YZ125K Rebuild

[Paulos]

Have you had a look at different needles rather than pilots?

[alexbrown64]

Roscoe, the rack is good.  I always use 4 tie downs on the bars just in case.  For one bike its an easy way to take a bike out.  Paulos, i have not looked at different needles... thats a can of worms as it took me ages to work out the 6f4 that i have.  From what i have read, most spooge comes from the pilot being rich.  Anyway, i shall find out tomorrow as i am keen to take the yellow US bike out for its first spin with all its goodies on.

Cheers,
Alex

[alexbrown64]

Well, took the US bike out to run it in and what a disaster.  Had a flat front tyre from the word go... no worries, its sandy so i will keep going. Then she nips up once and frees up, then does it again.  I let her cool down and check the plug.  Oily which is good.  I take her out again and in third gear at about 1/3 throttle she seizes up and i have to push her back to the car.
When i first started her up, it only took one kick from cold. It ran clean and crisp and revved well.
New fuel and oil at 25/1.
Bike was running great with old cylinder and piston.
Fitted the new cylinder, Wossner piston, a smaller pilot and replaced the reeds with identical ones.
Bike was running fine in first and second, but when going a little faster and in 3rd/4th she nipped up and released.
Any ideas?

I will strip her down tonight and have a look.  I never checked ring end gap etc.. when i fitted it all up.

I have the options of fitting the old cylinder back on and checking it all out, and once checked out, fitting the new cylinder/piston combo to the Euro bike.

Cheers,

[Lozza]

Well, took the US bike out to run it in and what a disaster.  Had a flat front tyre from the word go... no worries, its sandy so i will keep going. Then she nips up once and frees up, then does it again.  I let her cool down and check the plug.  Oily which is good.  I take her out again and in third gear at about 1/3 throttle she seizes up and i have to push her back to the car.
When i first started her up, it only took one kick from cold. It ran clean and crisp and revved well.
New fuel and oil at 25/1.
Bike was running great with old cylinder and piston.
Fitted the new cylinder, Wossner piston, a smaller pilot and replaced the reeds with identical ones.
Bike was running fine in first and second, but when going a little faster and in 3rd/4th she nipped up and released.
Any ideas?

I will strip her down tonight and have a look.  I never checked ring end gap etc.. when i fitted it all up.

I have the options of fitting the old cylinder back on and checking it all out, and once checked out, fitting the new cylinder/piston combo to the Euro bike.

Cheers,

There's a hint

[alexbrown64]

Thats a big hint Loz... i thought Wossner had a good rep?  Anyway, i pulled it all down and here are the results:

The piston "gripped" on 3 of the 4 corners.  The cylinder seems ok, just some scuffing, but the piston i think is kaput.

My Yamaha manual recommends a ring gap of 0.35-0.5mm.  I checked the end gap several times and i could only manage nearly 10 times less than that at 0.05mm. 

I dont have the gear to accurately measure the piston and bore, but guessing by the ring end gap, i think the tolerances are a bit too close for comfort.  I will drop it all off at the bike shop tomorrow for analysis and repair.

But what is the view on here... my forkup for not checking ring end gap and tolerances, machinist for not checking (Eric Gorr), Wossner, or something else.  I dont think its jetting or oil, dirt, etc..  i think its mechanical as in piston, ring, cylinder...


Cheers,
Alex

[Lozza]

Looking at your piston its a classic 4 corner seizure, either piston expanded to rapidly, too lean or too hot a plug. If it was another brand I would rule out the piston. A old rule of thumb is 4 thou(0.1mm) per inch of bore for the end gap. On a 56mm bore that is about 10thou or 0.25mm. You can easy measure piston to bore clearance with a feeler guage. Ring end gaps are easy increased. Nothing wrong with that piston is good to go after you file the 4 corners slightly with a points file and a rub over with wet and dry. What was your run in proceedure?

[alexbrown64]

Thanks Loz,
My ring end gap was 0.05mm which is 5 times tighter that 0.25mm so maybe that was the problem.  The piston has no heat marks under the crown or any where else.  Bike ran fine with the other cylinder and Wiseco piston.  My run in procedure was to go for a 5-10 minute ride up to 3rd gear and alternate slow to medium revs, then come back and cool it down and go a little harder the next time etc..  I didnt get that far.  I was only 5 minutes of poodling around in first and second, then when i went into third at medium revs she locked up.  TTS as 25/1, standard plug for bike.
Roughly, what size feeler gauge should i get between the cylinder and piston on a 56mm bore?

Cheers,
Alex

[sleepy]

No piont trying to measure the piston to bore clearance know as the piston will be out of shape.
My gut would tell me that if the ring gap is only .05mm then the bore was probably undersize.
Only time I've seen an issue with a Wossner is when the recomended clearance is ignored either by ignorance or poor workmanship.
Find someone with a micrometer and a bore gauge to measure the bore for you. It should be spot on or a little over the intended bore size. If the piston is for the stock 56mm bore then the bore should be minimum 56.00mm and quite often end up .02 bigger to get the correct clearance.
Can be other causes but first step is to find out what the clearance was and go from there.

[Lozza]

A 0.05 or 6mm feeler I have had so many of these type dramas with small bore Wossners I refuse to use them. You should be able to see any evidence of the ring ends butting together on the ends themselves and the ring peg. If there is no obvious rub marks it is the piston.
Break in is 5 min up to 1/3 throttle, let cool for 10min 5 min up to 2/3 throttle let cool down go for the doctor, or at a race meeting 2 laps 3/4 pace and slow ride through the pits and then go for the doctor. Never a constant throttle. But that is a piston problem for sure. You should be running a 9.5 or 10 plug in a 125

[alexbrown64]

Thanks for the input fella's.  I ducked out to the shed and worked a feeler gauge around the piston from all sides and top and bottom, also rotating the piston etc seeing if there were any high spots.  I could run a 0.05mm feeler all around.  I could just jam my 0.08mm feeler in but it was really tight.

[80-85 husky]

over on café husky forum there is a big consensus on Woosners. they require more clearance than the recommended figure. this nip up is typical for a woosy that has been bored to spec. get the hone out and let some more air in.

[Lozza]

Filing the high spots on the piston is a better idea. Idealy what you need to do is run the engine for a few minutes pull the cylinder and check for high spots on the piston. The problem is much much worse on <50mm pistons

[Tomas]

Had a couple o cylinders done by Greg Ball when I lived in Sydney an both have clearance around 0.9mm. Actual bore sizes are 55.55 and 56.05. I checked both with bore gage just to know what the cylinder guy did. Never had any issues. Cylinders are runnig wossner pistons.  If you can just push in 0.8 feeler around top of the piston you cylinder is too tight at the bottom (skirt). Take any piston a measure it with a micrometer when you get a chance. You wil be suprised to see where the size of the piston that is printed on the box comes from.

[sleepy]

Go to your local tool supplier and buy a cheap 50-75mm micrometer and matching telescopic gauge. The feeler gauge method is quite inacurate.
I have bored a lot of cylinders (50+) to suit Wossner pistons and have not seen one nip up, I have had several turn up here that have nipped up after bore jobs from other shops and all have been bored well undersize.
Your ring gap tells a story. The rings that come with Wossners are good quality and if you only have .05 gap when it should be .25 your bore will be undersize by .2/pye which is about .06mm to small. I always measure the pistons and never rely on the size on the box, most measure .02 bigger.
If you get a micrometer and telly gauge send me a PM and I will explain how to use it. 

[alexbrown64]

OK, i have decided to take it all in to the bike shop to measure and machine if necessary.  I go away to work tomorrow for a while so cant fuss around too much.  If the ring gap is as precise as Sleepy says, then there is no use in me mucking around.. the piston is probably a bit tight. 
I will however buy some cheap gauges so that in future i can check this sort of stuff out.
Will these gauges be OK for the home handy man to measure the occasional piston and bore...

Also, just for future reference.. if the piston was a whisker tight, can that be honed out or does it need to be bored?

http://www.ebay.com.au/itm/Outside-Micrometer-50-75mm-x-0-01mm-Caliper-Precision-Measuring-for-Workshop-lab-/331711159770?hash=item4d3b869dda:g:fn0AAOxyAs9SOrBr

http://www.ebay.com.au/itm/141535518535?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Cheers,
Alex