Air/Fuel screw

[wi_400EX_2001]

I was wondering how many turns i'm supposed to do on the air/fuel screw. What's wrong is that i keep getting carbon on my plug which in turn won't start my quad. I have a 2001 400EX w/ a Pro Circuit pipe, K&N filter, the top is off of the air box, it is jetted but i don't know what jet is in it b/c it was bought this way. I took the carb off tonight and tuned the screw tight and it went 3.5 turns... is this correct? if so should i put a pre filter on my K&N or get an air box top? thnx for the help

[GPracer2500]

3.5 turns out is quite a bit. That might actually be beyond the maximum useful adjustment range of the screw. If your carb/engine does indeed prefer 3.5 turns out then your pilot jet is too small.

The best way to find the proper setting for the pilot screw (aka fuel screw) is the following. This method can also indicate if your pilot jet is too big or too small.

1) Warm up the engine to full operating temp.

2) Turn up your idle a few hundred RPM (basically you want a fast idle). This will make it easier to hear small changes in RPM. Watch for overheating--pointing a big shop fan at your engine will help it from getting too hot during the fast idling.

3) Turn the fuel screw IN until the idle starts to drop and miss.

4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.

The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to drop at 1 turn out and starts to drop at 2 1/2 turns out then 1 3/4 of a turn out should be the correct setting.

If the peak/smoothest RPM is reached somewhere between 1-3 turns then your pilot jet is correct. If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one. If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up (bigger) on your pilot jet.

Typical fuel screw settings are in the 1 1/2 to 2 1/2 range.

It is quite difficult to adjust the fuel screw on a stock 400EX carb while the bike is running without a 90 degree screwdriver. I bit the bullet and bought this one from MotionPro. It works well but I'm sure there are others available. I think others have had success making a tool out of coat-hangers and such.

http://www.motionpro.com/images/groups/144.jpg

[wi_400EX_2001]

So since my plug is getting all carboned up that means it's running too rich correct?

[GPracer2500]

Originally posted by wi_400EX_2001
So since my plug is getting all carboned up that means it's running too rich correct?

The fuel screw is only part of one circuit within the carb. There are two other circuits, the main jet and the needle (and if you include the accelerator pump that would be the fourth circuit). What I'm getting at is you need to figure out which circuit is rich.

I think there are some jetting how-to's in the "How To's" section of this site. That might be a good place to start.

[wi_400EX_2001]

i figured out today that i have a 170 main jet and what appeared to be a stock pilot jet b/c it didn't have a # ontop of it. So since it was rich i moved the clip on the Needle up one notch and the Air/fuel screw is out 2 1/2 turns. When i went to start it, it started like backfiring through the carb does this mean it's not timed right? how do you time the motor. I have a K&N with the airbox off, if i didn't already say that above and i still have a Pro circuit T-4 pipe, and i live in southern Wisconsin, and the plubg is still black

[wi_400EX_2001]

i just ordered a 160 main jet, is anybody running a 160 main, stock pilot, needle 3 notches down, a/f screw 2 1/2 turns out, and a k&N w/ airbox off i remeber reading it when i searched now i cant find it

[GPracer2500]

The pilot jet should have a number on it. It will be on the side of the jet, not the end. If for some reason it doesn't have a number on it, junk it and get a genuine Keihin jet. A 40 or 42 is probably about right (stock is 38).

Also, is the main a Keihin jet or Dynojet or ??. A Keihin will have a "K" in front of the number etched onto the side. Keihin jets and Dynojet jets use a different scale, i.e. K170 does not equal a Dynojet 170. Bear that in mind when trying to compare your jetting to someone else's jetting.

[wi_400EX_2001]

the main just says 170 no k or anything, i'll have to try pulling the pilot out again, i stopped b/c it wasn't coming out very easily so i didn't want to strip it or any thing..

[zlam27]

what is your elevation?

i have an 01' with a PC T-4 slip on, K&N filter, and outerwear airbox lid and im running a 155 main and 42 pilot. i dont think the jetting is spot on but it runs great and starts better than most other 400ex's. im at about 800ft so if you are lower then you might need to bump up the sizes just a bit (opposite if you are higher in elevation).

[wi_400EX_2001]

My elevation is 957 ft. i think, but i put a 162 in yesturday and a 42 pilot and moved the air/fuel screw to 1 1/2 turns and it runs awesome now..