83 YZ125 K Restoration

Been a while since I've done much to the bikes as I've been doing house Reno's. Now they're all done it's back to finishing this basket case.

Im actually looking for an expansion pipe hanger / mount if anyone comes across a good used one ping me.

I'm sorted piston and ring wise and thanks regarding the exhaust hanger.

There's a NOS one on ebay but it's crack pipe money 💰 

Bit more progress while hunting for a few odd ends before the motor assembly begins.

Next step will be to have the cylinder bored out to suit the 58mm wiesco piston, the power valve clearanced and the mating faces of the cylinder and head lightly skimmed 

Maching work done.

-Cylinder has been bored and honed to suit a 58mm piston.

-head machined to suit the 58mm piston

-crank rebuilt using NOS conrod, big end bearing, thrust washers and pin

-shift drum bearing replaced

-power valve clearanced for the 58mm piston

Getting the donk ready for some rattle can love...😄

Painting it with some high temp semi gloss sating black, then will bake it the oven to harden 

Note I'm still in the process of masking here...

Also whilst waiting for some engine parts to arrive decidecided to get the pipe sorted out.

This is the pipe Brad bought back from Adelaide .

How it was when I got it although I've blasted it here..

How it turned out... once again didn't go to town on it as its for the rider..

Donk painted and baked..

Fresh out of the oven.

Is there an issue with these forums? My posts keep doubling up and the photos are not working... ?

Mikuni VM34 for the rider with an arrangement of pilot and main jets. 

The Miulkuni VM34 is the same size carby as OEM. In fact, both the mikuni and OEM are almost identical and parts are retro-fitable between the two.

I'll post later with some size by side pics of the mikuni vs the oem as I have both and show the minor differences.

Just as a reference in case anyone is wanting to know here are the stock oem jet specs:

USA spec:

needle: 6F21-3

Main jet: 280

Pilot: 60

Slide cutaway: 2.5

Float height: 24mm +/- 1mm

Euro spec:

As above however

Main jet: 290

Pilot: 65

Here is a side by side of the oem carb vs the mikuni vm34.

Key difference

-choke style however oem one fits the mikuni

-slide in terms of throttle cable attachment

-idle screw on different side

- lid one is threaded the other has a thru hole (pretty sure didn't look that close but i recall it being slightly different)

I think apart from that they're identical and the vm34 works fine on the 83 yz125k.

Here's some.progress shots of the donk

Being a rider I haven't gone full blown in sanding and fixing case imperfections etc. 

Mechanically it's had a full rebuild top to bottom with a full set of genuine yamaha bearings, genuine yamaha seals, genuine rod kit, nos flywheel, all genuine nos fasteners, Athena gasket set, wiseco 58mm piston, cometic head gasket, head machined, bored, honed and decked, power valve clearanced etc.

Can't wait to get this build finished and go for a rip and not having to worry about stone chips, general ware and tear etc.

Donk is finally finished. Not 100% happy with the squish as with one base gasket it was under spec, .75mm, and with two it's over at 1mm. I'll run it like this and see how it goes for now.

Have also ordered 2 NOS base gaskets in case I want to try get it closer to spec.

Next is to get it fitted up in the frame; maybe this weekend, carb on, coolant hoses on, etc

Also curious, other 83 yz125k owners, what 2 stroke ratio are you running and with what oil.

I'm debating between 32:1 or 40:1 using motul 800..

Bonus points if you know your jetting specs

Donk back in the frame

I'm in need of a usable white fuel / gas tank if someone knows of one.

Mine took a shit on me today...

Will try repairing it but who knows.. 42 year old plastic.

Well I got her fired up a few days ago. The rattle you hear in the video is the rear axle washer vibrating around as it was loose at time of filming.

The bike is pretty close to being finished, biggest hold up at the moment is the fuel tank.

Long live the yz! 😄 

Well i found a replacement fuel tank in portugal for the rider. Whilst I wait for it, I thought I'd tidy up the silencer.

Bit of paint stripper, a coat with some high temp semi gloss black and job done.

Bit more progress on the rider k.

Still waiting for replacement fuel tank to arrive, should be here next week.

Tank and seat just sitting loose in below pic.

Donk is aussie slang for motor / engine.. it can also be used to reference a man's tool if you know what i mean haha 😄 

Well my replacement fuel tank arrived from portugal. It needs a good clean but should do the job nicely for my rider. If I get bored enough I might even sand and polish it.

As for the tank that cracked, I've ordered some HDPE plastic welding rods so will attempt to repair it for a spare.

Oh, I also found a front number plate for the rider... should clean up nicely also.

Some more random bits for the rider...

Just wanted to share some info, that might help others out in the future, regarding the exhaust stay bracket.

During the build, I couldn't find an original one whilst googling, however I came across this website:

https://oemdirtbikeparts.com/product/dirt-bike-exhaust-pipe-mounting-bracket-mod/yamaha-1983-yz125/

Take a note at the price.. $69 bald Eagles + shipping to me in australia.

Anyway I did some more research and found what appeared to be the exact same item for alot less monies.

https://a.aliexpress.com/_mNStEYP

I purchased one and tried it out and it worked on my rider yz125...

Then I stumbled across this and the rest is history lol...

Well the rider is finished, motor ran in and I've been taking it for a few blasts.

Its absolutely ripping and going well but I'd like to know from jetting experts their thoughts on this plug after about 30 mins of riding. To me it looks rich. Currently at 1.5 turns out on the pilot screw, 290 main, 65 pilot and needle circlip in the centre position. Fuel is 32:1, using 98 RON fuel and motul 800 oil. Any advice greatly appreciated.