Broken trans?

I did a quick search and didn't really find what I was looking for.


I've got a 96 yz125 and it just recently flipped out on me. I was putting around my yard after lowering the needle in the carb. I was pulling the bike into my garage and put it on the stand(still running) and turned the gas off. I was gonna let it run the gas out of the bowl so i just let it idle. Now I know that the rpms raise a little when about to run out of gas, but my bike went full throttle. I hit the kill switch and it didn't work, I opened and closed the throttle in case the cable was hung up, still nothing. I waiting a moment to see if it would run out of fuel, but it was really making me nervous screaming at full throttle. I jumped on it and mashed the rear brake and jammed it into first gear. I immediately fired it back up, and it idled fine, and the kill switch worked fine.

Fast forward to today, and I crank it up and put it into first and putt around the yard again. I notice that it was making an awful clank noise when I let the clutch out. I pulled the clutch cover off and everything seemed to look okay. I pulled the chain off and ran through the gears( with the bike on the stand) and it made no noise. So I guess it has to have a load to cause it.

I really don't wanna split the cases, but I fear I must. Any ideas what could be it?

Could have bent a shift fork, or done hard part damage. You can drain it and flush it to see if any parts fall out, but it sounds like you are gonna have to pop the seal

Ok, finally started tearing into the yz. I noticed this noise after I pulled the clutch halfway out. The trans is in neutral and there seems to be a lot of slack in this motion. The clunk happens when it runs out of slack and I reverse direction of the sprocket. Any more thoughts as to what this could be before I split the cases?


Wow, that sucks. Why did you want to drain your carb like that? It causes your bike to run super lean, and then the killswitch don't work because it don't even need spark to ignight when it is pinned like that.

I've always drained my carb like that. The rpms usually pick up a little, but never went WOT. I don't ride this bike every weekend, sometimes it could be months before I ride it, so I like to run the fuel out of the bowl so it doesn't gum up the carb. After a while, the ethonal in the gas nowadays turns to goo.

Having been through this years ago (bike sitting a while, didn't want fuel in it) I went to the problem of draining the tank after each use, even had a setup with an extra length of fuel line and a special cap for the gas can that the line plugged into. I don't think the amount left in the bowl would be that big of a problem, as it has the oil in it.

I do believe that leaving gas for high performance engines in plastic containers, fuel tanks, gas cans, etc, is a really bad practice and will get you 75 octane gas after about a week or so. The goodies they put in gas to make it high octane migrate right through the plastic, leaving only low grade petroleum residue behind. I have all of my gas in steel containers, some I've had for years. It seems to be good for about three months in the steel ones. I only mix up the amount of pre-mix I need, otherwise the can for the weed wacker gets more and more full. I used to have a lawn Boy 2 stroke mower where some of it could go, but no longer. Any pre-mix over about two weeks old is suspect.

The engines in the YZs and similar high performance two strokes have a lot more than listed compression ratio in reality: when the pipe and reeds are at full sync, you almost get a supercharging effect from the sonic action in the intake and exhaust system. So you are probably getting about 12-14:1 compression under those conditions. So crap gas doesn't cut it. Sorry for the wordy post, I read too much Gordon Jennings as a kid.

I wouldn't sweat the gas in the bowl if you use it more than once a month. If it bothers you, pull the jet access cap and drain it onto a rag. But I wouldn't store any gas in the tank if it sits for prolonged periods of time.

Ok, finally started tearing into the yz. I noticed this noise after I pulled the clutch halfway out. The trans is in neutral and there seems to be a lot of slack in this motion. The clunk happens when it runs out of slack and I reverse direction of the sprocket. Any more thoughts as to what this could be before I split the cases?

First up: the "play" you feel there on that movie is normal: it is the amount of "freewheel" for lack of better term that comes from the engagement areas of the transmission. That isn't a problem. Every gear should have the same amount, or very close. They are all like that. If you don't believe me, go find a guy with a street bike that has a center stand: put it on the stand, put in in gear, and grab the back wheel: you'll get anywhere from 5 to 10 degrees of wheel rotation. I just tried it with my road bike: easily that much in first gear.

Next: Do you have any play the shock absorbing part of the clutch basket? You were holding onto the clutch hub, not the basket. Should have no motion that you can feel between the two. Really check that clutch basket. If you keep the spark plug in, try and rotate the clutch basket and see if it moves a lot before the crankshaft starts to turn. Should have a very small amount of rotation, but only as much as it takes to load the cushion system. Certainly not clunk-clunk back and forth.

Before I spilt the cases, I would do this:

Turn the trans shaft and run through each gear, try and feel if anything is not smooth motion in each gear. Have someone hold front sprocket with a rag or something to provide resistance. Don't cut their fingers off. The Rube Goldberg setup is needed as you really can't tell any problems with transmission without some load/drag. That's why running it on the stand with the chain off didn't show anything. If there's a gear problem, you'll no doubt feel it this way. It has to be somewhere, nothing is nowhere.

But what you found in the movie isn't a problem.

Edited by Yamaguy55

Thanks yamaguy. I will definitely try that when I get home after work. As for the gas in the bowl, my bike was in storage for about 10 months and the gas in the bowl had turned to syrup. Drained the tank and had to soak the carb in cleaner for 2 days to get the sticky stuff out of the carb. But I guess i'll be draining it the proper way from now on. Thanks again.

Finally some answers! Luckily I can get individual gears. Too bad its about $55.


Teeth! We don't need no steekin' teeth. I like the crack better than the missing tooth. Actually, since you have it, it isn't missing, is it? Check to be sure you have no case cracks as well. Lots of ways, but easiest way and cheapest, it to completely degrease it, then put in a container of gas ( outside, so you don't blow yourself up) then pull out, wipe off. If cracks are there, gas will seep out. works surprisingly well. I doubt you have any, but you never know after that episode.

Of course, if you have access to dye penetrant or Magnaflux testing, use it instead!

Don't forget new seals, Yamabond4, and fresh gaskets. While you're there, Check use going in twice. Check the shift forks: if one side is more obviously worn more than other, replace it. $55.00 is cheap, even adding gaskets and seals to it.

Again, do it right the first time saves greater expense and aggravation later. No one I know NEEDS more aggravation. I certainly don't. Let us know how it turns out.

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