Well, I'm back.
The wonder bike, that I wonder how long it'll stay running, died again. It was working great, then came to a skidding halt.
My son took it apart and found small shreds (the size of a fingernail) of black plastic under the piston, on the crankshaft. I'm guessing it's off the side of a bearing, but I don't know if they have a plastic shield, or if there's something else it could be.
The piston looks fine, but it turns over hard (without the jug).
What am I into now fellas? I guess it's time to split the case?
How about new seals in the forks while we're at it, major or minor work?
I appreciate ya,
Dad

Don't know what year your bike is, but many YZ's has plastic stuffers in the crank. If you have a rod bearing or even a main bearing starting to let go it could easily cause this problem.

I am not sure what the mystery stuff is possibly the crank shaft stuffers not sold separate from yamaha . some companies offer them in metal rather than plastic :Zip Ty Racing (http://ziptyracing.com/products/products.aspx?id=MTAyNw==)
and :Specialty Products - Mongoose Machine and Engineering - Cylinder replating & specialty machine shop services for motorcycle, snowmobile, watercraft, marine, ATV and kart dealers (http://www.mongoosemachine.com/specialty-products.htm)

but I agree it sounds like your crank is the root of the "no run" situation.

Edit: I didn't read Bpro's post close enough I should just have agreed with what he said . LOL

3rd vote for it being the stuffers.

that's a crank rebuild bud. Plastic caught up in roller bearings = one hell of a mess.

I've been out of the loop for a few years, but the Hot Rod Cranks were a pretty good way to go not too long ago.
Hot Rods Products (http://www.hotrodsproducts.com/)

They seem to cover a fairly wide range of years on the 125 so hopefully you can get back on the track sooner than later.

Excuse my ignorance, what in the heck in a plastic crank shaft stuffer?

yours doesn't have one, don't worry.

Excuse my ignorance, what in the heck in a plastic crank shaft stuffer?


Their designed to take up voids or space in the crank case to improve flow through the crank case. They are mounted in most cases to the crank shaft and don't fail unless they get hot from a big end rod bearing failure etc.

and 4-strokes don't flow charge thru the crankcase, and you want as much volume in the crankcase as possible to keep the pumping losses down.