Do I need a smaller Pilot Jet?

[AdrianXXX]

Hi guys!
I have read that if the pilot screw has to be turned more than 2.5 turns out for best idling, the next smaller size pilot jet will be needed. I have my pilot screw set 4 turns out, because I have ran too lean. But what is the logic of this? :confused: :eek2:
I thought the leaner you run the bigger jet you will need.
BTW I have Suzuki LTF 250
Thanks!

[bwamos]

There's 2 types of circuits.. some are air.. some are fuel.

Not sure what type of circuit the LT250F has.. but most 4-strokes are fuel circuits.

{edited... see below}

[AdrianXXX]

Yes it does. That’s what I thought too. But here (http://www.vinjapeuromcclub.org/techtip_carbs.shtml ) is written that if I have more than 2 turns out, I should change it to a smaller one (read the Pilot Circuit part). And also Mikuniś Tuning Manual (http://www.nightrider.com/biketech/download/Mikuni_manuals/tune_mikuni_carbs.pdf) declares something similar. (read on third site in the Note section on the bottom). So all of these are opposite of what we are thinking of.
BTW the LTF250 has a Mikuni BS29 carb and the pilot screw regulates the fuel (not air)
Thanks

[ohsobad_chevy]

Ive always went with 3 turns out as my rule, but who knows.
At 4 turns out, in your case, I would definitely install one size larger on the pilot.

[bwamos]

Well I'll correct my previous statment. My appologies.

Both are correct... in their own circumstance.

It all depends on wether you have an idle mixture screw that is a fuel or air bypass.

The pilot jet and idle mixture screw are in seperate locations. So if you're idle mixture screw is a fuel bypass then you are adding fuel as you screw out. and more than 3 turns out you need a richer jet.

If your idle mixture screw is an air bypass you are adding air as you screw out so if you are more than 3 turns out it means you are too rich and need a leaner jet.


Now on your carb you need to determine wether it is a fuel or an air bypass.

Most 4-stroke carbs are fuel bypass, and most 2-strokes are air bypass.

Don't ask me why.. and that's just the tendency. There are always exceptions.

Updated my previous post for clarity.

Location hints are fuel mix is usually on the engine side of the float.. and air mix is usually on the air cleaner side as a general rule. Some carbs even have both.

[AdrianXXX]

Bwamos, thank you for this useful info. I have fuel bypass, so it means that I will have to change it to a bigger one. One more question is still not clear for me: why do we need the throttle stop screw? I know it is for setting the idle speed. But how? What is the rule of this when there is the pilot screw which sets the idle speed too ? (Here is the picture of my carb)
http://www.tirinda.sk/adrian/quad/carb/Idle.jpg

[bwamos]

The idle mix screw sets the mixture at idle. You basically adjsut it to the highest RPM availiable and then make it a tad richer for saftey.

The Idle stop as you put it holds the slide open a hair and sets how far open/closed the slide is at idle. This is what you use to set the idle speed.

[Honda4life05]

Ok im also a little confused, i have a 300ex an its idenenticle to that carb in the pic. So if i wanted to adjust the idel which screw would i adjust? I have adjusted the throttel stop screw an it seems like after a day of riding it goes back to stalling.

[GPracer2500]

Originally posted by Honda4life05
Ok im also a little confused, i have a 300ex an its idenenticle to that carb in the pic. So if i wanted to adjust the idel which screw would i adjust? I have adjusted the throttel stop screw an it seems like after a day of riding it goes back to stalling.

The throttle stop screw is for adjusting the idle speed.

A fuel (or air) screw is not used to adjust the idle. As bwamos said, it is used to adjust the a/f mixture of the pilot circuit (i.e. 0 to about 1/4 throttle). Part of the procedure for finding the correct a/f mixture for the pilot circuit involves turning the mixture screw until the highest, smoothest idle speed is achieved. But once you've found that setting, you re-adjust the throttle stop screw to the correct idle speed.

http://www.exriders.com/vbb/showthread.php?s=&threadid=229923

[GPracer2500]

Originally posted by Honda4life05
Ok im also a little confused, i have a 300ex an its idenenticle to that carb in the pic. So if i wanted to adjust the idel which screw would i adjust? I have adjusted the throttel stop screw an it seems like after a day of riding it goes back to stalling.

Oh and your idle is probably erratic because your pilot circuit is either too rich or too lean. I'd try adjusting your fuel screw and see what happens.

[Honda4life05]

ok, there are two screws that look like the adjuster screw, one is under the carb an one is on the sid ethe choke is on. If i twist out the one on the side at all fuel spills out of the oerflow. i have a 300ex

[GPracer2500]

Originally posted by Honda4life05
ok, there are two screws that look like the adjuster screw, one is under the carb an one is on the sid ethe choke is on. If i twist out the one on the side at all fuel spills out of the oerflow. i have a 300ex

That's the float bowl drain screw. The fuel screw is the one underneath.

[AdrianXXX]

I think the ex300 has a Keihin CVK32 carb. If so, then here (http://myural.com/Keihin%2032CVK%20Service%20010.jpg) you can see the fuel mixture screw. (This is a bottom view).
And here (http://myural.com/Keihin%2032CVK%20Service%20003.jpg) (on the right side of the carb) is the idle adjustment screw (the sloping one).

[Honda4life05]

ok..thanx for helpin me out on this one.

[AdrianXXX]

Originally posted by GPracer2500

http://www.exriders.com/vbb/showthread.php?s=&threadid=229923

GPracer2500, thank you for the great post! It helped me so much! According to this I will need a bigger pilot jet. Thans!

[bwamos]

The 300ex has a funky Keihin carb.. pretty much made just for Honda. I forget the actual model tag.

But here a pic of the bottom of the carb.

Blue circle is the idle mixture screw.
Orange circle is the idle speed adjustment.

Note.. this is actually a 400ex carb .. but they are dang near identical minus a few extra vent tubes.