Chunky Countershaft

Hey! My sons 04 yz125 recently had the rear wheel lock up after the chain and countershaft got totally packed with gooey,thick clay,and gravel.We cleaned everything out right away,but there is now a notchy stop/go spot on the shaft.With the chain on in neutral,it will cause the rear wheel to almost stop as it goes around,and it won't go around twice.With the chain off,it turns completely freely by hand,until it gets to that spot,and then it takes alot of effort to get it past the hard spot.My mechanic said he didn't see anything too bad in there with a scope,and that it could get better or worse if he rides it again.My boy took it out again and said he could feel the rear wheel tightening up at low RPM.And I could tell the rear wheel was spinning less freely in neutral.The chunk spot is getting worse.Any thoughts??

Do I understand you to mean there is a "tight" spot with the chain off, and you are referring to the wheel, right? If you're referring to a binding in the transmission as it rotates, I have to say gears, bearing, shaft, in that order.

Id suggest replacing them. Dont know why the rear wheel would still be rough at certain spots unless the bearings are jacked from the force of the stop, same with the CS sprocket and shaft. Somethings jacked up pretty good if wheel doesnt spin freely through one rotation.

Agreed: if it rear wheel as well, I would replace wheel bearings. Beats pushing.

Everything I'm talking about is,as I stated,at the shaft.Wheel is fine.Sprockets and chain are fine.It's definitely something internal.Thanks

Sorry I misunderstood. I would guess that some teeth on a gear are somewhat "smeared" for lack of a better term. I wouldn't trust any bearings either, they're probably brinnelled, which means a dent in the race from the ball or roller. While you're there, do the seal.

I am kind of amazed a piece of gravel got in there. I do more "rock riding" than anything else, and haven't had one yet. Now that I've said that, I'll pick one up. For mud and goo accumulation, you can make a flap out an old inner tube with a hole for the chain, and it will help keep some of it out. I went to a really open countershaft cover to allow everything that got in to get right back out. I am not riding a YZ, but the concept is the same.

Thanks,Yamaguy.I don't think a rock got "into" the shaft or cases,it was just totally packed all around it.My friend said he saw what you described as "smearing" on a couple of gears,and thought they might clean up with use and oil changes(kind of like deburring).There were some shiny shavings and a couple of small chunks when I first drained the oil,less the second time,but the third time my son took it out,the rear wheel(because of the countershaft)was really dragging,and I figured we ought to split the cases.Have not gotten to it yet,as my friend is really busy,and I just had a bad crash last Sunday where I broke 3 ribs and collapsed a lung.So you guys are stuck with me as I take my sons bike apart in my mind and here at Thumpertalk.WHOOOOO!!!!!

Thanks,Yamaguy.I don't think a rock got "into" the shaft or cases,it was just totally packed all around it.

I know, I wasn't talking about inside, but into the chain/sprocket interface. I'm still amazed it happened. I guess it could, we have nothing but rock, rock, more rock and clay around here. I tend to avoid the clay, as nothing good ever comes of it. Like 6ft deep mudholes that swallow cars and bikes.

Have fun splitting the cases. Buy the right tools for disassembly and reassembly, and change all seals and gaskets while you're there. And use the yamaha stuff for the case halves, not RTV.

Definately something wrong inside the gearbox. You will need to split the cases.

Have you removed the counter shaft sprocket and checked behind it. I'm sure you have.

ya definatly dont use rtv. engine will get hot and explode at 74mph just as your hitting a tripple jump :confused:

been using rtv for years on cases, cylinders and everything else. i guess when your a real mechanic you can do things the lesser folks cant :thumbsup:

Put a dial indicator on the countershaft.

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