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1998 KX 250 downshifts on hard acceleration

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I know this is going to sound crazy.  I bought a 98 KX 250 about a month ago, haven't got much time to ride it, only a few times.  But when I go out my long driveway and get on it hard, right when the powerband hits and the back tire starts to spin, it will downshift.

 

I've never had a bike do this ever before.

 

 

As a side note, I've been riding bikes for years, i'm not accidently hitting it with my foot, or pulling in the clutch.

 

At first the chain was really loose, and i've tightened the chain(actually had to take a link out of it) and now it isn't as bad but still does it.  If I ride it like an average rider, it won't do it, its only when I push it hard that it will downshift once the tire starts to spin...

 

 

 

Any ideas would be great.  I've googled and searched everywhere, and haven't had much luck.

 

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Your shift forks maybe worn to or past their service limits. Mine does it occasionally and I know that that is my issue.

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Pull the right side cover and look for broken spring on the shift mechanism.  

Ok will do.  Thanks for the info.

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Your shift forks maybe worn to or past their service limits. Mine does it occasionally and I know that that is my issue.

I was hoping it wasn't this.  Wouldn't have thought a worn shift fork would do this.  I thought they would just cause issues shifting when they were worn.  Thanks for the reply.

Edited by Doyle670

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I was hoping it wasn't this. Wouldn't have thought a worn shift fork would do this. I thought they would just cause issues shifting when they were worn. Thanks for the reply.

I would do what J suggests first. Easiest things to check first. My thought wouldn't be the fingers of the shift forks but the pins that ride in the shift drum.

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Pull the right side cover and look for broken spring on the shift mechanism.  

I assume you mean the clutch side?  Do I have to pull the clutch off too in order to see?

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You say that after you shortened the chain it got better? Post us pictures of the teeth on both sprockets.

 

Will do...

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Look at the photos on this page, you probably only removed the OUTER clutch cover.  http://www.southbayriders.com/forums/threads/108443/page-2

 

A few posts down you see the shift detent mechanism.  There's a star (has 6 nubs on it) with a detent.  That's what locks the trans in gear. The end of the shift shaft is where the round, white bushing is to the left.

Edited by Jeekinz

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Look at the photos on this page, you probably only removed the OUTER clutch cover.  http://www.southbayriders.com/forums/threads/108443/page-2

 

A few posts down you see the shift detent mechanism.  There's a star (has 6 nubs on it) with a detent.  That's what locks the trans in gear. The end of the shift shaft is where the round, white bushing is to the left.

Totally got you now.  Thanks, hopefully can tear into it this weekend.

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Pulled the clutch off and found some wear on the top tip of the mechanism that turns the 5 ear sprocket. But if I'm thinking right this isn't what holds the transmission in a particular gear. So shift forks is the next suggestion?

image.jpg

image.jpg

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That star is bolted to the shift drum, so yes, it's what holds it in gear.  Take a pick farther back, are all the springs on there?

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Hi

You didn't say which gear you're in when it down shifts?. had the same deal also once,  would jump out of 4th under hard acceleration and shift into 3rd. problem was the dogs on the gears were rounded off.

General abuse and neglect ended in this issue appearing. I'm guessing you'll need to split the cases to see what's worn-out in the trans. What holds the bike in gear is undercut on the gears dogs not the shift drum indent spring or shift forks.

 

All that stuff does, is engage the gears in the correct position. Now if there's an issues with shifter mechanism been worn, bent or any damages that may cause the gears not to be in the correct position,

 

Hence the start of problems

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