Posts
208
Joined
6/2/2015
Location
Lancashire, GB
Edited Date/Time
5/29/2016 5:10pm
Hi all i wonder if someone can please explain this issue and why it causes the rear wheel to lock up
Hi all, having a little problem with 1st gear on my Yz250 1987 model
The engine has just been rebuilt completely new clutch, crank and 95% of the gearbox replaced
The engine shifted perfectly on the bench
If i try to select 1st from neutral it grinds when letting the clutch out the rear wheel locks up and the shifter lever locks up too, rocking the bike back and forth or kicking the engine over releases the locked up shifter and rear wheel
With the clutch in 1st gear clicks in and engages no problem as soon as i let the clutch out it starts grinding if i kill the engine thats when everything locks up
Yet if i ride the bike setting off in 2nd and go up through the box to 3rd, 4th or 5th and then come back down through the box and into 1st gear with the bike free rolling then 1st gear works no problem at all without grinding it stays engaged
All other gears work fine and so does the clutch
3 new shift forks, shift shaft have all been replaced
And everything was assembled as per the clymer manual and parts diagrams
The picture in my profile pic is the engine in question
My main concern is the transmission and rear wheel becoming locked up
1st gear wheel on the countershaft had ever so slightly rounded female dogs but not major wear, i could not source a replacement gear at the time cause the bike is so old it was one of the only gear box parts that didn't get replaced
I did replace 4th gear mating wheel on the countershaft, the one that engages with the 1st gear wheel
I have now sourced a new 1st gear wheel and another new 4th gear mating wheel
But i can not understand why worn dogs would cause the transmission to lock up
I have experienced worn dogs in the past but it normally just causes the gear to slip in and out, not lock up the transmission
Please someone explain why its locking the transmission and put my mind at rest
I have done a lot of research online and only found one person with a very similar issue his was an rm250 but his was third gear locking up the rear wheel and shifter and it turned out to be 3rd and 4th gear mating dogs both worn
But he didn't explain how this caused his lock up
Hi all, having a little problem with 1st gear on my Yz250 1987 model
The engine has just been rebuilt completely new clutch, crank and 95% of the gearbox replaced
The engine shifted perfectly on the bench
If i try to select 1st from neutral it grinds when letting the clutch out the rear wheel locks up and the shifter lever locks up too, rocking the bike back and forth or kicking the engine over releases the locked up shifter and rear wheel
With the clutch in 1st gear clicks in and engages no problem as soon as i let the clutch out it starts grinding if i kill the engine thats when everything locks up
Yet if i ride the bike setting off in 2nd and go up through the box to 3rd, 4th or 5th and then come back down through the box and into 1st gear with the bike free rolling then 1st gear works no problem at all without grinding it stays engaged
All other gears work fine and so does the clutch
3 new shift forks, shift shaft have all been replaced
And everything was assembled as per the clymer manual and parts diagrams
The picture in my profile pic is the engine in question
My main concern is the transmission and rear wheel becoming locked up
1st gear wheel on the countershaft had ever so slightly rounded female dogs but not major wear, i could not source a replacement gear at the time cause the bike is so old it was one of the only gear box parts that didn't get replaced
I did replace 4th gear mating wheel on the countershaft, the one that engages with the 1st gear wheel
I have now sourced a new 1st gear wheel and another new 4th gear mating wheel
But i can not understand why worn dogs would cause the transmission to lock up
I have experienced worn dogs in the past but it normally just causes the gear to slip in and out, not lock up the transmission
Please someone explain why its locking the transmission and put my mind at rest
I have done a lot of research online and only found one person with a very similar issue his was an rm250 but his was third gear locking up the rear wheel and shifter and it turned out to be 3rd and 4th gear mating dogs both worn
But he didn't explain how this caused his lock up
See the link below this guy had the same problem
See the last 2 replies by xdriver72 on page 1 and his last reply on page 2
This is the same problem i have
I just need to know why this would cause the counter shaft and rear wheel to lock up ?
http://www.thumpertalk.com/topic/1101103-2005-rm-250-having-problems-wi…
If yes then I'd look at the shift drum not notching in the first gear slot for some reason.
Let me ponder on it a minute,early and my mind aint right yet.
The only part that was slightly worn was the 1st gear wheel female engagement dogs on the counter shaft like the pictures in the link on the rm250 on thumpertalk on page 2 although the wear on mine was no were near as bad as the rm250
The Shop
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Just trying to find new shift fork number 1 as thats probably worn now due to the slipping gear
Thanks for the reply though any advice you guys can give is appreciated
If it isn't the shift drum detent wheel,shiter area not going to the 1st gear notch for some reason. Since the shifter is locking I'd suspect it is there and not in the main cases .
If it goes wrong after that sidecover is put on then one hopes it is the shifter hook that pulls the drum or the drum sticking halfway between the neutral notch and the bottom of the 1st gear notch.
Hope you dont have to split it again.
http://www.cmsnl.com/yamaha-yz250t-competition-1987_model9337/partslist…
I know part 16 was slightly worn on the female dogs and part number 17 was new although i suspect its now worn due the slipping issue
Shift fork number 1 moves part number 17 up and down to engage with part number 16
I just don't want the hassle of putting it all back together and its still not right
I fear a case split is the only way to fix it
It's engaging two gears at once - we typically see this when a retaining clip has popped off, or in some rare cases people put gears on backwards in some engines where this is possible - or on some engines often two gears look alike but are 1 tooth different etc.
I would make sure all shims are located where they should be - and not all aftermarket manuals are perfect...
I suspect my 4th gear male engagement dogs are getting wedged against the 1st gear female engagement dogs the actual raised area on the 1st gear not recessed area where the dogs would lock together when working normally
Which is causing the shifter and transmission lock up
If you look at the pics it should make sense
im just trying to aquire as much knowledge, info and new parts
Before i tear into it again
If the shafts aren't perfectly in line with each other the gears will 'walk' on the shaft when under load. I had a trans that ate a bearing, the previous owner had it bored to use an over sided bearing. The machine shop bored the hole off center and it would try to engage two gears at once when you put some load on it.
It's a long shot, but if you have to split the cases I would look.
Problem is the cost of this build/restore was not even funny i could have bought a new 2016 Yz250 and still had some money left over
It cost well over 7K and thats uk money about 8.5 for you guys
Pit Row
Since these pics were taken its a new pro circuit works a new fmf powercore silencer and a blue afam chain fitted
Here is what it looked like from the factory as you can see i prefer more of a custom look
Andy, I have built many of those engines working at a Yamaha dealership 1986-92.
Currently going on 27 years experience and I've seen problems like this, were everyone in the shop gave up and I kept looking and figured it out.
Every time it was something assembled wrong. Your bike shifted fine before someone rebuilt transmission.
Very easy to make mistake with Trans if everything was removed and not replaced at the same time piece for piece.
Of course you want to check the parts behind the clutch assembly first, but the shift drum STAR does not come off on the 87 model so it can't be that it is misaligned.
My best guess with all your testing notes is that a washer in the trans is in the wrong place, or a gear you replaced has one tooth more from the wrong year, but you'll need all the old parts to compare what went sideways.
It's strange the clutch seems to make the problem happen, but your clue is it locks up the shift shaft, that is from the drum getting locked up inside the engine.
If any of the Trans end washers get mixed up it can cause your problem, also if the shift forks on the longer shaft are reversed this kind of problem can occur.
It is not a worn gear dog, worn parts slip, not usually lock up, don't buy any more parts until you disassemble engine, it takes 1 hour to have engine on bench, you spent way more that that already with research.
But 1st gear never worked when i bought the bike the shifter just Wouldn't go down from neutral whatsoever
As for the shift forks i did get them the wrong way round when i first installed it as the transmission was binding at first, i then swapped them around and it was working fine after that on the bench
I'll bear your tips in mind when i split it down asap, thank you mate
So at least i can rule that one thing out
Have you looked at the thrust washer between pressure plate and clutch basket? Look for any scoring.
You can move the shift drum by hand with the clutch removed, I would try duplicating the problem on the stand, turning the rear wheel to help engage the inner trans gears with the star, if you can get it to lock again the problem is inside, if not maybe clutch/thrust/primary gear?
Tricky problems suck, I've fixed many.
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