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Old 01-24-2015, 08:31 PM   #1
doorman OP
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yz125 resurrection

I've found myself a new project. A 200$ yz125 that hasn't run in years. I thought I would start a thread here to ask questions, and to show my progress to anyone who's interested. Here it is right after unloading it



Yes, it has no front wheel. That is the least of my concerns though.

The list of problems I know about are as follows: The motor was locked up when I got it, the carb is still locked up, the airbox was stuffed with mouse droppings and other goodies, the fuel tank leaks, the forks leak, the stator wires were all cut and the stator cover is missing, many bolts are missing, the exhaust was held on entirely by zip ties, The brakes all need to be gone through/replaced, plastics are cracked or missing, and I'm certain there are other issues that don't come to mind just now.

I wanted a project, and I sure got one.

The first order of business was to get the motor freed up. I started by pulling it from the frame





Half the mounting bolts were missing, so it was a breeze

Then the teardown began

Cylinder looks like it is in decent shape? I measured the bore, and it is within spec.



Lower end was still locked up at this point. I don't know what a 2 stroke piston should look like, but I think it's in decent shape? Diameter was in spec. and the ring gap was correct.



I pulled the cases apart to get everything unstuck and found the problem. Rust on the crank. I wiped it up, brushed it off, and everything spins free now.







So now my questions:

Would it be advisable to sand down the rust on the crank, or should I just leave it as is? It spins freely right now.

Should I replace the piston and ring even though they are in spec? I really just want to see if the thing runs before dumping too much money into it.

Anything else I should check while i'm in here?

Thanks in advance for any help. This will probably be a slow project, but I will update it whenever I make progress.
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doorman screwed with this post 01-24-2015 at 08:42 PM
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Old 01-25-2015, 05:40 AM   #2
woods wizard
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You might get more traction in the 2smoker section of Bikes. Lots of ole Yamaha guys there. GLW the overhaul.
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Old 01-25-2015, 07:28 AM   #3
Kawidad
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I'm always in for a good rebuild/rescue thread.

My advice, which is worth very little, is change out the piston and rings. The piston is showing wear on the skirt. Also while in there check all of the bearings and replace all of the seals. And, yes, get rid of the rust. A clean engine, is a happy engine.
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Old 01-27-2015, 07:13 AM   #4
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Looks like a good time Is that a 94? Should be a lot of fun when it's finished.
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Old 01-27-2015, 11:11 AM   #5
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I've never seen a rod look like that, check for up and down slop on the big end bearing on the crank, it may have gotten real hot. If crank was rusty chances are the bearings and races are rusty too so they will be bad, maybe not right now but soon if you get it running. You can see outline of ports on piston skirt from wear.
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Old 01-27-2015, 11:56 AM   #6
stainlesscycle
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i have seen a rod like that - failed inner head gasket lets coolant into combustion chamber/bottom end. if you' run it while this is happening (it'll run, just not well), it'll overheat everything and rod will corrode like that very quickly.

if it was mine, i would replace:
piston, rings, rod and bearings, crank bearings. i would consider replating cylinder too....


if you leave it as is, the bearings won't last very long. you did say it was locked up right?
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Old 01-27-2015, 01:57 PM   #7
doorman OP
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The bearings for the crank weren't locked up. The crank was corroded to the bottom of the case from a little bit of water that got in. All the bearings feel like new. No play in any of them. I checked the connecting rod for up and down play and found none.

I know it makes more sense to just replace everything while it's torn down, but I think I'm going to throw it all back together with new gaskets, and see if it will run. If it does, then I won't feel so bad about pouring tons of money into everything else on the bike while I tear the motor back down to rebuild it. I swear that doesn't sound as crazy in my head

On another note, I am in need of a new carburetor. The original is totally seized up, and unsalvageable.
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Old 01-28-2015, 10:01 AM   #8
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Your "see if it will run" plan makes sense to me. Measure crank free play, don't run the bike if it is out of spec. Same for piston to cylinder clearance.

Make sure you enjoy the rebuild process, because you will not come out ahead in the end compared to buying a $1000-$2000, running, newer, YZ125.

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Old 01-30-2015, 08:00 AM   #9
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I agree with the last statement. Be honest and clear with well defined goals for this. If you're wanting into a dirt bike for cheap, this is certainly not gonna happen. In the end, you'll have a 2500 Yamaha, worth 1200.

That said- if your goal is sincerely to host a project, enjoy the journey, and end have a nice dirt bike to keep, then you're on the right track. I'm sure you know you could've bought that dirtbike in running, riding shape for about half what you're about to put into it, but hopefully, that wasn't the point
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Old 01-30-2015, 08:03 AM   #10
aklein318
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And yes, I like the idea of putting back together to see if it running is actually an option, just for that warm and fuzzy that from here out, you're moving up.

Before you do, get every spec of rust you can off that crank. If not it'll loosen itself, and make a trip through the 2-cycle exchange process, leaving its mark everywhere it goes
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Old 01-30-2015, 12:13 PM   #11
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I do a lot of projects like this for profit on the side, and I always do well with them. This one, however, is just for my own enjoyment. I know I probably won't benefit financially, but I get a lot of enjoyment bringing things back from the dead, so it's worth it to me (within reason). If I ever finish this thing I will post a breakdown of the final cost.

Now back to the bike. No pictures, but all corrosion has been cleaned, all specs/tolerances checked, and the lower end is back together. Just waiting on gaskets for the top end to get it all buttoned up.

I scored a front wheel on ebay for 25$, and am now on the hunt for a decent carburetor.

Things will probably progress pretty slowly from here, as I have to sneak all purchases past the accounting department (wife).
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Old 01-30-2015, 01:18 PM   #12
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Old 01-31-2015, 11:39 AM   #13
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I spent a little time cleaning up the exhaust today.

No before of the expansion chamber, but here is the after



Muffler before



After



It can still use some polishing, but it's good enough for now.
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Old 01-31-2015, 04:11 PM   #14
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Ladies and gentlemen...we have spark

Also, I haven't done a compression check, but the thumb test was very promising.
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Old 02-01-2015, 04:22 PM   #15
doorman OP
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I had some free time today, so I recovered the seat.

Off came the ratty purple cover



Foam is in decent shape



And on goes the new cover







Much better
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