yzf450 4th gear studder

I am posting this question for my neighbor. He has a 2004 yzf 450. The jetting is stock except for the clip being dropped one notch on the needle. The bike runs cleanly through all of the throttle in all the gears but 4th. In this gear the throttle response is crisp through the pilot. Once you go past 1/4 throttle the bike surges like crazy. This does not happen in any other gear. My first thought was jetting, but because it is gear specific I was wondering if it could be something else. The air filter and carb are both clean.

Any thoughts.

Thank you for your time in advance,


Are you sure it's missing? If the engine pulls well in 3rd at heavy throttle, what you may have is a worn 4th gear/bent shift fork problem letting the trans jump in and out of gear under a load. The locking lugs on the side of the gears round over and will no longer hold themselves together under a load. The fork will be bent either as the cause or as a result of this happening.

He took the tranny apart a couple of weeks ago and all is well. Shift fork and cogs look great. It feels like a bike with an airfilter that is caked with mud or like a bike with jetting waaaaaaaay to lean, but only in fourth gear. Any other thoughts.


...like a bike with jetting waaaaaaaay to lean, but only in fourth gear....


There is no way that the jetting could be single gear specific...RPM-Throttle position specific, but not gear specific. If the problem only happens in 4th, its probably in the transmission.


My 05' 450 did as you described. Acted almost like the clutch slipped in fourth. Grayracer called it.. although the damage to the gears can be very hard to see unless looked at by someone who has been there. I replaced third and fourth gears along with both forks. One fork showed obvious damage. I'd bet that is it. Best.

I'm agreeing with the above. If it's solid as a rock in every gear but 4th then it's most likely a tranny issue IMHO. Ask him how closely he checked the gear dogs. Especially if he likes to shift under power without the cluth, the gear dogs can get rounded which can cause the dogs to pop out under load. It might be another part, but I'm putting my $1 on worn gear dogs. :cheers:

my 05 has done this in 4th since new(i've tried about every jetting scenerio possible)... its only under steady load between 1/4-1/3 throttle,only in 4th. near as i can figure the bike seems to run best anytime else rather lean,when i richen it up the stutter in 4th goes away but runs worse any where else. so my solution has been to live with it because the only time it appears is on connecting pavement and cruising fast dirt roads,i'm sending my carb out to zip-ty here soon we'll see if that changes anything. i'm 99.44 % sure its not my trannie:excuseme:

But, yours clears up in 4th when you open it up, right? It sounds like the OP is saying his acts up at everything more than 1/4 in 4th.

ya, if you get up on it past 1/2 throttle it goes away. but when it is happening its rather annoying too say the least. doesn't do it in any other gear though

Grayracer's diagnosis is correct. I had an 03 that I used for desert racing and had the exact same problem. It only takes a slight rounding of the engagement dogs and your bike will pop in and out of 4th. You'll need to replace 4th gear and the shift fork to solve the problem.

One other thing you can do is undercut the engagement dogs just a bit. This will provide a much stronger engagement of the gears and keep them from slipping apart under load. Have someone who knows what they are doing do the work as you don't want to go too far as it will be tough to get out of 4th if done incorrectly.


Mine did the same thing 1/2 throttle and up in forth. It was jetting. Changed main jet and can fully pin forth with no problem. Try changing the main jet, their cheap enough to experiment with.

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