You need to use a 3rd party site like imgur to post pictures.

I can safely say without looking that a 1980 yz125 jug wont fit.
I had a YZ125G new in "80. Good way is to eyeball the base gasket between models.
Something like an AT2MX should fit.
They're all 56X50 bore/stroke.
I mentioned somewhere about the inlet noise baffle in the air boot where it clamps to the airbox.
I had mine on the windowsill in the shed for years, but has gone walkabouts.
Just see if yours has one but if ya running a pod filter then that doesnt apply. Cheers.

Thanks rt 

yea never thought of looking at base gaskets you’re right thsrs really clever. 

no chance yz would fit.  

looks like a rs125 cylinder will fit. I looked at specs irs rated 15hp looks like slightly wider taller exhaust port for the extra peak gp over the rx125 stock 13hp output. 

really the later style rxz135 cylinder porting looks better. Much larger transfers. Slightly narrower exhaust port. Wider boost port. And they claim 18.5hp from factory with a 28mm carb 

found a later style rxs135 port map which shows asymmetrical port timing and less exhaust width and height than the earlier rx135 that were 14hp. 

realistically I think I’ll just tidy up ports and spend all my energy building a good expansion chamber 

 

I contemplated boring my RX to a 135 piston which was 58mm by memory but i didn't bother in the end.
135 cylinder wont fit as are big around the base & held down by nuts.
RS125 of those early years will fit & i don't know if we get better less restricted porting down here in oceania but the one i had absolutely flew.
Only had a 4 speed trans but didn't need 5 as had bulk torque but also revved to the moon with power all the way.
My one was given to me years ago by an old fella [probably younger than i am now at 78 haha] & i used to service it at work from new [yamaha shop] then he gave it to his grandson who destroyed it.
So out of the blue oneday he tracked me down & said bring a trailer then pick me up & we'll go for a drive.
Was a worthwhile drive that was.
Exactly like this random pic.
www.authenticmotorsparis.com/project/yamaha-rs-125-1975
The RS has a squish head & a slightly convex piston.
The RX has a non squish head & a dead flat top piston.
Side by side the two pistons will line up but you can see the slight covex top on the RS.
That's my experience anyway.

Mine  is a 79. Rx125 and it has a squish head. 


I’ve looked at porting. Rs has slightly bigger transfer mainly the boost port is wider and the inlet is not bridged it is just a massive big oval port. Exhaust port wider too. I’ll do a bit of work on the porting but focus on the expansion chamber to get most out of it. 

will be good to get it running. 

 

Must be differences we didnt get down here.
Just google imaging i see an RX125 cylinder bolted down at the base.
Is yours like that or has long studs with long nuts holding head & cylinder on together--which is how ours are.

Yea the barrel sits on studs and the head bolts hold both down. 

 

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Does anyone know how to remove the reservoir from the master cylinder body. I know some other posts have suggested it’s possible. There is corrosion and grime
i wanted to clean that I can’t get to with the master reservoir in place. I’ve tried everything else. 

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RT I found the restrictive in the airbox. Must be to reduce intake noise. Don’t need that ?. 




 

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I rebuilt the master cylinder. The old seals were absolutely trashed. The bore seemed ok there were a few small scores I lightly polished out with 1200 grit. Don’t know if it’s going to work. Might need to buy a proper aluminium
suitable ball hone. 

Will rebuild the caliper too and see if the system holds pressure. 

the rebuild kit was from yambits and went together well. Will just flush a heap of fresh fluid through after rebuild and hope it works. I’d prefer to keep the original style master.