Carb Mixture Screws

[maxim9691]

What do these really do...

Various Setups tell me to Screw them out 4 and half turns or 2 Turns...
But I really need to know what they really do ?

I've taken various Types of Carbs apart and Rejetted and such.. and eventually tinkered with them , untill it worked.
But I would like to know more... to make my Carbs work better.

[flynrider]

They regulate the fuel/air mixture in the idle circuit. Your carbs will only use the idle circuit in the low rpm range. Midrange and high rpms will use the carb jets for mixture control.

[mydlyfkryzis]

Most idle screws control the air portion of the idle circuit. Some do control fuel. Keihin carbs use the air control. So the more unscrewed the idle screw is, the more air you are allowing.

[maxim9691]

So if I unscrew them when my carbs are running rich.. will that balance it out. Or if they are running lean, tighten the screws .

is that the Jist ?

[mydlyfkryzis]

So if I unscrew them when my carbs are running rich.. will that balance it out. Or if they are running lean, tighten the screws .

is that the Jist ?

Yes, but only at idle. So if your idle is too rich, this will adjust it.

Once you are off idle, the carbs use the midrange jets. At full throttle, there is a full throttle jet. So if you are lean mid throttle, the needle in the slide can be adjusted or changed to a different taper, or the mid range jet changed. Changing the jet enriches the whole midrange operation. A Taper change in the needle can customize to correct a lean spot in the midrange. Moving the needle up in the slide (so it is pulled out of the jet a little further) tends to enrich the bottom end of the mid range and leave the top about the same.

A dyno and exhaust gas analyzer are the best way to get a good tune.

Reading plugs can be used, but it is not as accurate and you need new plugs to check changes.

Here's a good explanation: http://www.motocross.com/motoprof/moto/ ... rb101.html

[maxim9691]

Thanks for the Link...
That is exactly what I was looking for