2002 YZ426 part throttle stumble

I've done my research and I am usually REALLY good with diagnosing carb issues but this thing has me baffled. It sucked a valve and that's when I bought the bike and rebuilt it. The engine is fine, cam timing is spot on from what I can tell. The Yamaha manual shows the mark as a big line on the flywheel but apparently the '02 markings are different?

The bike starts on the first kick one day and the next I have to push it down a hill to get it to fire off. It idles fine and runs fairly well wide open but anywhere between is horrible. My first guess was clogged jet/passage way but it still runs the same. Took the carb 100% apart, checked every single passage way and cleaned every single part, found nothing.

The accelerator pump shoots a nice big stream, brand new NGK spark plug, I am lost.

I know people have issues with off-idle bog when they snap the throttle, high RPM breaking up, etc. but I can't find anyone with my same issue.


Here's a video of what it's doing.


Here's a picture of the bike for those interested in seeing something awesome.

Edited by TheSlicer

is that a dent in your header? if its a big enough dent that could be causing the problem

Dent is big but not big enough to be a problem. The bike has moments of clarity and will do power wheelies in 4 gears. I've still got a few things to try but it's driving me up a wall.

Sounds electrical to me. Maybe a ground loose on the frame. That's where I would start. Sound to "perfect" to be carb. Carb problems usually erratic sounding if that makes sense.

The bike looks great. I like the gold hubs.

I'm gonna ask the stupid question. I'm pretty sure you did, but when you installed the header and pipe, did you rejet the bike? Does this problem drivability problem have any intermittences? Hot? Cold? I would perform a cylinder leak down test on it. If the valves aren't sealing correctly, it may very well cause the drivability problem your having.

The jetting was correct for the altitude. It had a 152 main, 42 pilot, 3rd from top needle position and the bike never gets ridden under 8,000ft. I ended up putting the needle all the way down (top clip position) and the bike cleared up a bit but still wasn't right. It does 5th gear power wheelies with the 42 pilot, second needle position, and a 140 main jet but it dies randomly and will not start after riding unless I let it cool down enough to stick my hand on the header without it burning me. I know it should be lean as hell with a 140 main and the needle as far down as it is however it will not run with the jetting that it SHOULD have at this elevation. When the guy behind me on a '03 450 is running a 158 main, 42 pilot, and 3rd clip position without any issues and mine won't even accelerate with that jetting tells me something is wrong. I've tried 3 different spark plugs as well.


I rebuilt the engine after the last guy blew it up and it should have run fine. There is virtually NO leak down on this engine, valves and piston rings seal extremely well. Valves are adjusted, cam timing appears to be correct. Yamaha shows the timing mark on the flywheel as a big line but when I look at the flywheel there is no such mark so I had a bit of a laugh at that.


I am running out of things to check. I pulled the top end apart again but I have not found anything wrong. Checked the coil and CDI, both fine. I'm going to keep digging through the bike until I find something out of place. It's rather annoying when it runs great but won't start if I shut it off or stall it.


At least I still have other more reliable bikes to ride in the mean time.





Edited by TheSlicer

If you can't locate the timing mark, pull the plug and find TDC by probing for the piston. 


Normal sea level baseline jetting would be about a 45 pilot and a 165 main.  At 8000 feet, that should go to around a 42 pilot and a 155 main.  


Do this: Check for wear at the top of the main nozzle ("needle jet").  If it looks oval instead of round, that's one problem.  Check the vacuum release plate on the engine side of the slide to be sure the seal is there, and the the corners of the plate aren't worn or damaged, and that it's installed right side up ("square end" should go down).


If that checks out, try it once with the TPS disconnected.

I am having the same issue. Went to Rainbow Falls yesterday and the low end stumble was driving me nuts.

Not to interject with a useless comment, but that is a beautiful 426

Not sure if you got this fixed, but I just recently had some "stumble" issues right off idle (02 yz426). Not when really cranking on the throttle, but when you give it a gradual crack open. It would studder/miss. It would go away, then randomly appear. I unplugged the TPS and the bike instantly runs better. No stumble, and even more power throughout the rpm range. I guess my TPS is going out or it got misaligned.

Edited by RBI

Apparently these engines are made out of cheese and I didn't notice when building the engine. Well, that and I guess Yamaha didn't notice either and screwed up writing the service manual. 0.10mm clearance for intake valves does not seem to be a suggested minimum valve clearance at room temperature but instead the point where the valves hang open when it warms up.


I tore the engine apart piece by piece measuring everything 10 times and finding nothing so I decided to see what the valve clearances were when the engine was still hot. I could not believe it. I could fit a 0.025mm feeler gauge on the right hand side valve and nothing on the other two meaning they were hanging open! They were all right around 0.08-0.11mm cold. I find it hard to believe everything expanded that much! What the heck is up with that?


I set all the intake valves to 0.15-0.16, took it out for a spin and everything felt good but I did not trust it for trail rides. I put some motard wheels on it and took it to the track. Flawless. Started first kick hot or cold and I was quicker on the straights than any of the 450's out there (other than my Aprilia). The jetting was a bit off due to prior troubleshooting but the thing ran perfectly.


All the rest of my bikes in my personal collection have intake tolerances in the range of 0.05-0.12mm. The Yamaha tightens that much just when it warms up?! I am starting to see why so many of these bikes have valve issues.



On second thought, at least it looks awesome in motard trim sitting next to my Aprilia.


Edited by TheSlicer

I would love to motard my 426....looks like a ton of fun

Glad to see its running great!


My previous suggestion did not actually cure my problem. It came back. The AP squirt nozzle was getting clogged up and now is permanently clogged with something. The squirt of fuel was in two streams and very weak, but also 7+ seconds long. Carb sent back to Zip Ty racing. I am awaiting it's return.

Zip Ty did all the carb work on mine before I bought the bike. The AP squirter acts like a freakin' fire hose and shoots a stream of fuel half way across my shop when pulling up the slide to remove the needle.

The little thing does well even against big bikes on straights!




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