YZ125K Rebuild

[alexbrown64]

Thanks fella's... i can pick up an OEM full piston kit for $120 AU delivered.  Thats  bloody good.  Will try that with a few other mods to keep the piston well lubricated and cool and see how we go.

Cheers,

[80-85 husky]

quite a few husky 500's have had woosner seizes. a mates 430 did one as well. quite a few run good and I think they may just get a little more honing to assist. they seem to expand a bit more than expected when new.

[alexbrown64]

Still waiting on the new OEM piston kit to arrive.  Have also purchased another white fuel tank for the Euro K.  Will be taking the boy out on his RM80N over the next few days, then hope we can both ride together on the my 83 K and his 79 N.

Cheers,

[alexbrown64]

Well, that didn't go down well.  Just got my OEM piston kit and what a pile of junk.  Rings were totally missing and the piston pin was so rusty it had gouges in it.  Junk.  I have requested a return.  May go with a new ProX after that experience.

Cheers,

[alexbrown64]

Had a look on Ebay, and picked up a brand new ProX piston kit with rings, clips and pin for $113 AU delivered.  Should of went this way the first time around.....

Now its a waiting game to get it, then send it up to Perth to get Ray Easson to fit it up.....

Cheers,

[alexbrown64]

Hi guys, just an update.  The boys RM80N is still not running right, so i got him a JR80 to ride in the interim.  He loves it.  Its a 2010 model, and they still use the 1977 5 speed RM80 engine.  Just detuned and with a quiet exhaust.

I took my white YZ125K for a ride with my boy.  He loved hooting around the bush with me.  Only problem was, my clutch went south.  The lever has gone soft and there is something rattling around in the left case where the basket is.  Haven't stripped it down yet, but my guess is adjuster nut has come off or something.

I have the yellow YZ125K cylinder/piston etc back from the shop. Just have to put it all back together. 

I have joined the Classic MXWA club.  Got number 224 which is Ron Lechiens works 125k number.

Cheers,

[alexbrown64]

Came back from the gold mines and found time to rip my clutch cover off.  Yep, the big nut had unwound and the basket was barely hanging on.  Not sure how it happened as i thought i had used loktite, torque specs, nibbed washers etc.. but it came loose anyway.

Also, i had left the fuel in the tank while i was working away and guess what happened..



Thats nasty!!

Luckily i did suspect that the tank was going bad, and i found a spare on ShitFace for a hundred dollars.  Looks to be in good condition, but needs a sand and polish and decals.

Well, kids holidays are coming up so i want to get her all back together and running right so i can go for a ride with the boy.

Cheers,

[80-85 husky]

all going on there!

[Lozza]

Clutch lock nut should be nipped up with a rattle gun and fold the lock washer over a flat on the nut.  I don't think anyone ever came up with a solution to stickers not bubbling or unpeeling on a plastic tank. They even did it new IIRC.

[alexbrown64]

I didn't use a rattle gun, but i referred to the manual, and torqued the nut up to 58lbs.  I also used a little thread lock and bent the washer over against the nut.  I think this was the original problem, as i did not bend the washer over previously.

Put it all back together and it works like new again.

Next stop was the fuel tank... i replaced the one with the six inch split with the one i got on Facebook.  The replacement started leaking from the petock i put on... cranked that tighter and it fixed that leak... Another leak is coming from a hairline crack next to the mounting brackets.

I looked up YouTube and its an easy fix to plastic weld the polyethylene  to repair the leak.. https://www.youtube.com/watch?v=Lqu3NVqFNCU  I will use my old tank as the donor plastic...

Will do that in the morning.. Will also have to start on the Yellow YZ and put the ProX piston kit and cylinder back in...

Cheers,

[ITDOG]

try super glue and baking soda...yes it works

[alexbrown64]

Thanks ITDog.. the original tank was knackered with multiple splits..  The second tank i got had a pin hole leak near the lower mount.  I used a heat gun and some strips of plastic from the original tank and got it fixed.  So far no leaks.

So today i took the white YZ for another blast.  The clutch and tank etc.. all held up well.  I had added some oil to the front forks because they felt soft on an earlier run.  I also had wound up the shock spring and set the sag.  The bike rode really well and felt brilliant through the whoops and sandy corners.
Something wasn't quite right with the engine though.  It ran great at full throttle, but 1/4 to 3/4 it would run rough.  When i pulled up, there was fuel pissing out the carb.  Time to pack up again.

Pulled the carb at home.  Float height perfect at 24mm.  Slightly sticking needle and seat.  I had a Keyster carb kit in the shed so i will swap out the parts.
One other major problem was the carby cap.  It would drop down onto the threads of the carb body about 3/4 of the way and then finally tighter up with only 2 thereads to go.  Obviously something worn out. 
I found another 34mm VM in my parts bin, and it ended up being the body was knackered.  So i swapped it out and now the cap and carb work perfectly together. 

So, next step will be to clean everything up, check the specs and get it all back together. 
Next test run in a few weeks.  Still have to bolt the yellow YZ engine back together.

Just got and email back from Eric Gorr, who ported the cylinder.  I gave him the yellow YZ carb specs and he said it sounds like the main is too lean (280).  He said modern 125's are running up to 400 mains.  He said try a minimum 310, and also richen the needle.  I will get a 360 and 330 (i have a 310) and run it super rich to run it in, and then tune it backwards from there. Hopefully a richer bike and Prox piston and the bike will not nip up next time!!

Cheers,

[80-85 husky]

lots of heat cycles to start with on dicky engines

[alexbrown64]

Will do Husky.  I'll put a bunch of heat cycles on with the richer jetting.

OK, got to work on the white YZ.  Rebuilt the carb with a different body and lots of new parts and gaskets.  Fitted it up and it runs great.  Sound a lot more crisp that before.

Next problem is the overflow.  The rads seemed to of lost about 200ml from the last ride. I topped it up and after a bit of a warm up, it started coming out the overflow again.
It does this on my yellow YZ as well.

I have the original caps (very old) and a chinese GPI one.  They all say 1.1 bar.  I am thinking new Yamaha OEM cap, and/or get a cap thats 1.6 bar to increase the pressure a whisker.  Anyone had something similar with a fix.

Last but not least, i got hold of another white tank. We shall see how that one is.  I have ordered a new petcock for it.  The whole valve is only 18 delivered for an exact replica of the Yamaha one complete with all gaskets.

Cheers,

[alexbrown64]

Finally got to work on the yellow YZ.  Refitted the Eric Gorr ported cylinder with a new ProX piston that was checked and everything cleaned up by Ray Easson.  I changed the jetting.  Main gone from 270 to 310.  Needle gone from second from the top to second from the bottom. Third kick and she burst into life.  Sounds crisp.  I started the white bike at the same time and proceeded to rev them both up in unison and fill the shed up with smoke.  I also fitted my race numbers.  Me and a bud joined the Classic MXWA club, and i asked for 224 just to give the bike the Lechien theme.  Got the new numbers fitted and both bikes are now ready to go riding again.....just gotta run the yellow one in.. lots of heat cycles..

Pic taken in a smoke filled shed.. smells fantastic....


Cheers,