YZ125K Rebuild

[80-85 husky]

using castor 927 I hope

[alexbrown64]

I should change to that perfume, but i am running good old Castrol TTS

[80-85 husky]

the only better thing than running the castor is the bike in front running castor...

[alexbrown64]

I've been hit by the greed of photobucket. Never fear, I have downloaded all my pics and reuploaded them to my one drive. I will be updating and rebuilding this thread..will just take a while.
Cheers,
Alex

[alexbrown64]

Finally got a chance to update the K page. With 20,000 views, i think a few people were interested in seeing the pics updated.
I have taken both K's out a few times now... and everytime i have been plagued with minor and major hassles.  Went to ducks nuts ride park a month ago.  Even though i was taking it easy, both bikes blew head gaskets. I fitted new oem cylinder studs, nuts washers and head gasket.
Today i had a ride with a mate and siezed the US model again.  This is after a new ProX piston fitted and checked by Ray Easson and given the OK.  Also rejetted for extra rich. 





Cant explain whats going on with the cylinder.  I can run the other cylinder on the US bike and thrash it, and its fine. But put the ported one on, and it plays up.  I am going to have to swap parts on bikes to try and track down the problem.

On another note, i may be picking up a third K in the morning. This could get interesting.....

Cheers,

[alexbrown64]

Didnt pick up the 3rd K. It was a piece of shit.

As reported, i seized the yellow bike again.  Bike seemed to run fine with the richer jetting, but seemed to get a bit spluttery until i revved it up to clear its throat. It also felt slower than before.

Ripped the cylinder off the K and what a mess.  It has the 4 point seizure, but also the aluminium at the front was melted. The ring was fine.  I cleaned the aluminium off the cylinder and there is no scoring. In fact, the hone marks are all still there.

There are no overheating marks under the piston.
I had jetted extra rich. The plug looks Ok. Not dark, but not white.  Kinda light grey. Bike is still spooging.
Bike would run fine at 3/4 power, but nipped up under a few minutes of full power.
All gaskets Oem and new.







Just want some advice on what to do next. This is the second piston, and multiple siezures. 
Bike seems to run fine when i put my other cylinder on with the 58mm oversize Wiseco piston.

I cleaned the cylinder with acid and there are no gouges or marks... it was just melted alloy and all the hone marks are still there.  Can i hone again and use another new piston? or is it rebore  time? I dont care about a loss of performance at the moment.  I would rather not keep reboring if possible until i sort this mess out. I can get another standard size prox piston if necessary.

Should i get rid of the 36mm oversize carb and go back to the standard carb and jetting?

Should i run this ported cylinder in my other standard bike to see if it reacts differently?

Should i run the white bikes cylinder with Wiseco piston in my yellow bike and see if that still runs fine? as it used to work great.

Any other things i can do? I emailed Eric Gorr again as he did the cylinder originally. Before i ran it this time, he had recommended the 310 main jet, which i installed.

[80-85 husky]

serious seizure....back to basics...air leaks? base, case and seals ok?

[Lozza]

What heat range plug? Have you a pic of the piston crown? Looks slightly lean to me. Do you ride on sandy tracks (I assume that being from WA)?

[alexbrown64]

It using the standard B9EV range plug.

Bike did not exhibit any sign of air leak.  No hunting, no running rough etc. Started second kick and idled perfectly.
I did less than an hour on the piston. I can take a pic if you like of the crown.  Bike shop said it looked like some detonation.
Does look a bit lean, but plug wasnt white.  It was grey and bike was still spooging.  I just checked the MXA report for 83 and they recommend 280 main on the 36mm carb. I was running 310.
I had inspected the piston when i ran it in, before i opened it right up, and it did show signs of scoring on the 4 corners at 45 minutes of gentle running in time.

I may get a new piston kit, and run the cylinder in my stock K.  Gently run it in and check the corners again.

I will also run my stock cylinder in the modified bike again and see if it exhibits abnormal signs.

I will also make a leak tester to check both bikes for air leaks.

I am a little stumped...

Cheers,

[Tomas]

I would say that you may want to go next size up on needle jet and also pull needle up a notch. Swaping only main jet may not help as needle jet may restrict fuel flow and that will lean fuel mixture. When you go up on main jet you also have to adjust everything else. Worked for me. Also I would go through at least a couple of tanks of fuel before going a full throttle. Check Mikuni web site. There is a chart that explains how needle, main jet, jet needle etc affects fuel mixture depending on in which position your throttle is. Also I would go back to stock carburetor. These bikes are already tuned to higher power output from factory

[80-85 husky]

needle jet seize sounds like its on the money if the bike is airtitght. oil spooge tells you that you "didnt" forget the to mix the oil..

[Lozza]

It using the standard B9EV range plug.

Bike did not exhibit any sign of air leak.  No hunting, no running rough etc. Started second kick and idled perfectly.
I did less than an hour on the piston. I can take a pic if you like of the crown.  Bike shop said it looked like some detonation.
Does look a bit lean, but plug wasnt white.  It was grey and bike was still spooging.  I just checked the MXA report for 83 and they recommend 280 main on the 36mm carb. I was running 310.
I had inspected the piston when i ran it in, before i opened it right up, and it did show signs of scoring on the 4 corners at 45 minutes of gentle running in time.

I may get a new piston kit, and run the cylinder in my stock K.  Gently run it in and check the corners again.

I will also run my stock cylinder in the modified bike again and see if it exhibits abnormal signs.

I will also make a leak tester to check both bikes for air leaks.

I am a little stumped...

Cheers,

The is a LOT of deto there. A 9 is to hot in a 125, in a modern well cooled 250 we run 9.5's a 10 or 10.5 in hot weather or sandy tracks is more like it. You should never be gentle running in, let it idle for 10-15 min then go for it.
I would after running in pull the cylinder and look for high spots (polished areas on the piston) then hit those areas with 600 wet and dry and the a green scour pad all over the piston. Rinse repeat. I would also look closely at the top edge of the exhaust port window to see if that is finished correctly.

[rocketfrog]

Wouldn't the spooge be a sign of over fuelling? Perhaps you are literally washing the bore with fuel. Oil should cling to the cylinder wall in order to transfer the heat and provide a bearing surface.

 I would reset to stock carb and jetting and a new piston, go through the std run in procedure and then tear down the top end and inspect the piston and the cylinder for high spots for signs of metal transfer, localised heating etc, and correct as necessary.

Kawasaki recommend the piston is run to break in (15 mins of low throttle running) and then tear down the top end and carefully rub down the high spots on the piston if there are any - True this is a Kawasaki procedure recommended for electrofusion bores but I cant see how it couldn't help and save you roaching pistons. Besides, you must have this top end off job down to F1 pit stop standards by now Alex, what is a few minutes work for the peace of mind?

You should not be having this much drama, don't overthink it - go back to basics. Basically what Tomas and Lozza are saying would be my 2 cents on the topic.

[justanothercgrader]

The sponge could be a sign of richness at other throttle openings,
You need to go back to some "base settings"

i would install a stock type cast piston, and Check the squish distance,

 install the stock carb and jetting, maybe raise the needle and up a main size or two.

if i remember you have a funky ignition?, If so why not install the original ignition, or at least you must check the settings.

Another thought is that the cooling system, pump, is actually pumping, you say there is overflow, or is it overheating because there is no flow?

As they all say air leak test to be 100%

I feel for you….

[alexbrown64]

Thanks for the input fella's.
Yep, it shouldn't be this hard, but i am not a tuning whizz and the bike had so many mods at once, that its hard to know whats going on.

I am getting the gear together to make my own leak tester for the K.  I will run a leak down test. That will eliminate, or show up that problem.

I am ordering a new Prox cast piston kit.  Bore still looks like new in the cylinder. Will check the squish and compression psi numbers.

I am assembling the engine, but removing it from the bike and taking it up to Ray Easson Motorcycles.  He wants to run some compression ratio checks and measure everything up.  He can machine the head etc to modify squish if necessary.

I got a message from Harry Klemm who is quite a tuner, and he says their is a lot of detonation there and to run race fuel.  I dont want to go down the race fuel track, and would rather detune the engine to go back to a regular octane fuel.  Hopefully by timing and head mods. 

I may go down the track of getting a new Mikuni 34mm carb with stock jetting.  Also add my stock exhaust and stock ignition.  Then, using the advice from you guys, run it in and keep checking the piston.  Sand any high spots.  Hopefully jet to get it nice, and then start adding my trick parts back and checking the piston and plug regularly.  Could also change the heat range of the plug as suggested. I may change my brand of oil and go to a different ratio.  Maxima at 32/1 or something like that.

Lots of options open, and again its going to take some time to go through all this...

Cheers,