'01 YZ426 Problem - Help!

I just got my new 426 a couple days ago and things started out fine with it but have gone downhill..

I've only rode it for a total of about 1 hour. When I am going slow (from idle to about 1/4 throttle) and give it gas fast, the bike bogs and will die if I don't let off. If I give it gas slowly it works fine. Is this jetting or the accel pump problem that I've read about? Help please!

More Info Needed:

Where do you live? Altitude and Temp.

What gas are you running?

What is your current carb settings?

What plug do you have in it?

Have you changed the air filter? Did you let it sit overnight.

Do you use the choke to start it?

Do you let it warm it good after a wash?

Does it clear it after a bit of riding?



01 YZ426F #85 Vet C

Sorry I didn't give enough info.. I live in Maine. The temp has been in the 50's, altitude is 500ft.

I have premium pump gas in it now (still the first tank of gas in the bike). My carb settings and plug are the stock settings. I've only ridden the bike for an hour since I got it. The air filter is clean/oiled from the dealer and it was dry when I first started the bike. I looked at it yesterday - it is clean and doesn't seem to have too much oil on it.

I have been using the choke to start it, and I haven't washed it yet. The problem doensn't go away after riding it for a bit.


I would suggest you take it back to the dealer. You could have one with a bad CDI or carburator. I have read that some carbs have hair line cranks from the factory.

You have a 30 day warrantee on the bike so get the problem correct before it expires. I would print out all the topics here on plug fouling and poor perfromance. Especialy the one's that deal with the CDI issue.

That way you have proof of a problem when you go to your dealer. It could be something simple, but I would get it in to the dealer just to have documentation that the problem existed before the warrantty ran out.

If the dealer does not help, then you will have to sort out your carb and ignition.

Frist, pull your plug to see what it looks like. Should be able to generaly tell if your lean or rich.

Check your fuel screw. Sould be 1.5 to 2.0 turns.

Also, note the exahust gas and smell when it happens. This is also clues to the problem.

Then take your subframe off and turn the thorttle to see when your accelerator pump activates. Do not want it to hit the slide.

Take the carb off, clean and verify the settings.

Do a search on plug fouling, there is a lot good info there.

Good Luck,



01 YZ426F #85 Vet C


-Start by getting a very short screw driver and check how many turns out the pilot screw is. It's on the bottom of the carb in the deep recess in the float bowl. Try turning it out to richen the mixture a little. Adjust only 1/4 turn at a time and recheck the response & ride a little to feel the difference. This is probably all you need to do. At 2.5 - 3 turns consider a bigger pilot jet.

The stock pilot is on the small side (#42) which is conservative to avoid plug fouling. Many riders use a #45 or even a #48, but you have to be careful not to turn the pilot screw out too far and cause plug fouling.


That is a good start. But, my point was inform your dealer so you can get the waranty if you need it.

Like, I said it could be very simple. Just the fuel screw.



01 YZ426F #85 Vet C

Thanks for the info! I will try the pilot screw tommorow morning to see if it helps.

We still have about 2 feet :) of snow on the ground here, so I'm not in a huge hurry to get this sorted out, but I was considering the warranty period as forloop mentioned.

Again, thanks for the help! I'll let you know how it works out..

I tried a couple things, but no good luck yet...

I adjusted the pilot screw richer and the problem got a little bit better, but didn't go away completely.

I then took the subframe off and made sure the accel pump wasn't hitting the slide. It wasn't, so I took the carb off and took the needle out to move the clip intending to make it richer, but I moved it the wrong way (oops! - I moved it towards the top of the needle). I realized this after I got it all back together. But I tried it anyways. It doesn't seem worse, (actually maybe a little bit better) but now it backfires badly when I let off the gas.

Should I take the carb back off and move the clip in the direction I intended to in the first place? Should I even be messing with the clip?

Obviously this jetting stuff is all new to me.. Thanks for your help!

yes, move the clip the other way. You want to richen the needle. I raised mine 1 clip position from stock. That means lower the clip one position to raise the needle. Raising the needle richens the mixture in the middle. The fuel screw works the idle and low end. Main jet affects the top end. Also remember they all overlap a bit. I backed my fuel screw out a 1/4 turn and it seemed to help (richened the mixture). Always take notes of your original settings and your final settings.

A carb with an accel pump on it has different characteristics than a non-pumper. You will have to learn to not wack the throttle on as abrubtly on a pumper carb. Experiment with how fast you can roll it on and remember the technique.

Yes, definitly play with it. I am surprised you need to go richer given your environment. But, you never know. Did you have a look at your plug? This is a good indication as to what is going on.

When I first leaned mine out some. It popped some until it got all the carbon out of it.

I did try and experiment with mine. If it is idling and I just wack the throttle wide open it will bog and die. But, at the track it is perfect. Like Scooter said, this may just be a learning curve issue.

Good Luck.



01 YZ426F #85 Vet C

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