Carb Settings for the lame Please Help...

Ok All,

I've been looming around here for a while and you've never let me down so far. I have been running bone stock carb settings with an FMF power core IV2 silencer/stock header. I am happy with the throttle response so far but I have always had a sputter when cruising at a steady throttle. I think I determined that this was a rich condition since it seemed worse at 4k feet. I usually ride at less than 1k foot elevation. so...

I recently tried a 50/50 mix of 110 and 91 (californias new reduced octane super unleaded) in an attempt to lean out without messing with the carb. It seems slightly better but not quite enough.

Long winded way to the real question. Can or should I try turning the fuel screw? I believe in or tightening = leaner. Is this correct? I'm thinking 1/4 turn clockwise. I would hate to mess this thing up and have had bad luck once I start messing with the carb.

Thanks for any advise...




Rip the knobbies clean off :)

due to the underwhelming response, I am replying to my own message so it will rise to the top of the list once again. Please throw me a bone. I think I know what to do but am nervous. I don't know much about carbs and don't want a bike that never runs right...





Rip the knobbies clean off !!

The sputter at steady throttle you mention:

What year/model of bike is this happening on?

What throttle position?

What approximate rpm?

What is your current jetting?

If you have a 98/99, you can use the jetting in my signature. It is spot on thanks to James Dean and has been tested/used by many.

Otherwise, as requested by Hick. You'll need to provide more specifics.



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, de-octopussed (Thanks Clark) EKP #4, 50PJ, 175MJ, 100PAJ and 65 SJ at 500-1000' Thanks James Dean!

I guess the model and year ARE important details. I'm riding a 2001 YZ426F. The carb is factory set with no modifications or adjustments...except for the occasional twist to the idle speed knob. So this is stock jetting on a stock bike...the only difference is that I have an FMF PowerCoreIV2 pipe with the stock header. I never adjusted the carb when I put the pipe on (day one).

The sputter is at low to med throttle like when cruising along at a slow speed. I don't notice it when I'm on the gas hard. Just when trying to maintain a steady speed and throttle position.

When I went to Foresthill at about 4k feet it seemed more pronounced so I assumed it was due to a rich condition which gets worse at altitude. I just tried running a 50/50 mix of 110 octane and super which is 91 octane here in CA. It seemed a bit better but not drastically better. My thought is to now try to lean the mixture a bit but hopefully avoid jetting or even opening the carb up. (yes I'm still scared of that level of maintenance/adjustment.)

I now defer to you all...much more knowledgeable than myself...

Thanks for bothering to make it this far in my long winded post.!!




Rip the knobbies clean off !!


I live/ride at 4k feet, for that elevation I would suggest you try lowering the needle (raising the clip) and swapping the next smaller pilot (# 40). Sorry, but both require that you remove the carb.

From your description I doubt you can improve things by changing the idle mix only.

What elevation do you normally ride at? Just about everybody who has changed the jetting on their 426 has gone richer, I wouldn’t know myself but you could also be overly lean, not rich, at lower altitudes. Sometimes the only way to know for sure is to try something.

To that end, I think changing the clip position on your needle is worth trying. This is something that should create a fairly noticeable change, and getting it out is not all that difficult or time consuming.

To avoid having to remove the throttle cables from the slide pulley, which can be one of the more difficult aspects to someone unfamiliar with this carb, you can loosen (don’t split it apart, just loosen the two screws) and remove the throttle housing from the bars, you may also have to loosen your bar clamp to move the bars to the left. This will create enough slack in the cables to allow you to move the carb around a bit.

After you loosen both the intake and airbox clamps from either side of the carb (phillips head in back, small allen up front) you should be able to rotate the carb to the left side of the bike (you may also want to remove the fuel line and tps wire). You need to gain access to the two small allen-head screws (same size as intake boot) that secure the top of the carb. Remove that and you will see another allen-head fastener on top of the slide (larger, maybe a 5mm), the needle is underneath this. To get the needle out you can use a magnet to pull it out or gravity to dump it out. Carefully put the clip up one notch (leaner) or down (richer) and reinstall the needle.

This shouldn’t take too long, even the first time you try it.

Hope this helps.

Thanks Hick,

I guess I just need to quit being a whimp and go for it. I have done my fair share of wrenching but I treat this bike like a precision instrument and don't want to blow it. I guess if I make a change like that it won't be tough to go back to where I started.

One last must not think that turning the fuel screw would help or that I should try that first??

Thanks again for the great details...

Ride on,


By all means try the fuel screw first! From what you describe I don’t think that is the problem, but you are the one riding the bike. Maybe we should have a general rule here where everything I say is taken with a grain of salt...

You will need something very short and compact to get at the idle mix, I use the metal bit-end from one of those cheap bit drivers. You know, the ones where the metal driver end comes out and fits inside the plastic handle for storage. Whatever. The screw should turn easily, and is found in a recessed hole on the underside of the fuel bowl towards the front of the bike. The screw angles down from inside the carb about 15 degrees away from vertical towards the front of the bike, so angle your tool accordingly.

Stock should be 1.75 turns out. The first thing you need to do is make sure that is where you are by bottoming the screw and counting the turns. To avoid confusing myself I make a mark on the tool I am using to make sure I’m counting the turns correctly, but you may be coordinated enough to do without that.

To finally answer your original question, clockwise is leaner for the fuel screw, opposite of the air screw found on most two strokes.

Hope this helps.

[This message has been edited by Hick (edited September 19, 2001).]

eBayThumper: Your problems seem very similar to mine but my elevation is much higher than yours so it would be hard to believe it was rich but still possible. I'm not sure what affect your aftermarket muffler is doing but usually they recommend increasing jet sizes with these pipes unless they actually are more restrictive than stock. Hick gives good advise so try it out. It should get better/worse by moving the needle clip.

I do have one other thing.

Hick: You said clockwise is richer for the fuel screw and I believe you meant counter-clockwise (unscrew). It is opposite of an air screw and clockwise is richer for an air screw. When I get decel pop I turn the fuel screw counter-clockwise (unscrew) to richen it.


I live and ride in Truckee so my average elevation is 6000. I too had many reservations regarding jetting and working with this apparently complicated carb. I have since changed all jets and did the bk mod. Here is what I learned.

01 426 w/ fmf pwr cr4 sq. I had stock settings when I received the bike and stock pipe. Ran rich but strong. I got the spark arresstor and it ran like Sh++. I promptly followed my local shop orders and leaned the clip position, and leaned the mjet to 155. Result was it ran much better! I then did the bk mod (a lot of fun to do actualy) and got lean readings as expected. I then bumped my mjet up to 158. So far so good. STRONG off the bottom and pulling hard through the top.

Let me know if your interested in riding further up 80 than Fhill.

Originally posted by vaughn:

Hick: You said clockwise is richer for the fuel screw and I believe you meant counter-clockwise (unscrew).

Ooops, yes you are correct Vaughn, turn it out, or COUNTER-clockwise, to richen. I guess I should edit my post and fix that.

One of my favorite movie quotes (from Fandango, great movie), Kevin Costner as Gardner Barnes in a very well done mild Texas drawl:

“No, no son, lefty-loosey, righty-tighty, remember?”

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