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Old 04-05-2003, 09:02 AM
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Question Hi all I am a new member with a 02 426 and questions

First off let me say hey to all this look like a very cool site. I am a CR 500 rider for the past 15 years and the switch to this amazing thumper is just short of a heaven. I LOVE IT. But I am having some issues I put the PRO Circuit T4 Full system on it they told me I would not have to jet it so at first I didn't. I have been riding at 3 to 6 thosand feet and at 3000 it was spuddering and at 6 barley running so I went down to a 158 and it got better but right on the throttle it still has a little blurp but overall the bike pulls harder tha I ever thought a thumper could. Any thought or recomendations for fixing this bulrp. Someone said I should install a P-38 on the carb? Thanks for any help
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Old 04-05-2003, 12:20 PM
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Talking Welcome to All Things Moto

Diamond,
Welcome aboard to All things Moto. If you have any problems please ask senior mod or a forum moderator. Just P.M. or email one and we will try to help you as quickly as possible.

As to your 426 and its jetting, I would go to the Garret Michael Berg Memorial Foundation forum and link to his website; pay a visit to the Motoman 393 web site for some jetting charts and the article on the BK/GB mod to the spray duration in the carburetor. This site is also accessible through the knowledge forums. The jetting charts there are really good, Thumper Talk is gracious enough to host the site since Garrett was killed last year. the site has a wealth of information and technical articles for the 426.

I am at sea level and my '02 runs crisp with stock jetting so I will not venture a guess on the needs at altitude. FFRacing11X can tell you alot about jetting the singles. Youy will get several tips through this post from some western riders that have high altitude jetting work on their YZ 400F/426F. the BK mod is similar to the P-38 in nature. It was developed by Brian Kinney for Timmy Ferry's factory 426. Garrett corresponded with Brian quite a bit and was a big proponent of this mod for the 426.

Good luck on your jetting adn please do not hesitite to P.M. me if you need anything here or to talk about the YZ 426F. It is my favorite subject.

Bill Barnard
Senior Moderator
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  #3  
Old 04-13-2003, 12:46 AM
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Default Re: General Jetting Info (Very Good)

Diamond - the '02 bike responds very well to just jetting changes. It is not necessary to do the GB/BK mod or the P-38 unless you like to tinker.

The air/fuel mixture on your carb is throttle position sensitive.

The pilot jet and fuel screw from 0-1/4 throttle
The needle from 1/8-3/4 throttle
The main jet from 1/2 throttle to WOT

Note that the areas overlap each other.

The needle and main WILL slightly affect the mixture everywhere but the effects are most pronounced at the ranges given above.

You need to run a leaner mixture for your altitude and you have really only done that for about 1/2 - WOT by changing the MJ.

You might also need a smaller pilot jet and 1 clip position higher on the needle to lean out your jetting over the entire throttle range. But before you change the pilot jet, tweak the fuel screw IN (leaner) and see if you can get your "hesitation" or stumble to go away without introducing a "hanging idle." If so great.

As for the needle, after you get the pilot/fuel screw where you want it, put the clip one position higher and see if you can feel a stronger pulling engine. If not, then put it back since there's no reason to run a leaner engine if you can't tell the difference.
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Old 04-13-2003, 10:12 AM
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Default Re: General Jetting Info (Very Good)

Thanks a ton that,s what I am going to do I will let you knw how it comes out.
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Old 04-20-2003, 03:43 AM
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Default Re: General Jetting Info (Very Good)

Diamond,
All of Mohtks points are first rate info. Just be sure you only make one change at a time. Write down the original settings / jets and how the bike performed. Then record the same information for the change. This way you keep a base line and a reference to return to if a change does not work out. Good luck and please keep us up to date on how your tuning goes.

Bill
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Old 04-22-2003, 12:22 AM
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Thumbs up Re: Hi all I am a new member with a 02 426 and questions

Quote:
Originally posted by Diamond426
First off let me say hey to all this look like a very cool site. I am a CR 500 rider for the past 15 years and the switch to this amazing thumper is just short of a heaven. I LOVE IT. But I am having some issues I put the PRO Circuit T4 Full system on it they told me I would not have to jet it so at first I didn't. I have been riding at 3 to 6 thosand feet and at 3000 it was spuddering and at 6 barley running so I went down to a 158 and it got better but right on the throttle it still has a little blurp but overall the bike pulls harder tha I ever thought a thumper could. Any thought or recomendations for fixing this bulrp. Someone said I should install a P-38 on the carb? Thanks for any help
As was said, welcome to ATM!

You will (and already have) found some good info here!

We hope you stick around and make this site your home!

I also want to extend a warm welcome to mohtks also!

Welcome aboard both of you!
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Old 04-24-2003, 12:47 AM
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Default Re: General Jetting Info (Very Good)

Quote:
Originally posted by mohtks
Diamond - the '02 bike responds very well to just jetting changes. It is not necessary to do the GB/BK mod or the P-38 unless you like to tinker.

The air/fuel mixture on your carb is throttle position sensitive.

The pilot jet and fuel screw from 0-1/4 throttle
The needle from 1/8-3/4 throttle
The main jet from 1/2 throttle to WOT

Note that the areas overlap each other.

The needle and main WILL slightly affect the mixture everywhere but the effects are most pronounced at the ranges given above.

You need to run a leaner mixture for your altitude and you have really only done that for about 1/2 - WOT by changing the MJ.

You might also need a smaller pilot jet and 1 clip position higher on the needle to lean out your jetting over the entire throttle range. But before you change the pilot jet, tweak the fuel screw IN (leaner) and see if you can get your "hesitation" or stumble to go away without introducing a "hanging idle." If so great.

As for the needle, after you get the pilot/fuel screw where you want it, put the clip one position higher and see if you can feel a stronger pulling engine. If not, then put it back since there's no reason to run a leaner engine if you can't tell the difference.
Uh, I don't know what the rest of you think but that is some very good info on jetting.

This whole jetting thing is a mystery to me. I still haven't jetted my CRF for the 4200 feet I'm at. I don't have problems though, and can barely ride it as it is, so I haven't messed with it, but I will one of these days.

Anyway 5 stars from me. DANG GOOD Info.
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Old 04-27-2003, 10:44 PM
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Location: Texas
Posts: 21
Default Re: General Jetting Info (Very Good)

Thanks Woody for the thumbs up on the jetting tips.

Basically, it all makes sense if you think about what happens with this carb.

(1) When the throttle is closed the needle goes into a little hole and blocks the main jet orifice. In this case the pilot jet sort of acts as a bypass. So the pilot jet regulates the fuel when the throttle is closed (or very small openings).

(2) When you open the throttle, you pull the needle up a bit and fuel is pulled up past the needle through the main. The needle taper and clip position thus greatly affect how much fuel is pulled in.

(3) Once you get to about 1/2 or more throttle, the needle is no longer blocking the main jet orifice so it is only the size of the orifice that regulates the fuel.

(4) Note that almost all the fuel goes through the main so that changes in the main DO affect areas other than 1/2-WOT. Same idea with the needle is raised or lowered with the clip position. Even if you don't crack the throttle, lowering or raising the needle by changing the clip affects the fuel that can be pulled through the main.

REAL experts may notice that I neglect the effects of fuel screw, accelerator pump and air jets. But the concepts outlined in 1-3 are a good start into understanding it all!
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Old 04-28-2003, 02:20 AM
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Default Re: General Jetting Info (Very Good)

Quote:
Originally posted by mohtks
Thanks Woody for the thumbs up on the jetting tips.

Basically, it all makes sense if you think about what happens with this carb.

(1) When the throttle is closed the needle goes into a little hole and blocks the main jet orifice. In this case the pilot jet sort of acts as a bypass. So the pilot jet regulates the fuel when the throttle is closed (or very small openings).

(2) When you open the throttle, you pull the needle up a bit and fuel is pulled up past the needle through the main. The needle taper and clip position thus greatly affect how much fuel is pulled in.

(3) Once you get to about 1/2 or more throttle, the needle is no longer blocking the main jet orifice so it is only the size of the orifice that regulates the fuel.

(4) Note that almost all the fuel goes through the main so that changes in the main DO affect areas other than 1/2-WOT. Same idea with the needle is raised or lowered with the clip position. Even if you don't crack the throttle, lowering or raising the needle by changing the clip affects the fuel that can be pulled through the main.

REAL experts may notice that I neglect the effects of fuel screw, accelerator pump and air jets. But the concepts outlined in 1-3 are a good start into understanding it all!
That's what I'm talking about. Very good information. I can honestly say I had (before I read this post anyway) no idea how this all worked. So thanks again for the info.

I have a question for you since you obviously know you jetting.

If my bike was jetted from the factory for sea level, and I'm at 4200 feet, would my bike be more than likely rich, because of the lack of oxygen?

So I should lean it out, but going smaller on the Main, and needle?

Is that right?
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  #10  
Old 04-28-2003, 09:21 PM
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Location: Texas
Posts: 21
Default Re: General Jetting Info (Very Good)

If your bike was jetted for sea level, then gaining altitude (at the SAME temperatures) will make it run rich. But if you lose temperature when you go from sea level to altitude, it might be a wash. Lower temperatures will make you run leaner.

Here is what I would do, assuming your temps are 60-70 degrees or so and you have no off idle hesitation (i.e., your pilot jet and fuel screw are dialed in). Go ahead and change the clip position one step higher (leaner). If you feel a positive difference (i.e., bike runs like a rocket) then you are good!

Then change out the MJ one step leaner too. Same idea. If you feel it, great. Keep it. If you don't feel it, put it back.

Either of those changes may require slight adjustments to the fuel screw position.

Unless you are popping or backfiring badly you will not hurt your bike so just try it!
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