2002 YZ426F carb problems! Need high alt tuning advice

Hey guys and gals. I'm new here. I bought a brand new 2002 off of eBay last year. The bike only has about 30hrs on it now. I've never been able to tune the carb right. It's a Canada model so the carb is a little different jetted but not much.

I'm trying to find someone who knows how to jet them for high altitude. I dropped the main from 162(stock) to 152, and I've played with the needle (up and down) no luck. It's VERY choppy on the bottom end. So bad I can't ride it normal. I've also set the floats to what I think is correct. I've also turned the air/fuel screw, no good luck either.

I had a 2000 426 and it was tuned my Bruce Sass, Vickery Motorsports in Denver and it ran like a scalded dog. I'd love to know what he did.

What I'm guessing he did was, put bigger air jets in it. Becuase I took apart my carb on that bike once and it had a 160 main jet. I dropped to a 152 in my '02 and it still runs like crap! Any help!!! Thanks!

You haven't replaced the pilot jet? That's the jet that controls the off-idle response, the needle comes on at about 1/8th throttle, then the main takes over at 1/2 to 3/4 throttle. So if your "bottom end" runs rough, you need a leaner pilot and needle clip change.

Stock should be a 42 pilot, I would guess you need at least a 40 and more likely a 38 depending on your elevation. I would lean the needle one or two clips as well.

Your main may be OK at 152 but a 158 should be on hand...

skthom, thanks for the info. I figured the pilot jet was one of them. Now, by going down in numbers is that going increase air flow? The reason why I ask, is fuel jets, the bigger you go the more fuel, correct?

Go here and see what the Altitude recommendations are. I'm not sure what the differences are between the 2001 and 2002 carbs are but this should get you close. http://motoman393.thumpertalk.com/ Under Tech Articles go to Engine, click on YZ426 Jetting Tuning spreadsheet. There is alot of great info at this site!

ripntear :cry:

Hey, thanks ripntear. That chart is pretty neat. Sounds like I'm close on the main jetting. I'm at about 5200ft not 6000. So, I'll go in the 5000 chart at 60deg day.

So, it says I should be:

156main jet

40 pilot jet

3rd down from the top needle position

I'm still confused about the numbers on the pilot jet. is the lower the number the bigger the fuel hole or vise versa?

What about any other jets, such as starter jet? Or Pilot air jet?

The numbers on the pilot jet are the same as your main jet. The highr the # the larger the opening. Higher is richer lower is leaner. I don't know enough about the other jets in the carb. I have never had to jack with any of those. Make sure your carb is clean. If you have to take it off and clean it, do it. Make sure your plug is good and gapped properly. Make sure you are using fresh fuel. Read this article. It might help shed some light on your frustration. http://www.4strokes.com/tech/4sjetting.pdf

ripntear :cry:

Thanks again, ok, I understand the sizes now. I was confused at first, thanks for clearing that up.

So, what your saying is if I just adjust the normal main jet, pilot jet, pilot screw, and needle it should run good :cry: I've never heard of anyone messin with any of the others so you must be right. I just wanna know what that guy did to make my '00 run so good.

Yes....if it's your carb that's the problem. More than likley it is, but you have to make sure that it is in sound working order and jetted properly. Make sure you have a New Plug and Fresh Fuel also. Those are just as important when you are having a hard time getting the bike to run properly.

Good luck and keep us informed on how it turns out!

ripntear :cry:

I don't know if this will help but,I bought my 02 yz426 in Glenwood Springs CO,(before moving to az)It came set up from dealer with 42 pilot jet,152 Main jet,and EJS needle ,it ran pretty good,unfortunately I didn't spend much time riding it before I moved,Jim

jimn, yeah that sounds about right. I called Rocky Mountain when I first got my bike and they said all they do is jet it down to 152. That Bruce guy I was talking about does sidejobs outside of Vickery mtrspts, and he tunes them a lot better than the dealers do.

ripntear, I've had good luck running one step hotter plug in everything at this alt. I'll try and get it apart tomarrow and tell you guys how it goes! Thanks SO much!

ceetwarrior,The ejs needle I was refering to is not the standard needle,according to my manual it is the leanest needle listed(ejp is standard)This caused me all sorts of trouble when I moved as I assumed it was the ejp because all I thought they changed was the main.Mine also had the hotter plug.Good Luck!,jim

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