need transmission help on an 03 yz450 FAST!

5 replies to this topic
  • srfer71

Posted April 25, 2007 - 05:06 AM


OK, well my 03' yz450 is having transmission problems.

As of right now i have the whole clutch basket/clutch out of the motor. When i put the tranny into 1st gear, 2nd gear, andd 3rd gear it binds up. The rear wheel won't spin. It acts as if its catching on something. If i rotate the wheel hard enough it will move a little more. It feels that there is something binding up in those gears.

However, when i shift it ito 4th it is as smooth as butter and works fine.

What could it be? Im getting ready to go inside of the cases tomorrow and I'm double checking to make sure it is not something simple. Which I have already figured that its not, but it doesnt hurt to double check.

Also, can someone tell me how this tranny works? I need to know why it binds up in 1-3 gear and not in 4th.

I know there are 2 shift forks in the tranny, is 1 shift fork for the first 3 gears and one shift fork for the last 4th gear?


  • grayracer513

Posted April 25, 2007 - 07:35 AM


One fork is for 1st and 3rd, the other 2nd and 4th.

Your trans has 4 pairs of gears, one for each speed. One of each pair is on the main shaft, the other on the output, or counter shaft. To avoid damaging the teeth during shifting, the gear pairs are always in mesh with one another. To make it so that they can be engaged and disengaged, one member of each pair is splined to the shaft it rides on, and the other spins freely on its shaft. To engage any of the gears, a shift fork moves the splined gear next to the freewheeling gear of the desired speed over against the free gear, where it engages a set of lugs on the side of the splined gear with lugs or slots on the free gear. That binds the free gear to its shaft, and the power then flows through that gear pair.

The locking lugs are in either 3's or 4's, so there is a narrow range of about 10 degrees every 90 or 120 degrees where the gear pairs will line up with each other. That means that the trans shafts must be spinning in relation to each other in order to align the gears so that they can be completely shifted. Often, when on the stand, this will require some manual juggling of the wheel, and with the clutch off, you may have to manipulate the main shaft itself to get it to actually shift. With the clutch off, you also allow the main shaft to drift in and out along its axis far more than it does when the whole stack of parts is back in place, and that can throw off the internal gear spacing, and lead to some odd binding/grinding noises (temporary) unless you pull the shaft back out against the outer bearing, as it is in operation.

Try it again with all this in mind, and see if it doesn't behave better. If the trans is in fact shifted cleanly into each gear, but binds as you say, you probably have some missing gear teeth. Did you have some symptoms of trans problems before yopu opened it up, or was this something you noticed while working on the clutch?

  • srfer71

Posted April 25, 2007 - 10:31 AM


I noticed when it yesterday. I had started the bike up, let it warm up, and i was just going to roll it forward about 3 feet. I shifted into first, let out of the clutch slowly and it didn't go. It ended up stalling, then i tried it again and it did the same. So i put it up on the stand and tried to get it into neatrul, no worky. It didnt want to go into N. So i took the clutc plates out and it went into N. With the clutch plates out the whole tire/tranny turned fine/smooth in N. When i shifted into 1st it started to lock up. The wheel would spin about 1/4 way around and lock up, i turned it 1/4 the other way and the same thing. It does this in 1st, 2nd, and 3rd gear. If i force it it will turn but just barely. Then I shifted into 4th gear and it spun freely like it should.

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  • grayracer513

Posted April 25, 2007 - 12:38 PM


Doesn't sound good. Looks like time for a tear down.

  • srfer71

Posted April 25, 2007 - 09:27 PM


I just finished splitting the bottom end apart.

it tunrs out that the 4th pinion gear on the main shaft seized. it is suppossed to be a free spinning gear and it siezed itself to the shaft. Probably due to running low on oil.

I guess this is what i get when i buy a used bike. I didn't even get to ride it yet.

  • srfer71

Posted April 30, 2007 - 08:08 PM


updated, 3 bearings were also bad. Its alright since i have money to fix it ill will have a completely clean back after im done.

Plus it gives me a chance to really give the motor and whole bike a good cleaning. Since most of the places i ride is dunes it doesnt get to dirty over the winter months. However summer is coming around and ill be doing some track riding.

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