Idle Adjustment

Guys, I know basically nothing about carburetors/jetting etc. I do, however, want to increase my idle speed. The manual, under "idle speed adjustment", says to adjust the pilot screw first. The problem is that the "pilot screw adjustment" section just before it is very vague. It simply discusses turning the screw without much about why your turning and what to look/listen for when adjusting it.

Any information you can provide regarding the relationship between pilot screw and idle would be greatly appreciated. By the way, my machine runs great, doesn't foul plugs, and throttle response is awesome. I simply want to turn up the idle to help guard against the dreaded mid moto stall. Thanks in advance for your expertise.

426Pilot, There are many here more qualified than I am to answer your question, but I'll try and share a few things that may help. First, if you check in your manual starting on 7-1, it explains all about carburetor function and tuning. Now to answer your question. The Pilot or fuel screw controls the air fuel mixture from approx 1/8 to 1/4 throttle opening. You state that your bike runs fine (ie: crisp throttle response, etc.), therefore I don't think you need to do anything with the pilot screw. If you want to bump up the idle a bit, just turn the idle adjuster knob clockwise until you reach the desired idle speed. Hope this helps.

As an addendum to what dirtdad offered, here is my advice on adjusting the pilot(fuel) screw as it relates to idle. The first to know is the label "pilot screw" means that this small needle screw adjusts a small amount of fuel in the very low RPM range. That is, this screw allows fine tuning of the air/fuel mixture at idle and just above idle to about 1/4 throttle. The best method to set this adjustment is start, warm up, and then ride your bike for about 15 minutes to get it up to normal operating temperatures. Don't baby it...ride it hard. When I'm ready to fiddle with these adjustments, I have a big fan ready to blow air across the radiators as I adjust the pilot screw because this is done with the engine running and it will definitely get pretty hot while you are doing it. Anyway, after getting the engine to operating temperature, leave the engine running and and in neutral. Now, with that fan blowing at the front of the bike, adjust the big black knob next to the red hot start knob so that you have a rather fast idle...about 2300 RPM's..if you can guess about what that would be. It's not important to be precise here...just want an idle that's fast enough to make the engine run smooth and steady. Start by turning the pilot screw out(counterclockwise) until the engine starts running bad....then start turning it back the other way until it smooths out again...continue turning it until it starts running bad again. What the aim is here is to find the middle point. This should be where you get the highest and smoothest idle. Be patient and don't worry that you have to keep turning the screw back and forth until you've found that spot. By the way, the pilot screw is found inside a little metal tube just in front of the float bowl. From the shifter side of the bike, look at the bottom of the carb, between the float bowl and the engine and look for a round tube-like machining that is open at the end. You can take a small mirror and a flashlight and look up inside this tube and see the slotted end of the pilot screw. the screw has a small spring on the needle end that keeps tension on it so that vibration doesn't make it fall out. You can't see this spring since it's between the screw and the seat. Once you've found the smoothest idle by adjusting this screw, you've through with this part. Now you just adjust the idle where you like it. Personally, I set my idle to where the engine will barely idle in neutral. I do this to take advantage of the engine braking characteristics. The downside is that it's easy to stall coming into corners and I have to adjust my riding style accordingly. I've noticed that hardly anyone sets their idle as low as I do. Believe it or not, I rarely stall the engine as I have learned to blip the throttle often anytime I pull in the clutch. Besides, it's fun to make the guy in front of me nervous.

Also, it's a good idea to make a written note as to where you had to end up with the pilot screw adjustment. Do this by counting how many turns in(clockwise) until the screw bottoms out. This is a brass screw so don't seat it too hard. The benefit of doing this is that this will let you know if you have the correct pilot jet size. I don't have the manual in front of for this range but I think it's something like this: if you had to put the screw at less than a half turn out from bottom, then your pilot jet is too rich...if you had to turn it 3 turns or more, your pilot jet is too lean. Keep in mind that turning this screw counterclockwise adds more fuel(richer) to the pilot circuit. If you have to turn it out more than 3 turns, the spring may not hold the screw in place and could vibrate out. That has happened to a few riders. My '00 is at 1 and 3/4 turns out. Riding altitude affects this setting. Hope this helped and I didn't ramble too much... :)

[This message has been edited by Boit (edited 03-10-2001).]

Once again, ask an engineer (or senior technician) what time it is and they'll tell you how to build a clock :)! Every thing he said is good advice, but sometimes it's best to keep it simple and leave well enough alone. "If it ain't broke, don't fix it!"

[This message has been edited by dirtdad (edited 03-10-2001).]

dirtdad: You picking on me? I'm getting sensitive in my geezer age... :)

Unreliable server...double post

[This message has been edited by Boit (edited 03-11-2001).]

Boit, What ever gave you that idea!? :)

Boit gave you good advice. I just want to add this. You can tell pretty quickly if you fuel screw is off.

When you rev your bike if the idle hangs a little before settling, your are lean. Turn the fuel screw out a little.

If your idle drops then recovers you are rich. Turn your fuel screw in a little.

What I have noticed is when I get it right I get a nice glow from the header pipe.

I will post later how to build a clock. :)

Good Luck



01 YZ426F #85 Vet C

Thanks for the advice, I'll play with it some and let you know.

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