Author Topic: YZ125K Rebuild  (Read 63270 times)

0 Members and 1 Guest are viewing this topic.

Offline sleepy

  • Legend
  • *****
  • Posts: 697
    • View Profile
Re: YZ125K Rebuild
« Reply #225 on: November 29, 2017, 10:38:04 AM »
Using the BTDC method for the stock ignition is the most accurate method but you still need to check that the retard at the top end is working as it should or you could have another failure. Simple check for that is aim a timing light at the marks and if they don't line up at 9k it is faulty.
With the PVL until you know exactly what the timing curve is with your 5000 wind stator any sort timing check below 8 or 9k is asking for trouble.

Interesting how you had the same failure with a cast piston as you did with the Wossner, guess that it wasn't the fault of the Wossner after all.

Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #226 on: November 29, 2017, 11:45:02 PM »
Thanks for all the input guys. I'm working away for a while so I've had to put the bike aside. I did achieve a bit though. I reassembled the engine with another new piston kit, and took the lot up to get measured and checked over.
Also ordered a genuine stator and coil, and have a spare Yamaha cdi.
Will do leak down test, timing light check and use my new standard size carb with Yamaha spec jets. The newer Mikunis may be a whisker different, but I am sure I will be in the ball park.
I will do all these mods, together with the recommendations and mods that Ray Easson is doing in regard to squish and compression.
Hopefully I will have a runner in the new year..

Updates later...
Cheers,

Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #227 on: April 30, 2018, 04:43:30 PM »
Just a quick update.  Will be putting her back together soon.. just had a million and one other projects on the go..
Anyway, according to Ray Easson she was detonating, and the reason he believes is because she was built for high octane race fuel by Eric Gorr. He measured everything up and has added an extra base gasket.  Still wants me to check ignition etc..

I have everything to take her back to stock.  Stock size carb and jets, stock ignition etc.  I will put her back together with all the stock parts and slowly go from there..

Cheers,


Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #228 on: May 20, 2018, 06:47:53 PM »
The Curse Continues...

There was a ride day at Coastal Park today.
I quickly cobbled the Euro YZ back together with a new 2mm oversize 58mm head gasket and new base gasket and some other fresh parts and got her running last night.

So me and a mate hit the track this morning.  About 100 bikes there by the time the fun started..



Bike is running like a champ for the first half of the warm up lap, but i do notice it feels a bit weak in 4th at high revs..
Then after 1 lap i hear a pop and she comes to a stop....
I push her back to the pits and notice that the radiator hose has popped of and no fluid.  I have cooked her. I obviously forgot to really tighten one of the hose clips, as it was hanging off.

Damage has been done..



I think the cylinder is OK, as there are no scratches, just light scoring..



OK, so i know i am up for a new piston kit, gasket kit, hone etc.. But... is it a tear down motor job.  Will i need to tear it apart and rebuild the big end, replace main bearings etc..
Or is there a shortcut? There was some small aluminium pieces around the edges of the cylinder.

Cheers,


Offline Hoony

  • Legend
  • *****
  • Posts: 4260
  • Melbourne, Vic.
    • View Profile
Re: YZ125K Rebuild
« Reply #229 on: May 20, 2018, 07:36:25 PM »
its a total tear down i reckon. its not worth the risk NOT replacing the big end and mains.
i think you know that anyway but are hoping for the impossible.

it they let loose they you are back in the same boat.

thats a very tidy K you have there.
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)

Offline GMC

  • Legend
  • *****
  • Posts: 3691
  • Broadford, Vic
    • View Profile
Re: YZ125K Rebuild
« Reply #230 on: May 20, 2018, 08:27:38 PM »
While I agree with Hoony I did have a KX 250 that melted a piston twice in 2 years (Curse you Sea Lake)
A flush out and new piston and it was fine for several years.

You been a bit quiet of late, are your projects ramping up again now?
G.M.C.  Bringing the past into the future

Shock horror, its here at last...
www.geoffmorrisconcepts.com

For the latest in GMC news...
http://www.geoffmorrisconcepts.com/8/news/

Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #231 on: May 21, 2018, 08:06:34 AM »
Thanks for the input guys.  I might not risk it, and just order some new parts and rebuild her properly.
Yep, work and other projects have been getting in the way. I am fifo, plus i am separated and have my kids the whole time i am home.. bit of a juggling act.
I have the yellow YZ engine ready to go back in, and a timing light to check out the ignition.

I also have the G 125 project going, so hope to strip down the engine soon. Then hit you up Geoff for the pipe...

Cheers,

Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #232 on: August 13, 2020, 03:20:45 PM »
Time bloody flys.
Anyway, finally got the Euro K running beautifully. Persistence pays off.
My boy is big enough now to ride a 125, so I gave it to him for his 15th birthday.

After the piston meltdown, the bike was parked up. I then pulled the engine out recently, and it would need cases split, new rod, total rebuild, piston etc..

I stripped the engine down and put the crank to one side. Bore was OK, but needed acid wash and another hone.
I then got the engine from my other K. Top end blown on this one too, but bottom end all new.
So, the good bottom end was used, and with fresh gaskets, piston, ring, bearings etc..she was all fitted back together. A leak down test from my crappy home made leak down tester revealed 4 air leaks. One from the leak down tester haha, and three from the engine. Base gasket, reed block and oil side main seal. Clutch off and new seal fitted (the other one had gone hard). The other leaks were where new gaskets had been used. So I got some copper spray gasket, applied it to the new gaskets and got a perfect seal.

All air leaks fixed, ring end gap adjusted, squish band checked...everything torqued to spec, timing set at 1.88 btdc and then engine back into frame. A few more days spent cleaning, adjusting and prepping her for startup and riding.

She started OK, but wasn’t running crisp. Carb had been cleaned, but she didn’t have any mumbo. Almost sounded like a weak ignition. It still ran good enough to run her in, but the boy was disappointed.

I had my suspicions that it was the original carby.  My yellow  K has never run perfectly, and one of my ideas was to put it all back to original spec, as it was too highly modified. I had purchased a genuine 34mm Mikuni from MikuniOz and all the original spec jets including a 6F21 needle.

I set the new carb up to spec, checked float etc..and installed it.

Bang!  The K fired up and idled beautifully. Revved and run perfectly and had heaps of grunt. Sounds perfect too. Nice deep braaap...
The boy took her for a test ride and came back with a big grin and thumbs up.
Finally this K is running amazing.

The other crank has gone to the bike shop and I have started ordering parts to rebuild the yellow US K. We will be running both of them stock, and slowly adding modified parts.

Long journey this one....


Offline rocketfrog

  • Legend
  • *****
  • Posts: 559
    • View Profile
Re: YZ125K Rebuild
« Reply #233 on: August 13, 2020, 04:04:29 PM »
Thanks for taking us on the journey Alex, your bikes are swish! I always like to start with a stock set up and tweak from there, stock is usually predictable and reliable. Old worn out carbies can be a wild goose chase to tune and air leaks are destructive. Sounds like you have a great philosophy for tracking down gremlins methodically - remember 99% of electric problems are fuel and vice versa!! What brand are the fork gaiters on your sons 125k? They look like they fit well.

Cheers Jason
Political correctness is a doctrine,fostered by journalists and politicians, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end."

Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #234 on: September 02, 2020, 12:17:14 PM »
Hey Jason,
I couldn’t find the brand I used on these forks.
I’m swapping them out anyway, as they are a bit big and tend to rub.
Will let you know how the new one fit, and what brand they are.

The K is still going great. We had it at the Mx track and it’s going well.

On another note, the yellow K is just about ready to go back together.
Has rebuilt crank, new piston and ring, new VM34 with all correct jets etc.  I just have to build it all back up with new seals and gaskets.

Even my new 2T Motion Pro leak down tester has arrived, and it’s a beauty.

Will keep updating as this one come together too...

Cheers,



Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #235 on: April 12, 2021, 06:09:41 PM »
OK, been a while, but still busy with bikes.
The poor old US 125K has been sitting dusty and broken for quite a while, but behind the scenes i was ordering parts and getting things built like cranks, spare cylinders etc.

The Euro K has been going gangbusters. Finally i think we have a great K. Took ages to iron out all the bugs, but i have learnt a lot and i seem to be able to build a robust engine.
We swapped pipes the other day from the OEM version to the Jemco cone pipe and DG muffler combo. The boy loved it.. Here is a short video.. she sounds sweet and really rips..

https://youtu.be/kCwyjg5IVDo

Here she is after her recent rebuild...



So this one is ready for VMX racing, and the boy loves it.. so its time for me to get the yellow K going to that i can ride as well.

First thing was to go through my shed and track everything down. I had a cylinder and new piston kit in one box over there... a crank under there.. bits and pieces everywhere.  Took ages to gather everything up and give it a good clean.

Now, heres the story.  When i blew a hole in the piston of the Euro K, i decided that the bottom end would need rebuilding. I had the good bottom end from the yellow K from when i seized that one up. So.. the new top end from the Euro K went onto the good yellow K bottom end, and thats how we got the Euro K running really well.

Now, with the yellow K.. i had the seized top end (Eric Gorr ported cylinder), and the ruined Euro K bottom end.
I needed to start from scratch. So, the bottom end was thoroughly washed out and blown out with compressed air. New main bearings were fitted, along with new main seals. A freshly built crank with new rod kit was fitted. This was all closed up with Threebond rubber gasket goo.
For the top end, the cylinder was sent away and came back with new piston kit.
I checked ring end gap. 0.5mm Perfect. I did some squish tests.. This bike also requires two base gaskets to get the required 1mm of squish. I used copper gasket spray on the two new base gaskets. I also copper coated the gasket at the reed block and used some silicon on the rubber block side.

I torqued everything up and then did the leak down test with my new Motion Pro leak down tester. Perfect.. no leaks from anywhere. All seals and gaskets holding tight at 6 PSI.



I then went through and bolted the rest of the engine together. I replace the PVL ignition with a genuine 24X stator and rotor unit. I set the timing to 1.88 BTDC using my dial gauge down the plug hole. Everything was checked and rechecked, oil added, and finally she all came together to make a nice fresh engine....



This will be going in the yellow K soon and run in.

Hopefully that will be the end of all the woes. Lots of lessons learnt over the time.
Main lessons were to check squish and adjust as necessary. Do proper leak down tests, as these old girls leak everywhere, even with new gaskets. Start stock, and slowly add mods.

I have a brand new genuine Mikuni and jets to fit up to this old girl.  Hopefully, once she is running nicely, i will slowly add the mods again. I have the bigger Mikuni to go on later, cone pipe etc.

I think one of the main reasons i kept seizing the yellow K was the PVL ignition. It was sold as a complete unit for the K, but i believe the curve was way to advanced.  I have a digital timing light and later down the track i will play with it using the stock ignition to get some base readings. I will then swap to the PVL to see what was happening there.

Anyway, thats it again for now.

Cheers,
Once

Offline alexbrown64

  • Legend
  • *****
  • Posts: 712
    • View Profile
Re: YZ125K Rebuild
« Reply #236 on: April 30, 2021, 08:54:12 PM »
So,  Finally got the yellow K running, and its running sweet. Ive only done 3 heat cycles and ridden it around the garden, but so far it starts first kick, idles and revs great.
Me and the boy are taking them out this weekend. Im pretty confident that these two old girls are finally running in tip top shape. Once i start doing mods, they will be done very slowly and one at a time.

Check them out running here.....
https://youtu.be/NZX3o-kkod4





I guess the next posts will be where we slowly add the mods back.. and ride feedback..
Hope to also post up some pics of some meets we go to this year if we have a chance,
Cheers




Offline Hoony

  • Legend
  • *****
  • Posts: 4260
  • Melbourne, Vic.
    • View Profile
Re: YZ125K Rebuild
« Reply #237 on: May 01, 2021, 04:17:19 PM »
awesome Builds, very cool you can share this with your boys.
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)