturi and lowering it removes fuel making the mixture leaner. The needle is moving up and down in the atomizer inside the venturi, at the bottom of the atomizer is the main jet. Unless you've done a lot of mods to the bike up to and including a high performance pipe, you should not have to change the main jet. You can just raise and lower the tapered needle until you achieve the proper level of lean and enjoy.

I couldn't find a diagram of your carb, but this is close enough for the discussion.

Diagram of a De'Lorto Carb

enter image description here

1 - air intake

2 - throttle valve

3 - tapered needle

4 - atomizer and needle jet

5 - main jet

6 - starting device

7 - venturi

8 - idle speed adjusting-screw

9 - idle mixture adjusting-screw

10 - starter jet

11 -idle jet

12 - float chamber vent

13 - fuel inlet banjo union

14 - needle valve

15 -float

16- float chamber

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