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YZ250F nonstop shifting trouble

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A brief history of my bike...

 

2009 YZ250F with 2 previous owners before myself. The two previous owners did not take the best care of the bike. One of the intake valve guides broke not long after I purchased the bike. I tore the entire engine down and rebuilt the entire top end after the intake valve guide broke. The bike shifted great before the very first time I tore the engine down.

 

Current time...

 

I have had nothing but shifting trouble ever since I put the engine back together. I have split the cases at least 5 times previously trying to track down and fix what is causing the shifting issues. I split the cases again tonight...this is getting absolutely ridiculous :foul:. From neutral, I cannot shift into 1st unless I damn near jump on the shift lever. If I put normal pressure on the shift lever when trying to shift from neutral to 1st, I hit a false neutral even while spinning the rear wheel. Shifting into 2nd from neutral is easy. Shifting from 2nd to 3rd, the transmission goes into a false neutral even while spinning the rear wheel. Shifting from 3rd to 4th is clunky but doable with excessive pressure. Shifting from 4th to 5th is easy.

 

My friend let me swap in his shifting mechanism (shift star, entire rachet system, etc.) into my engine today to test. His parts made no difference in the shifting. The transmission shafts and gears appear to be in great shape. The shift cam is in great shape. The shift cam bearing rotates like a brand new bearing. All the parts of my shift mechanism appear to be in great shape. All of the shift fork pin bores in the cases are in great shape and round. The long pin in one of the shift forks has a bit of side to side and rotational play (enough to feel and see). The two other long pins in my other two shift forks are nice and tight...no play whatsoever.

 

PLEASE help if you can. I will truly appreiate any help I get. I am at my wit's end with this thing. I'm over it :banghead:.

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The only thing I can see after inspecting the cases tonight that may be causing shift problems is what appears to be a little bit of wear where the shift cam rests in its bore in the case. Is the bevel around the bore's ID machined like that from the factory or is that wear? Looks like it's that way from the factory.

case.JPG

Edited by motoXfmX27

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Pull the crank out.  Assemble the tranny in the right case and shift thru the gears without the segment that has the pawl, pawl pin, segment, guide, and stop lever.  It should move into each gear.  Then push the left case half on the right half, and test movement again.  If it jams up, there may be something warped in one of the case halves and alignment when assembled.  If it still moves OK, next step.

 

Install the guide, segment, stop lever, pawls and pins, and shift lever.  I'm suspecting something in this section since I've had some trouble in that area before.  I replaced the pawls and segment. 

 

Also check the bearings with the shift drum in place.  it should rotate freely.  I know you said, but alignment is what we're testing now.

 

This mechanism is designed to shift under power, and will do it.  I don't recommend you do it, but it will.  It gets a lot pressure and will show wear in that section.

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The edge of the bore is machined, not a wear mark.  Be sure that you have correctly indexed the segment (indexing "star") to the cam.  There's a small pin in the cam that has to align with the segment. 

 

At this point, I think it's down to what Ron has said: get the crank out of the way and install part of the trans to see if it binds.  I'd start by removing everything but the 5th wheel from the output shaft and assemble that with the #3 fork and cam into the case and test for smooth rotation, then build up from there to see where you run into trouble. 

 

It is, of course very true that the trans will not shift as smoothly on the bench, or dead-engined in the bike as it does when it's rotating at speed, but you should still be able to isolate the problem this way. 

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Thanks for the replies.

 

I did as you both mentioned and assembled the transmission in the case half. The transmission eventually binds up and stops spinning after several rotations. I can get the transmission spinning again by slightly lifting the shift forks a bit higher out of their bores in the case. It's like my shift forks are not raising the gears enough. Is this a possible problem? The forks appear to be straight. I'm hesitant to buy new shift forks in the case they're not the issue.

 

To make things even more confusing and frustrating, the transmission shifts in and out of 2nd-5th OK (1st is still no go...false neutral without nearly jumping on the shift lever and gives me another false neutral between 2nd and 3rd about half the time) with the cases mated together while rotating the shaft(s). As I said above, the transmission binds when only installed in the left case half, and I have to slightly raise/lower the shift forks with my fingers to keep the transmission spinning. The only reason I posted this thread is because the shifting is just as terrible while riding the bike. I have no clue what is going on. I want to give up on this engine/bike and forget about it.

Edited by motoXfmX27

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OK, I sense a small amount of frustration going on there.   Easy man.

 

Jaming up when testing like that is normal from my experience, when you have out in just 1 case half.

What I was trying to determine is: was it a warped case, bent forks, bent fork shafts, or a bearing issue.

 

Just remembered another test for the fork shafts.  Roll them on a flat surface like glass top on coffee table or something.  You should be able to see if they wobble, a bad thing.

 

Put it back together w/o crank again, but before you do that, take a magic marker and put an index mark the fork shafts and try again.

If it still doesn't work, try twisting them 60 degrees or so and try again.

 

The only reason that I know of one not shifting properly is the drum not being indexed correctly, which is pretty easy to check by looking  at the tracks the fork tabs go into, OR using the wrong fork in the wrong position.  They are all different, look it up in the manual.

 

These trannys are just like a T5 in a Mustang or a Camaro, work the same, just scaled down sized from a car version. 

Edited by ronbuell

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