Pilot Air Screw

22 replies to this topic
  • e-z_e

Posted April 09, 2003 - 04:42 AM


So I've been playing with my jetting, needle clip position in particular. I finally got it to where my bike ('02 WR426) has never run better... almost. At idle, it will rev high and then low and then stall. I now have to use the hot start button more than ever to get it started. I think think is a pilot air screw issue. How do I know how many turns is enough/too much???

On an interesting note, I also put a new Hi Flow header on. After running it for a while, the header changed color from it's nice new aluminum shade to a almost reddish hue. Could this be a clue as to were to direct my attention???

  • ColoradoXR

Posted April 09, 2003 - 05:53 AM


I'm going through a similar problem. See my sig for my jetting. I have the Zip-Ty pilot screw and have tried anywhere from 1 turn out to 3 1/2 turns out to correct :usa:and can't get the low end perfect. The best I have been able to get it is at 2 turns out. The idle is good, but there is a "blip" when I snap the throttle. Every once in a while, it will almost stall. I can't get it any better though. Do a search for a post entitled "From Where?" and there is a post by Wicked Crash that I referred to. Like you, I'm starting to wonder about my pas setting of 65. I'm going to move it around a bit and see if I can get it better.

After all the jetting work I've done, this is the best I've been so far. The only problem I have is that there is a "blip" at the bottom when snapping the throttle. I'm still working on this.

  • LarryCO

Posted April 09, 2003 - 05:56 AM


See my comments on Mr. Lee's post (Help after mods?) regarding tests for the PAJ setting.

  • ColoradoXR

Posted April 09, 2003 - 06:20 AM


Figures, Larry the jetting God would swoop right in on this. I'm going to Thunder Mountain this afternoon around 3PM if you can make it. I'm going to do the tests/adjustment there, as my neighbors are probably sick of hearing me testing in the neighborhood. Not to mention, two of my buddies decided to go on a 2AM run on their two strokes Friday night. Remember that buddy of mine I told you about that has the kegerator? That thing causes normally ordinary men to turn into asses. :)

  • Wicked_Crash

Posted April 09, 2003 - 06:53 AM


I'll be at thunder mountain after work around 5:00 to test my fuel screw adjustment. I just got the zip ty fuel screw so now it's way easier to adjust. I'll be in a red chevy pickup. I'm sitting here at work with my WR in the back of my truck. It's torture! Come on 5:00!

  • ColoradoXR

Posted April 09, 2003 - 08:35 AM



Smurf blue F150 and a WR with a brand spanking new black YZ fender :)

I'll probably be leaning over my bike working on the jets.

  • Rich_in_Orlando

Posted April 09, 2003 - 09:26 AM


Hey guys, I'm sure e-z_e appreciates your help, but I think he's asking about the pilot air screw, not the fuel screw. If you turn your pilot air screw 3 turns out I doubt your bike will run.

e-z_e, am I right? Did you get a pilot air screw from Sudco or some other place? Your bike doesn't come stock with one. The pilot air screw (PAS) replaces the pilot air jet (PAJ) which is located at the intake on the face of the carb. If you did, it depends on your pilot jet. I'm running a 35 pj, so I have my PAS set to 1/4 turn out, equivalent to a 45 PAJ. If I recall correctly, a 38 PJ needs a 65 PAS (3/8 turn out), a 40 - 1/2 turn out, 45 1 turn out. Seems to be 1/8 to 1/4 turn out for each larger increment.

Now, if you aren't talking about a PAS, then you must be talking about the fuel screw. These carbs don't have an air screw that is externally adjustable. I emphasize this because air screws (like 2-stroke carbs have) and fuel screws work oppositely from each other. You turn an air screw out to lean the mixture while turning out a fuel screws richens it.

You optomize the fuel screw setting at high idle. Warm the bike up and adjust the idle up pretty high with the idle adjust knob. Now turn the fuel screw to where you get the highest idle. If you need to go more than 3 turns out, it needs a bigger PJ. If you need to go less than 3/4 turns out, you need a smaller PJ. Once you get the fuel screw set properly, then turn the idle back to where you want it with the idle adjust knob.

  • Wicked_Crash

Posted April 09, 2003 - 10:09 AM


Thanks for catching us on that Rich. Here is a link to the PAS chart. http://home.earthlin...screw chart.jpg

  • e-z_e

Posted April 09, 2003 - 02:07 PM


Rich - apparently you knew more of what I was trying to say than I did. You're right, it is the fuel screw. My carb is still stock excelt for the PJ, MJ and clip position. So I did what you said and at 2 turns out, it seemed to idle the best. I still have to get it in the dirt for the real test.

Until then, thanks!

  • Rich_in_Orlando

Posted April 09, 2003 - 06:02 PM


No problem. It sounds like you got it right in the middle of the range. A couple of guys I ride with on WRs (a 426 and a 450) adjusted theirs before we started riding this past Sunday. When they cranked the idle up, you could hear a little "missing" going on. They were able to adjust the fuel screws so that the missing went away--you could really hear the difference. You could hear when they went past the sweet spot-either too rich or too lean. The range was pretty narrow. And they seemed pretty happy with the results when we were out riding. (One of those Kouba T-handles really helps when trying to adjust the fuel screw.)

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  • FNG241_WR450F

Posted April 10, 2003 - 08:28 AM


Okay, Maybe I'm wrong here, but the article I just read on Fuel screw adjustmens in the March 03 addition of Motorcross Mag says that 4 strokes don't have air screws, just fuel screws. Anyone with a 4 stroke should read the article. It gives an in depth approach to adjusting the fuel screw. Helped me 100%.

  • Wicked_Crash

Posted April 10, 2003 - 08:42 AM


4 strokes have air jets. A lot of us are replacing the stock pilot air jet with an adjustable pilot air screw so we can fine tune things without having to buy different pilot air jets. If Motorcross Mag is reffering to specifically air screws, they are right (until we put one on ourselves), but if they are reffering to all air jets, maybe they should take a look at a 4 stroke carb again.

  • MorrisWR

Posted April 10, 2003 - 09:00 AM


e-z, I had exactly the same problem with my stock WR426 I picked up a couple weeks ago. I broke it in stock except for the GYT-R insert so I could get a baseline to start from. It ran way lean that day. The idle would stay high then drop to normal after a while. I couldn't adjust it out with the fuel screw. I tore it apart this week and did all the mods and was going to put a 45 pilot jet in for the 42 stock to richen the lower end but since I opened it up and did the BK mod I decided to raise the needle to richen up the entire range instead. That helped a lot. I now have it set pretty well using a tachometer to fine tune the fuel screw adjuster. I still might go to a 45 pilot jet because I have the adjuster out about 3 turns. Like Colorado, I have a small blip or hesitation from start that I need to get rid of. One other indication that my bike was too lean is it ran better with the choke on and I never had to use the hot start all day. My jetting is all stock except the needle clip pos#5 but it works at my low altitude. I decided to take the slow route and do one jetting change at a time to get it dialed in. It'll warm up here about 20-30 degrees in a few months anyway so that should richen it up a bit also. :)

  • racemile

Posted April 10, 2003 - 09:22 AM



Do the air jet changes make a large difference in how the bike runs, or, is it a safety thing for getting the correct ratio's of fuel to air?

I am fixin' ta order a PAS, a 200MAJ, kouba T-Handle, and EKP needle from sudco (and extra circlips) Any opinions?


  • Rich_in_Orlando

Posted April 10, 2003 - 10:08 AM


I know you asked Wicked Crash the question, but in my experience, the proper pilot air jet/screw setting is very important to getting the bike running right. As I wrote earlier, my PJ is a 35, which needs a 45 PAJ/screw (1/4 turn out). How do I know this, you may ask? I have to preface this by reminding you, the reader, that a larger PAJ/screw enrichens the pilot circuit because it allows the PJ to pick up more fuel (kind of counter-intuitive). For several months last year, my bike had been fouling plugs first thing in the morning. I was going through a new plug every other week. Starting was a bit erratic. Choke was acting funny, too. The fuel screw adjustment wouldn't change the idle one way or the other. I checked everything (so I thought) and could not figure out what was wrong. I finally checked the PA screw and it was turned out almost a half turn, almost double what it should be. I carefully adjusted it to 1/4 turn, then double and triple checked it and put it all back together. Haven't fouled a plug since. That larger PAJ opening was letting way too much fuel dump in at idle. The bike starts easier (1-2 kicks), the choke behaves like it should-it needs it for about 30 seconds first thing in the morning, and runs smoother since I fixed it.

  • racemile

Posted April 10, 2003 - 10:52 AM


That makes sense. I just ordered a PAS, and a 160 and 200 MAJ, along with a dmn, emp, and another needle. Someone bought sudco out of EMN needles!!!! All 30-40 needles?!?!? and all the circlips for needles!! Trying to get in touch with zipty to see if they have any fuel screws (adjustable) they could over night to me.

Thanks for the help


  • LarryCO

Posted April 10, 2003 - 11:13 AM


I'll save ya some time...maybe...

At altitude (5K), I tried and tried and tried to make an __M or __N needle work...to no avail. Too rich. The richest needle I could get to work was the EKP...and thus my jetting setups listed below. Perhaps you can by not using a setup anywhere close to the "recommended" PJ/PAJ ratios (as Rich explained above), but I wasnt able to find any that would work.

Let me know if you do some testing with them and do get them to work...


  • Wicked_Crash

Posted April 10, 2003 - 11:14 AM


Go to Fay Myers, they have a bunch of Zip Ty fuel screws, I just got one there the other day.

To add to what Rich said, the proper PAS adjustment combined with PJ seems to make the whole pilot circuit run a lot crisper. The combination in my jetting (very similar to LarryCO's with the exception of the 148 mj) runs great, pulls hard in all ranges and there is absolutly no bog. I think I can leave my jetting alone for a while until I go up to 13,000 ft, then I'll switch back to the EKQ #2. :)

  • e-z_e

Posted April 10, 2003 - 12:24 PM


I just want to thank you all for all your help. If I've learned nothing else it's that my WR is a little more finicky than my previous XR 600 pig... but well worth it...

  • racemile

Posted April 10, 2003 - 12:59 PM



You can't move dude!!! We have guards at the border. Thanks for the help!! I hope that the YZ timing may make a difference in my bike.....We will see!


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