YZ125K Rebuild

[80-85 husky]

governor? is it a diesel?

[alexbrown64]

Not exactly husky, but i got her going this morning and she runs like a scolded cat.  I have a few carb issues, but i gave it shit up the street and she hammered.



Cheers,
Alex

[80-85 husky]

Ideal outcome, get that go pro running

[alexbrown64]

In the pic above of the Euro K, i had to pinch the pipe and muffler from the Lechien tribute K to get it running.  This is because i only had a 1984 L pipe and muffler.  I have realized that a lot of L parts do not fit the K. I have now found a K pipe and while a little dented, its all original and fits like a glove.  I tried to fit the L muffler as its repackable, but again i was snookered as they don't fit.  I ended up using an old K muffler that i modified...more on that in a minute.  I have also been running the US K with the 58mm cylinder and Wiseco piston.  This has now gone on the white Euro model, so that i could fit the Eric Gorr ported cylinder on the US race bike.  Both  bikes are now almost there.....

OK, before i fitted the cylinders back on, i took some pics to compare standard to modified.  In the pic below, Eric Gorr has resleaved one cylinder back to 56mm and supplied a Wossner piston with it.  He matched up the power valve and cylinder head to suit.  He also did some port work.  I noticed grinding in the intake and exhaust, as well as some new holes added.  Ride report to come, but it will be interesting to compare bikes.



Then next pic shows what i had to do to the old K muffler.  At 33 years old, the K packing was knackered but they are a sealed unit.  In 84 they made a slightly different muffler with a replaceable cap.  I had to cut the tip off the K muffler and then cut back about 50mm of outer metal to expose the inner pipe ready to fit the 84 end cap.  I removed all the old packing which was just hard sludge, cleaned everything up, and then fitted new packing.  I fitted the new cap, drilled 4 holes and riveted it on.  Bingo, a K muffler which is now repackable. 



The finished product came out well and i have given both the pipe and muffler a coat of 1000F satin black paint.
I still have to swap pipes back as the US race bike gets the Jemco cone pipe and the white K gets mostly stock parts. 

A few other things i have noted.  The white K does not seem to like Iridium plugs. It was a bitch to start with an Iridium plug, then i put an old BR9ES in it and she fired up and run heaps better straight away.
I am having trouble with a decent gear lever.  I bought an aftermarket one that is for this model but it fits like shit.  I have ordered another one for a CRF50 that is supposed to fit.  They are both 11mm shaft and a similar bend, but the Honda one is about 20mm longer.

Cheers,
Alex

[alexbrown64]

Put everything together this morning and both bikes stared within 4 kicks.  A few minor idling adjustments and a quick blast around the garden on both.  The US bike is plusher and snappier with a very short power band.  The white K has a broader power spread.  I think the Jemco pipe has a lot to do with it as when it was on the white model, the power felt snappier.
Anyway, both these bad boys are ready for the track and some serious riding... ride reports, pics and go pro vids to come....



Cheers,

Alex

[Rossvickicampbell]

Alex - think you might be right re the Jemco pipe.  Tried a YZB one on my bike recently and changed the delivery in much the same way as you mention here - much snappier.

[Richo52]

Great looking bikes Alex.......bring them out to the classic MX club here in WA formerly the pre85 club,lot of rides and laid back approach to the rules.........ride and have fun......go as slow or fast as you want.:-)

[bishboy]

Geez they look good Alex, brings back some memories.  Back in the day I remember painting my frame white and the radiator shrouds red for that worksy look 

Here's few parts for sale if your looking for some more

http://www.gumtree.com.au/s-ad/wollongong-2500/motorcycles/1983-yz125k-parts-vmx-yamaha/1090931978

[alexbrown64]

Great looking bikes Alex.......bring them out to the classic MX club here in WA formerly the pre85 club,lot of rides and laid back approach to the rules.........ride and have fun......go as slow or fast as you want.:-)

Thats on the cards Richo.  I just have to sort out income protection, MA license, etc.. iron out a few bugs, get fit and i will be there. 

Cheers,
Alex

[alexbrown64]

Geez they look good Alex, brings back some memories.  Back in the day I remember painting my frame white and the radiator shrouds red for that worksy look 

Here's few parts for sale if your looking for some more

http://www.gumtree.com.au/s-ad/wollongong-2500/motorcycles/1983-yz125k-parts-vmx-yamaha/1090931978

Thanks Bish, i have seen those bits for sale but i think i have everything now.. and two of!!  I cant imagine what a white frame and painted red shrouds would look like..  not sure worksy comes to mind but i used to do all that stuff as well.  Used to make my own fox decals out of contact.. and i took my muffler off my yz80 to give it that "factory" sound 

Cheers,
Alex

[Rossvickicampbell]

if you wait until you get fit Alex may be a while 

[alexbrown64]

if you wait until you get fit Alex may be a while 

Ha! My job keeps me pretty fit but ride fitness is a different thing.  Roscoe, the Jemco pipes definitely pep the bikes up thats for sure...

[FourstrokeForever]

Nice pair of 125's Alex. they are a credit to your dedication and craftsmanship.

Now you just need to take them out and play with the others. No need to worry about being bike fit. That will come with riding them 

[alexbrown64]

Both bikes are ready to go... I chose the Euro YZ to take out for its first run.  Loaded it on the bike rack and headed bush. 



The cylinder on the bike is the one from the US bike, so it was kinda already run in.  Two kicks and i was away and i did some trail bike riding to let everything bed in and sort itself out.  Then it was time to give it some shit..... The two bikes are like chalk and cheese.  I knew this YZ would be the soft version but the difference is huge.  The suspension has all been rebuilt but is all original.  Its soft! In the sand it does not inspire confidence and twitches and wallows.  The US bike with its Race Tech springs, emulators and gold valves is stiffer, plusher and has a more modern feeling.  The engine was crisp when on the boil but 3 or 4 seconds of trail riding and it would take a second to clear its throat and be crisp again.  Perhaps some jetting or just ride it like a 125 and keep it on the gas.  The power is also completely different.  The US bike has the Jemco pipe, PVL ignition and larger carb.  This Euro bike is all standard including original K pipe.  Both have the same gearing but the Euro bike feels slower and goes off the boil very easily requiring more clutch.  I found the US bike had such a short power band that it was like a light switch.... you just twisted the throttle and it was "on".

Overall, i had a good ride and it was nice to do a bit of bush riding again.  The bike ran as expected and sort of feels a little old like me.

What went wrong.  The ride was cut a little short due to the inevitable gremlins after just rebuilding a bike from the ground up.
1. I noticed the wheel seemed to be closer to the right side and the tyre actually rubbed through the fork boot.  Not sure whats going on here... i may have miscalculated when i rebuilt the wheels and just need to pull the rim slightly over to the left.  But also, the axle and fork alignment does not seem quite right.
2. Front brakes are lousy and may need bedding in some more and then some adjusting.
3. I experimented with a CRF50 shifter but it is 15-20mm longer and is a pain on the toe.  Back to the original.
4. I was losing coolant.  Had to add about 150ml.  Its strange because it is acting exactly the same as the US bike.  They both seem to loose fluid through the overflow but bikes are not running hot or steaming.  One has GPI rads and cap and the other is original.  Both have silicon hoses.  I am thinking it is the cheap coolant i am using.  Its the cheap premixed glycol from SuperCheap.  A better coolant may help.\
5. I lost the bolt that holds the right radiator and main exhaust bracket on.  The thread is stripped so will require a Helicoil. I have a kit with all the sizes at home so no problem there.

Thats it for now... i may be taking the US bike out in the morning for its first ride on the Gorr ported cylinder with 56mm Wossner piston.  I have also changed the pilot out for a smaller one as i was getting masses of spooge.  More ride reports to come... and hopefully a GoPro vid.

Cheers,
Alex

[Rossvickicampbell]

Alex -slightly different note - how do you find the bike rack?  Have fitted one recently to my ute and think it is better than expected so far but haven't had much use.

cheers

Rossco