Stumble at constant throttle

I have a 2000 426 supermoto. At constant throttle the bike stumbles and hesitates.

I have played with the jetting 42,45 pilots, different needle settings and air mixture screw setting and cleaned the carb. Nothing seems to make a difference. I can feel the difference in how the bike runs when I change the jetting but the stumble is still there. With the jetting's correct the bike pulls very strong and idles well but will still stumble at 1/4 - 1/3 constant throttle. I need to check the accel. pump. anything else I should look for? In other post people suggested the slow air screw.

Any other suggestions? I have a Dakar dual sport that isn't working correctly, could it be electrical. Thanks for the help and the past suggestion in my Hanging idle post

Originally posted by Mike Walsh:

At constant throttle the bike stumbles and hesitates. I have played with the jetting 42,45 pilots, different needle settings and air mixture screw setting and cleaned the carb.

…With the jetting's correct the bike pulls very strong and idles well but will still stumble at 1/4 - 1/3 constant throttle.

Whenever I hear something like that I think Pilot Air circuit, provided you really meant idle fuel screw when you said “air mixture screw.” If not then I apologize for jumping to conclusions, but a lot of guys mistakenly refer to the fuel screw as an air screw.

The YZF does not come with an air screw, but you can buy one from Sudco, and that is what I would recommend to deal with your problem. I agree w/ Rich about the accelerator pump.

Additionally, I’m an on a mission from God to reach every TTalk poster that owns an ’00 YZF and convince them to replace the needle w/ the version that comes stock/std. in the ’01 and ’02. The best twelve dollar mod for every ’00 YZF. So please help me with my pilgrimage…

Greetings Mike, my 2002 WR had the same symptom. Your AP should not be a factor since your symptom is steady throttle. If its surging, not just missing, your probably typically lean at that point in your throttle. So far I have little improvement by doing anything with the pilot or main to resolve. Pilots would change low end idle and throttle response, mains woul change top end pull but no effect on steady throttle at 1/18 to 1/3 opening. The BIGest factor at that point in your throttle opening is the diameter of the needle. Ie DQR needle was stock on mine. I tried a EMM and the surge was gone but then had a rich miss. Went to far, I am now trying a EMN on clip 3, still working on it but it has greatly improved may still be a bit rich.

Hope that helps.

Ps. Third 3rd letter in the needle part# is the diameter.

Matt

I've noticed the same phenomenon on my '99 ever since I've had it. You say you have a 2000 426? So that's a YZF, right? I was going to suggest cutting the gray wire because that helped a lot on my bike (but it's not gone away completely). But you don't have a gray wire on a YZF. It shouldn't have anything to do with the accelerator pump. Have you checked your TPS? I've never tested mine to see if it's adjusted properly and now that I'm thinking about it, maybe I should see if mine is. Hmmm...

[ June 07, 2002: Message edited by: Rich in Orlando ]

Originally posted by YamaMoto:

Ps. Third 3rd letter in the needle part# is the diameter.

Matt

I should mention that is what changed from ’00 to ’01. They went two sizes richer on the straight diameter, this governs mixture just off the pilot jet.

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