Author Topic: YZ125K Rebuild  (Read 30504 times)

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Offline alexbrown64

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YZ125K Rebuild
« on: October 11, 2014, 07:27:16 PM »
Edit: Hi, well its July 2017 and photobucket decided to charge everyone 400 US just to host some pics... then they blocked all the pics on the forums.... so i am deleting photobucket... so bare with me as i download and re upload all my pics to OneDrive and then link them back here.. thanks, Alex

Hi all, well i haven't seen a lot on this forum about the 1983 YZ125K so i thought i would start a thread up.  I used to race the K back in the day and thought it was the best Yamaha i ever rode.  It just seemed to do everything right and i never had any hassles with it.  I also had the 125 F, G and J and they were woeful in comparison.  I also believe that the 125 L and N went pear shaped so the K was just the lucky one.
Below is a shot of me on my K at Noble Falls WA back in 83.


Anyway, as an old fart now a few of us have taken an interest in some VMX racing and i have decided to relive my teenage years and rebuild and ride a K.  I have a website i have created with all the information i have garnered for the project so far. You can get more info from http://yz125k.weebly.com/  They are a rare beast but i found a nice one in America and it is on its way as i write. In fact i think it is in Singapore at the moment.  Below is a pick of my yellow US edition.


Its been taking ages to organize the bike and get it over so i have had plenty of time to organize the project.  The idea is to set it up as per the 1983 Yamaha Race Report that came out.  I have put a copy up on the website.  I ordered a Jemco cone pipe from Jon Easton and it arrived a while back and looks pretty good.. below is a pick of the pipe...


Also on the list was the updated carby and specs as per the report.  That was hard as the specs call for a 36VM with a 6f34 jet needle.  These are impossible to find so i found a bloke that measured his NOS one and i made a spread sheet to calculate a suitable alternative based on available needle jets, jet needles, profiles and the area created between the two jets at 10mm increments.  I believe i found a suitable alternative in a 6f4 using an alternative needle jet to go with it.  Below is what the Race Report calls for and below that is what i have found.




I believe that a 30 year old ignition is prone to failure so i did some research and went with a PVL digital ignition... below


I then made a dial indicator for it so that i can set the timing when i eventually fit it....


OK... i have more to write but i will put that up next.....

« Last Edit: October 13, 2017, 09:27:04 PM by alexbrown64 »

Offline mudguard

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Re: YZ125K Rebuild
« Reply #1 on: October 11, 2014, 07:51:08 PM »
I'm liken your bike Alex 8) Don't give away too many hot up tips Alex. Just may give someone else the edge. Lol  ;)
I thought I was wrong once, but I was mistaken.

Offline 80-85 husky

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Re: YZ125K Rebuild
« Reply #2 on: October 11, 2014, 07:52:30 PM »
I hope the bike shows up!

Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #3 on: October 11, 2014, 08:06:25 PM »
I also wanted to sort out the cooling system so that i would not have any future hassles.  I read some reviews on the ally rads that are made by Winner and GPI and ordered a set of GPI ones.  My first mistake was to assume that the picture of YZ125K rads on Partzilla.com were correct.  In fact, they show rads for the L and N.  Below is a pic of the rads i received and an original set i purchased on ebay.


I contacted GPI and they said no worries, just send us the rads back with the originals and we will reverse engineer them and send you back a set of custom made ones together with yellow silicon hoses.  I have sent everything to them and the last response was that they have drawn them up. 

I have also been on the AllBalls website and ordered all the bearing kits for the bike.  You can just about get everything including steering stem, linkages, wheels and swingarm.

Partzilla.com was used to order new cables, brake pads, engine and transmission seals and bearings, conrod and other bits and pieces.

As for suspension, i found a bloke that raced a YZ125K in the vet class in the states and he told me to contact Matt Wiley at Race Tech USA.  Matt was kind enough to send through all the details of the suspension work that was done.  This is supposed to give the bike a really nice plush ride.  Below are some of the specs but the website has full details and prices.  When i am ready, i will send the suspension off to Terry Hayes at Shock Treatment as he is the Australian Race Tech guy.  Matt gave Terry my details and Terry contacted me and is glad to help out when i get the bike.

1983 YAMAHA YZ125K SHOCK RIDER 175lbs & WANTS PLUSH RIDE

VINTAGE SHOCK REBUILD

SMGV 4616S
SK GOLD VALVE 4631 SHOP
KIT
SPSC B5747
SPRING SIZING COLLAR- 23
EACH
SRSP 672750
RT SHK SPR 66.9x64x267 5.0kg
KIT
SYSH 4614
SEAL HEAD 46x14x19 no t/o YAM
EACH
SSBO 144439
B/O BUMP 14x44x39L SHOWA
EACH
US1 PT
US-1 LIGHT (pt) REBUILD
PINT
LS12

83 YZ125 FORKS
 
FRSP 305543
RT FRK SPR 30x500mm .43kg
PAIR set @ 5mm Preload
 
FEGV 3801 Gold Valves
EMU VALVE SPRING 40 lb BLUE @ 3 turns
4 Bleeds Valve Plate
15wt Fork Oil @ 130mm Oil Level

There are a few guys on this forum that ride the K and have helped out with advice after seeing my website.  If anyone else has any info, pics or advice to offer it would be great and hopefully we can all work together to get some great K's racing out there.

Cheers,
Alex
« Last Edit: July 23, 2017, 09:50:26 PM by alexbrown64 »

Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #4 on: October 11, 2014, 08:18:53 PM »
Don't worry Wayne, i dont think there is much out there that can touch your K.  Husky, i hope the bike turns up too.  I have the shipping docs so hopefully it should be OK.

Another thing i forgot to mention is the engine.  A full rebuild is in order and i have a bunch of parts like bearings, seals, rod, gaskets, clutch etc.  When i finally get the bike, the cylinder will need to be inspected and then sent to Eric Gorr to get bored, ported and a new piston matched up. 

Cheers,
Alex

Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #5 on: November 07, 2014, 01:14:40 AM »
Well, bike finally arrived in Fremantle.  Now i have a problem.  I dont have an import permit.  The website says you dont need an import permit for an offroad bike.  However, you have to apply for the exemption to not have a permit.  To not have a permit is a 10 page document with 9 attachments, fifty dollars and a 3 week wait... while my bike accrues storage fees at Main Freights yard. 
Wish i had looked into it more but the penny didnt drop... now i have to wait for my permit which authorizes me not to have a permit.
Bureaucracy in Australia is mental...

Offline Richo52

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Re: YZ125K Rebuild
« Reply #6 on: November 07, 2014, 09:44:26 AM »
Great looking bike Alex.I get any bikes I buy from the states partially stripped and bring them in as parts.....it avoids the problem you now face.Good luck with it all.Catch up some time Richo .
SOME BIKES AND CARS

Offline 80-85 husky

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Re: YZ125K Rebuild
« Reply #7 on: November 07, 2014, 08:07:35 PM »
Guys... since the unruly mob showed up at Botany bay, the greatest lurk to all involved is screwing punters to get stuff into the country...allways has been allways will.

Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #8 on: November 16, 2014, 11:56:06 PM »
Well, bike is in Freo.  Now, major problem.  When i read the Gov Dept web site originally, it says you dont need an import permit for a dirt bike as it will never be registered.  I left it at that.....big mistake.  You still have to apply to get permission to not have a permit.  It is still $50 and 10 pages, plus 9 attachments and a 20 day wait.... except they dont give you a permit... they just give you a permit to say you dont need a permit.  Now my bike is sitting in storage accruing fees....

Anyway, sun will be up tomorrow and im still breathing so its all good..

Cheers,
Alex

Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #9 on: December 08, 2014, 12:43:35 AM »
Well, "they" finally let me have the bike after extorting a fortune out of me.  I was expecting a fee to get the forklift to put it on my trailer.
Anyway, bike was filthy, decals bubbling off and at close inspection the old girl is a piece of shit.
Below is a pic of me finally getting the old girl home after a long trip from New Jersey.



Ive decided to build the bike after Ron Lecheins 1983 works OW125. Of course i cant have the rotary engine but i will theme it that way.
I special ordered some white rad shrouds from DC.  Also ordered white side plates.  The motor will be blasted and i will try to match the OW125 case colour. 

First things first... i wanted to check it all out and get it running.... Put some oil in the tranny, some juice in the tank, check the airfilter, check spark, 30 kicks later and she is running.  Sounds a bit rough but it hasnt run in a while...

Before i tear it down i want to test it with the new carb, pipe and ignition.  First i put the Jemco pipe on and run it... sounds good.  Next i play with the new VM36.  This bit took 2 day as it kept flooding.  I rang Tom at MikuniOz and he said dont touch the float...its factory set.  I tried everything but it kept flooding.  I went from 18mm to 22mm float height and it ran like a beauty...pic below


Power valve and comp test.  Power valve lined up well and a compression test was around 130 psi.  Later during the tear down i revealed a new 2mm oversize piston in a freshly honed bore. 





Next up was the new PVL digital ignition.... I ripped all the old crap off and fitted the adapter plate.  It should be locktited in.



Put the stator in next.  Just fit it so that you have a bit of play on each side of the attaching bolt so that you can slide it from side to side.


Next, you want to put a dial gauge in the head and zero it in at top dead centre.  The turn the crank anti clockwise so that the gauge reads 1.2mm BTDC



Now you can put your rotor in.  Leave the green tape on the rotor for now and align the mark on the stator next to the clockwise arrow with the mark on the rotor and fit it up.  If it all fits well, remove the green tape.  There should be a tiny gap between the rotor and stator.

You have just done your timing.  Now just fit up the PVL coil where the old Yamaha coil went... its a direct fit.  Make sure its ground well.  I put the new CDI where the old CDI went by just bending the tab a bit.  I will make a stiffening plate for it later.  The wiring is simple... There is a harness to plug in and a few black ground wires to the frame from the CDI and Coil.  Your kill switch has 2 wires...1 to ground and the other to the CDI red wire.  There are also 2 blue wires that plug together.



After i fitted it all up i checked for spark... bit fat blue sucker.  The next day i went to start it.  6 month old premix and second kick it bursts into life and runs like a dream.  I took it for a rap up the road and its like a buzz saw.  Go's through the gears well.

Once i knew i had all the systems up and running, it was time to tear it all back down ready for a rebuild....
Only took a few hours to get it looking like this.....



And then a few more to split the cases..



And finally, the suspension needed tearing apart and cleaning ready for Race Tech....


I think the forks were filled with mud.  The rear shock clicker just turns and doesnt click... not sure if that is going to be a problem...

Waiting on some emails from Eric Gorr regarding cylinder sleeves and porting and Terry Hayes at Shock Treatment regarding Race Tech emulators and gold valves.

Well, thats it for now... frame needs some welding and then its off to get blasted and powder coated.  Crank will go to get new rod..

All hardware is in a bucket and is going to get some acid treatment to remove the zinc then they are off to the zinc coaters...

I think its time to concentrate on Christmas and forget bikes for a while.....

Cheers,
Alex

« Last Edit: July 23, 2017, 10:25:15 PM by alexbrown64 »

Offline Tomas

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Re: YZ125K Rebuild
« Reply #10 on: December 08, 2014, 01:18:56 AM »
Great looking bike Alex. Only noticed that the crankcase looks a bit rough. Like the pictures and all the details. Good luck with rebuild.

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Re: YZ125K Rebuild
« Reply #11 on: December 08, 2014, 09:09:40 AM »
As usual, love your work Alex.  Having imported a few bilkes, cars and boats from the USA before, I was well aware of the need to an import approval.  I just hope the holding charges didn't break your bank.  I know of people that have imported cars without one and the costs have been astronomical .. so much so it was easier to ship the car to NZ whilst they sorted out the paperwork.

My question is, looking at your first picture, where is was taken etc, this looks like an ex East Coast Vintage bike.  Is that who you purchased it through?


Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #12 on: December 08, 2014, 10:14:01 AM »
Hey Tomas, up close the whole bike is a bit rough. Certainly got used in its 31 years.  Kick start looked like a pretzel and everything was either cracked or broken.  But she is a runner and thats the model i was after.  Simo, it is from East Coast Vintage MX but only after i found it on Craigslist and asked him to purchase it and get it shipped to me in Oz.  Yes, ended up costing me a fortune but i will put that behind me as a lesson learnt and move forward with getting the old girl race ready. I have a an endless supply of parts coming now.... kickstart, airbox, stator cover etc.. Im not sure what happened with some of the parts i ordered from Partzilla.  I never checked them and have had them stashed in a box for months.  Finally got them out and i have a Polaris front sprocket, YZ250 rear sprocket, YZ125H clutch plates and YZ80 Boysen reeds. (the 100 or so odd other parts are ok) I have already swapped the rear sprocket for some Renthal gold bars but either i was pissed when i was ordering or they have a buggy system or someone that works there that doesnt give a shit.

Cheers,
Alex

Offline bishboy

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Re: YZ125K Rebuild
« Reply #13 on: December 09, 2014, 05:57:32 PM »
Here's a 250 if you want a pair, I'd go for this if I didn't have the mighty KX  ;D

http://www.gumtree.com.au/s-ad/cedar-grove/motorcycles/vmx-1983-yz250-k/1063301083
79 KLX 250 parts for sale

Offline alexbrown64

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Re: YZ125K Rebuild
« Reply #14 on: December 09, 2014, 11:38:19 PM »
I think its gone Bish, as Gumtree says add no longer available.  To be truthful, i rode the 250K back in 83 at Bunbury and didnt like it.  In the thick sand and giant whoops, i found my 125 to be much easier to ride and at my age now i need easy :-)