2004 yz450f clutch/gear box issues?

Hey all, been lurking here for a while decided to register. I got my first thumper a few days ago an having some issues. I took it on trade, knowing it needed a few things, new hot start cable, valve adjustment, plug, clutch cable adjustment, chain adjustment, changed oil, and took it out for a test.

Noticed it had a stumble in 3rd and 4th at high rpm, so I started playing with the fuel screw. Found it to run best at 1 3/4 turns out but still has the stumble. Bike starts good, for a 4 stroke. Pills hard first, second, and bottom or third, but high rpm third and forth have a weird stumble them pick back up. I got it out on pavement and if I go wot in third or forth it feels like I'm feathering the clutch lightly then re-engaging. Can feel a slight vibration too when it does it. I'm pretty sure it isn't a carb tuning issue because it runs great and will rev to the limiter in lower gears.

I pulled the clutch cover and all the plates for inspection. All the plates are within spec, so are the springs. Boss looked ok, the clutch housing has some scarring on the outside of it. I changed the fluids with yamalube 20-50, I do not know what previous owner was using but he said it had the stumble before he gave me the bike. I'm afraid this is leading to splitting the case, it is 10 year old bike. Clutch doesn't stink, get hot, I even tried putting it in 4th, revving it a bit then engage the clutch to see if it would slip but no slip just stall the engine.

Chain and sprockets look good, I'm stumped. I've been told clutch can still be faulty even when the parts are in spec. Help!!!!

The problem is the transmission.  The locking lugs on the sides of both 3rd and 4th gears are worn to the point that they are forcing each other out of engagement against the forks.  Both shift forks will be bent because of this as well.

That's what I was afraid of. :(

Thanks gray

See the attachment.  Have you looked at the specific areas shown by the red arrows?  Those are the locking lugs on the 3rd and 4th pinions that engage 4th.  Rounding or polishing of these surfaces can cause your problem.  Check the forks for signs of wear near the centers of the yoke area, and for loosened follower pins where they track in the cam grooves.  The fork on the output shaft controls first and third, and the one on the input controls second and fourth.


Gear layout from right (clutch side) to left is 1-4-3-2.


What attachment?

:bonk:  I was multi-tasking when the phone rang.  Have it posted by ten tomorrow.

Lol. Ok


I couldn't find any damage there gray. They got nice shoulders on them, no polishing or rounding. Is it possible the clutch was the culprit the whole time even though the mic'd ok?

That would be very unusual, unless I completely misunderstood your description of the symptoms.  But if the locking lugs AND the forks all look good, I would say it wasn't the gearbox.

My interpretation probably wasn't very good, lol. Either way I learned a bunch, before this I wouldn't have attempted opening a case.

I did notice some scarring on the outer diameter of the clutch basket. The boss looked fine, and the basket fingers weren't grooved. Just had some scars on the outer diameter of the fingers. What causes that?

Wasn't as hard as it looks, was it?  Watch out for all the little hollow dowel pins.  They'll stay in one side or other of the joint they align when you take them apart, and then they'll drop out when you're not looking.  Good idea to pull them out and store them with the bolts.  Um, maybe wash the engine a little better next time you open it up, too. :busted:


If you think you have scarring on the outside of the clutch, look for matching scars on the interior of the case cover.  Could be something broke off or fell off somewhere and went through that space.  Bear in mind there is a small area inside the case that gets fly cut for clearance, but that looks like machine work, and not a gash.


If the clutch were weak, it would slip first in 4th, then as it got weaker, 3rd and 4th, and so on.  If it slips under a load, it could possibly produce a shuddering sort of effect, called "chattering".  This could be your problem.  You might also simply try a new spark plug, and also look the coil over closely for cracks. 

Chattering is EXACTLY what it felt like. I feel really dumb for opening the case, but normally the hard way is the only way for me. And yes the dowels are kind of a pain, especially the ones with o rings.

I pulled the kick start lever, it had a little dowel and a spring that fell out from some where? How does it go back together? I didn't even realize the lever was spring loaded. Also had a little ball.

Gary is spot on. When those pieces wore on my bike it felt like i was "popping" the clutch in 4th. You could hardly see any damage at all but i replaced the whole transmission and it was fine.

You probably put a new clutch in it too?

nope. I actually got on here and asked everyones opinion and Gary is the one who told me about the problem, so i replaced the transmission parts and boom. problem was gone.

I should have replaced the clutch first to rule it out. I didn't replaced it because the plates all mic'd to spec, now I can't find an issue in the gear box. I guess I could replace the whole bike with a new one, lol

hey, thats always a plausible option! haha

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