Yamaha Dirt Bikes Forum

I got a '90 YZ125 for $300. The guy who sold it to me could only get it running by pushstarting it. One of my buddies who rides all the time can get it to start after 10-20 kicks. I've never been able to start it but i'm  new to kickstarts. Once running it will never idle but pulls fairly well and hits powerband well (according to my buddy), my only experience is with streetbikes and my KT100 powered kart so i'm not sure how to judge power. Its pretty easy to start back up (~5 kicks max)after warming it up and riding it

I took off the top cover today and discovered that the piston had some nicks and gouges on the top of it as well as the interior of the top cover. The bore looked and felt very smooth.




I also took of the powervalve linkage cover to discover that the linkage had a lot of back-and-forth play at the top bolt.




Also I saw two wires that are just dangling and noticed that there isn't a brace in between the top cover and the frame.. shouldnt there be a brace there?



I'm planning on buying a new piston and ring set. What does everyone think about this, the wires, the linkage and the brace?  

allright those knicks look to be like they could be caused by peices of ring. was their chunks of metal in your exhaust? if there arent gouges in your cylinder hten hte peices had to go into your exhaust. as for the wires, a lot of wires are there for light hookups or w/e, some just dont attach to anything. as for your engine brackets. yes something should be there. my guess. the frame is off a bigger engined bike. ~pat  
PEACE GUYS ITS BEEN A BLAST!!!!! ~patrick

a friend of one of my friends blew up his yz125, but it was his first time time on a dirtbike, and he's used to a 1800cc road bike  
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03xr100 soon to be crf150r expert
id rather be a loser than a quitter

wow thanks for the quick reply! there wasn't any metal in the exhaust that I could find. in the ad for the bike it said that it had been honed recently, perhaps this is previous damage and the previous owner honed the bore and replaced the rings before selling the bike?

also what about the power valve linkage? i'm pretty sure it shouldn't have that much play because it would occasionally catch the locating pin hole just when I was playing with it with my hands

thanks again for the replies  

the last time i saw marks like that in the head and piston was after a lower rod bearing failure. bearing shards were blown up through the transfer ports into the combustion chamber. pull the cyl and inspect the lower end very carefully. the pitting on the head and piston are from something getting pounded around in the combustion chamber.  

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06yz125,kx60(2)

nice pics, and yes the head stay (brace is missing).  

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06yz125,kx60(2)

what should I inspect when I pull the clyinder? rod play?

I still kinda think this is all from something when the previous owner had it and when he rebuilt it, the piston wasn't replaced.  

Yes, there should be a triangular brace (one each side) on the head to frame.  

The marks are from some past unhappy event in the engine; broke ring/piston/ etc.  There should not be all that red RTV silicon either; my guess is they were re-using the old o-rings and used that to keep it from leaking (hopefulley not that the head is warped etc).

The hard starting can be a sign of a worn out top end.  Based on what we see I'd say since you have it somewhat aprt go ahead and do a fresh top end; clean the powervale etc and make sure to use new gaskets and o-rings.

Hopefulley you didn't get any ani-freeze down in the crank area; its always a good idea to drain the coolant before taking off the head.  
Dave H 2006 YZ250
(2008 YZ 125 son)

foCk:
what should I inspect when I pull the clyinder? rod play?

I still kinda think this is all from something when the previous owner had it and when he rebuilt it, the piston wasn't replaced.

I would see if any cross-hatching is left, and it probably would not hurt to take the cylinder to the shop and have is measured to make sure its still in spec.  The rod can have some side to side (they need that) but you should feel no vertical or up and down play in the rod.

I think the '90 used the older powervalve design. I would disassemble and clean it and look for major wear on it.

If you don't have a manual I would get one as it will have all the diagrams, torque values and things to check and examine.

What shape is the rest of the bike in?  
Dave H 2006 YZ250
(2008 YZ 125 son)

I'm going out today to buy another micrometer since mine isn't here, and buy a new piston kit. It doesnt look like they re-used any gaskets so i'm also going to get a gasket kit. What would happen if antifreeze got into the crank area? I was planning on doing an oilchange before and after breaking in the engine.  

I wish I had the extra cash to buy a manual but it'll be two weeks before I get another paycheck and I have just enough now for a piston kit and gasket kit.  

make sure you inspect the crank, it would be a shame to waste money and time on a new piston that will be ruined if more bearing material ends up in the combustion chamber.
rhe marks on the piston look fresh (carbon and oil residue would have covered them up)  

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06yz125,kx60(2)

make sure your measure the cylinder diameter.

i bought my bike last year and they said the top end was just rebuilt well like a month later it had a minor seize. i tore it apart and a peice of the top of the piston was chipped off but the piston looked brand new. so did the bearing on the top end. the cylinder was out of spec and it cost 200 dollars to get it replated.  
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Ride 2 strokes, buy 2 strokes, save the 2 stroke.

1998 cr125
2002 cr80

Wow looking at that top end show exactly why I do not run Klotz premix.  You have a very bad carbon buildup.  I would do a complete teardown and look for whatever has broke loose and did that to your piston.  It could be destroying otherparts of your engine if you keep running it.  

Bill H. 2000 CR250R

Those wires look like the attach to one another.  You have a male and female connector of a wire the same color.  

The head mounting bracket is missing.  On modern aluminum frames I have heard of riders omiting these to put more flex into the frame, but your old steel framed bike does not need that extra flex.  I would get a replacement before running that bike again.  

Bill H. 2000 CR250R