Does needle straight diameter affect idle mixture?

[Davehuge]

Here's something that's been bugging me for a while...

On the Keihin PWK carb, does the diameter of the straight portion of the needle affect the mixture of the idle circuit (pilot jet and air screw)?

Thanks, Dave.

[Jakobi]

Well, its a loaded question Dave.

The answer should be YES!

But it depends on where the idle screw is (height of the slide) and also the length of the straight section on the particular needle you are using.

Ideally at idle the straight section will be sitting in the nozzle/needle jet, restricting fuel pickup into the body. As you open the throttle and lift the slide the needle also rises in the nozzle and moves on to the tapers and increases fuel supply.

The pilot and airscrew are a totally independent circuit, however the combined effect still influences the idle mixture; therefore the diameter does affect this.

Times that it might not though, are usually when using a needle such as an N1EF which has a very short straight section before it leads into its taper. With this needle you can easily wind the idle screw in and lift the needle high enough to end up on the taper.

[(F5)]

As above of course and ideally there is only flow through the relatively high velocity closed throttle so all through the pilot jet. But some will of course come past the needle. If the needle is too thin it will be fat wet drops that don't atomise very well in limited volume flow.
Thus a rich wet mixture you can't cure with progressively smaller pilot jets the change needle you may need to go up in pilot if you see what I mean

[Davehuge]

Thanks guys, that makes sense.

All the best, Dave.

[uawelder]

If you want a visual representation, check out this thread from ThumperTalk in 2010. I have this saved to my favorites, I wish that they had included the effects of the air screw in the first chart.

Enjoy

https://thumpertalk.com/forums/topic/812931-jetting-a-little-closer-look/

[Davehuge]

Wow, thanks for sharing that uawelder, just spent 30 minutes reading it and getting my head around it. Very interesting.

Whoever hallsy is, he's definitely put some work into that post.

So assuming that info is all correct...

I can see that by fitting a larger diameter/leaner needle with the same clip position, and all other components remaining the same...the only effect it has is to lean off the first 1/16 of throttle opening! Displayed by the red dotted line.

To be honest, I was expecting a larger/leaner needle to lean off the mixture over most of the throttle positions, obviously I was mistaken.

chart.jpeg