Vintage Dirt Bike Q & A

I've owned my MR since 76, rode it as a kid, was stored many years, started riding her again about 10 years ago in "easy" vintage events. Bike is in great shape, never abused.

Never had a problem, until my teenage son did "something" to the tranny. He entered a corner, slow speed, I thought he hit neutral downshifting by accident... anyhow, bike didn't stop running, you can click the lever up and down, but no engagement inside. No big (or odd) grinding or banging noise at all.

Any thoughts? I haven't taken anything apart yet. I have a few "donor" bottom ends, but have not taken a motor that far down before. (top ends, yes). Always have done all work on all my bikes, so I'm not a pro, but I'm comfortable with tools.

Common problem? Any type of "testing" I can do to narrow things down?

Any thoughts are appreciated. Thanks

Since you'll almost certainly have to do it anyway, I'd pull off the clutch cover and clutch, then inspect the shift spindle, drum, stopper arms, etc. - all the stuff on page 29 of the shop manual. Maybe your son downshifted into neutral by accident and then the shift mechanism jammed or disengaged, leaving it stuck in neutral.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks, I'll let all know what I find.

All looks good, from the outside, I guess it's time to go in...

What happens in the photo when you upshift/downshift with the lever? (the countershaft will have to be rotating for the gears to engage, obviously). Can the shift drum be rotated by hand, or is it jammed in position?

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I burnt third gear up on my mt250 same tranny as the mr just a bit different on the gearing. I didnt have it fully installed in the crank case when I put it back together. The dogs rubbed so much they quit engaging third gear it was like I shifted to neutral. To me it kinda sounds like you broke a shift fork pin and now the fork is not moving. I think it will be pretty obvious once you take the case in half. Dont worry its not hard especially after three times lol. Once its split you can lay the engine clutch side down on a engine block and rotate the tranny while shifting thru the gears and you should see whats wrong. Just remember the shift drum is pressed in place and if removed make sure its all the way back in or third will burn up within a hundred miles, well mine did anyways.

What happens: when I shift up and down, the exposed mechanism ratchets and move the way it should ( I believe). When I turn the back wheel, I can see the gear and shaft move lightly. I say lightly, as when I hold either with my hand, they won't move, zero tension.

Everything inside looked good, even the clutch basket/plates look new! Years of Bel-Ray have done pretty well. I mean if you don't count whatever is broken!

I've never split a case before, guess it's time to learn!

Same here! I've got two that need splitting, and I've been putting it off as long as possible (this winter it'll get done, for sure).

It sounds like all the gears continue to freewheel regardless of the shift drum rotation. I think woosh is right, most likely some kind of shift fork or drum problem.

+1 on Bel-Ray.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

The only thing you need to worry about is getting the shift drum bearing fully seated in the case. This is very important and is how I burnt up my tranny. Glad it happened tho as I got a back cut jastco tranny as a result and couldnt be happier.

Update: Motor out now, and a discovery: the front sprocket was barely making a connection, as the splines on the shaft are pretty much sheared off.

Anyone ever seen this before or know the cause?

I have a few donor motors, so i'll be splitting the case and putting one in...

If the sprocket gets even a little bit loose, it can worry the splines to death.

Was the sprocket an OEM type with two bolts and 'fixing plate', or was it held on with just a circlip? I prefer the OEM, as it prevents the sprocket from cocking or sliding across the shaft, neither of which does the splines any good. Aftermarket sprockets can also vary in hardness and tightness of fit on the shaft.

Right now my MR175 is running a circlip type sprocket as I couldn't find a 16T with bolt holes, so I'm also using a RH crankshaft collar (Honda P/N 13616-360-000, used on all years CR125M/MR175/MT125) between the sprocket and bearing. If you find yourself using a circlip sprocket, perhaps this may help prevent shaft damage in the future.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray thanks for the collar tip. It was OEM style with the bolted plate (the plate "splines" were also heavily damaged.)

I bought a CT70H motor that had the same problem.It had no sprocket on it so I don't know how they had it on. At least the problem was obvious you didn't need to do any digging.Are you going to pull one out of a parts motor?