Idle Speed Knob on WR426F

[llamaloui]

My bike (WR426F) runs pretty high idle RPM's. It starts OK but it hangs after riding for a bit.

Just confused about which direction does what on the Idle speed knob and I need someone to confirm if what im thinking is correct.
I got info from here: <The dreaded &#39;Hanging Idle&#39; and how to eliminate it - WR 400/426/450 - ThumperTalk>
But there's still some clarification I need.

This pic is from the Yamaha manual.

http://img577.imageshack.us/img577/251/ihrz.jpg

Direction (a) increases idle speed (turn screw in)
Direction (b) decreases idle speed (turn screw out)

If I turn the screw IN and increase speed, does it mean I am taking in more air and making the idle mix relatively leaner? Too Lean = Fast = Burnt smell?
If I turn the screw OUT and decrease speed, does it mean I am taking in less air and making the idle mix relatively richer? Too Rich = Slow/Stall = Fuel smell = Backfire?

Also when I decelerate, the exhaust gurgles a little followed by 2-3 popping sounds. Its not a full on "BANG" backfire that I've heard others do before. Is that normal? The exhaust pipe is stock.


Cheers

[Crobbo]

The idle normally adjusts the throttle stops. Winding it in keeps your open throttle very slightly. So you aren't changing the fuel/air ratio.

[Shady7/8]

^^ What he said

[llamaloui]

Oooh OK. So if I wanted to lower the idle RPM's, I just turn it out.

Will it have any effect on my idle fuel mix whatsoever?

I read somewhere that the idle knob actually changes the "idle fuel mix"?

Is this the same thing or am I thinking of something completely different?

Cheers

[Shady7/8]

It's the same as slightly cracking the throttle open. Won't change A/F mix.

if you wind the screw in, you will increase the idle revs, if you wind it out, you will reduce the idle revs

[agrid]

The idle normally adjusts the throttle stops. Winding it in keeps your open throttle very slightly. So you aren't changing the fuel/air ratio.

^yes.

You might have the idle screw mixed up with the mixture screw. I didn't think they used the latter much on modern bikes...

[llamaloui]

OK I think I understand it more now.

So the screw holds throttle open. In the Thumper Talk thread they mentioned fuel mixes and essentially the 'mix' part means how much fuel the throttle feeds when your hand is off the throttle.

If I'm still experiencing hanging idle, even after lowering speed, could it be due to a fuel leak at the float valve in the feed line?

I have also noticed the smell of fresh petrol right after I turn off the engine. The smell is gone after a while though. Is this just unspent fuel escaping the carb? I always close the petcock if I'm not riding (even if its just to stop by the post office...etc)

[Crobbo]

OK I think I understand it more now.

So the screw holds throttle open. In the Thumper Talk thread they mentioned fuel mixes and essentially the 'mix' part means how much fuel the throttle feeds when your hand is off the throttle.

There are screws that do what you mention either idle air or idle mixture depending in your carby setup.

If the only thing you're touching is the one in the picture with the knob on it then you'll be fine.

[agrid]

Throttle cable with no play?

[Crobbo]

Throttle cable with no play?

This and does your throttle snap back nicely when let go? try it at full lock either way of the handle bars.

If you post up a video of your idle speed hot and cold with a few blips at each it would help.

Its hard to figure out if anything is wrong without being able to hear it.

[llamaloui]

Throttle cable with no play?

There is a small amount of slack but there is no effect on RPM. If I let go of the throttle it snaps back well but will land anywhere in the 'slack area'.

As far as the actual jetting, I haven't even dared to go near those. The previous owner had a full carb overhaul done and I've got the receipt for it too.

Ill try to get a video or sound clip over the weekend.

[Dirty]

Hanging idle is usually too lean on the pilot jet.
Try taking the pilot jet out and giving it a clean, then resetting your idle mixture using the ridiculously hard to get to mixture screw.
If that doesn't solve it, or you need to run more than 2.5 turns out on the screw you need to go up a size on the pilot jet.
Thumpertalk has all the info you could ever want on the carbs for those bikes.

- - - Updated - - -

I didn't think they used the latter much on modern bikes...

a WR426F is not a modern bike....
Carbs 4 life

[tinytoolmans274]

Make sure both clamps are tight at each end of the boot that connects the carbie to the head. Also make sure there is no splits or damage to said boot.

[smitto]

You may find its the slide thats worn, in particular where the throttle shaft meets the slide in the top of the carb. When the cable pulls the throttle shut there was movement that allowed the slide to open and close slightly so I could never set my idle

[darkfibre]

If it responds to the idle speed screw, then the throttle is probably not sticking.

Rough rule of thumb,
Blip the throttle
Idle hangs high - lean idle mixture
Idle drops and stumbles -rich idle mixture