YZ125K Rebuild

[Lozza]

Have you run a timing light on the engine while running?

[alexbrown64]

No Lozza. Never done that and not sure how.  I always set the timing using the tdc method and a dial gauge.
Can you show me some pics of how to do it and what parts i need.  I will also do that check.
Cheers,

[justanothercgrader]

TTS is a good oil may not be the best, but most older bikes run that in the "olden day's"
I know you said the other bike runs good, but what fuel are you actually using?
Also, this suggestion of sanding the high spots on the piston sounds extreme, i would be checking the cylinder for roundness instead.

Way back, we used to slap a new piston and ring, super fuel, Castrol TT, warm the bike up, cool it down at home, do 1/2 dozen slow laps, cool it down, then go out and rip the shit out off it,

I am weary of your ignition, not the quality, rather the set up???

[Lozza]

No Lozza. Never done that and not sure how.  I always set the timing using the tdc method and a dial gauge.
Can you show me some pics of how to do it and what parts i need.  I will also do that check.
Cheers,

If you bought an aftermarket ignition, it must have supplied a curve ie @4000rpm ignition timing is 20*
So you would have to make a mark on the flywheel at 20* BTDC against a reference mark on the stator. Then strobe with a timing light and adjust the stator to suit. Just need the dial gauge the torqsoft website, a copy of the ignition curve and a timing light.

Also, this suggestion of sanding the high spots on the piston sounds extreme, i would be checking the cylinder for roundness instead.

The piston is checks the cylinder for roundness ......when hot.

[OverTheHill]

No Lozza. Never done that and not sure how.  I always set the timing using the tdc method and a dial gauge.
Can you show me some pics of how to do it and what parts i need.  I will also do that check.
Cheers,

If you bought an aftermarket ignition, it must have supplied a curve ie @4000rpm ignition timing is 20*
So you would have to make a mark on the flywheel at 20* BTDC against a reference mark on the stator. Then strobe with a timing light and adjust the stator to suit. Just need the dial gauge the torqsoft website, a copy of the ignition curve and a timing light.

Also, this suggestion of sanding the high spots on the piston sounds extreme, i would be checking the cylinder for roundness instead.

The piston is checks the cylinder for roundness ......when hot.

Lozza, not sure if i'm having a bad day [what day is it]. Should the mark [you make] on the flywheel say 20 degrees, be lined up with another mark you'll make on say the case or something 'unmovable' so if it's firing away from there you can move the stator to bring the marks inline. If marks are made on the stator [talking strobe marks not factory marks] they'll move around together when strobing & stay apart if are strobing out of line in the first place that is. I'll shut up--i'm having a bad day. 3 days of 'school built' mini bikes racing does that. [close to 200 bikes & 110cc sidecars plus 70cc honda clone motards] Just about pulled me bloody shoulder out trying to start the single speed 50's 2 strokes [cheapy chinese motors] on the start line, can of either start ya bastard in one hand & a bucket to pick up everything that fell off at the start line, nut bolts footrests, mufflers, chains [& riders]. Anyway--!! forgotten the subject.

[Lozza]

It really doesn't matter where the reference mark is because you normally make a TDC mark on the flywheel so stator or reference mark/rotor marks line up at TDC. Then make another mark(s) XX* BTDC on the rotor. So you have 4 marks cases stator and 2 (or more) on the rotor. Move the stotor to adjust the timing against stator or case marks

[OverTheHill]

Ok, thanks for that. Had a 125k back in the day, damn good model too. Hadn't even looked at the heading [125k] so no doubt would be rotor not flywheel, if original.

[Tomas]

do not know much about ignitions but I have used Horse Powe Ignition of Belgium(I belive) CDI unit on my80 RM 125(They only offer cdi unit for 81 but i thought that they are more or less the same)  and it works well. It is mixed up with original suzuki rotor and stator that was rewinded by a local elecrician and set to what manual recomends. Ignition coil is an e-bay job for $25 delivered(made in china I guess).  I reckon you will probably be beter of  with stock ignition that can be set to what was tested by maufacturer and worked well. Only thing I would recomend to replace by a new unit is CDI(even nos CDI unit can be 30+years old and may not last). Most of new units for these bikes are just copies of original unit but they are brand new. Ported cylinder should not be an issue. Run it in for longer and it should be fine. I run Repsol 2t offroad oil at 20:1 and see hardly any blue smoke when running at full throttle. It is running on ritcher/safer side just in case. This is what manufacturer recomended for a new bike. Dont worry about all tat dribble about how good modern oils are and you can run mix at 100 to 1. Cheaper to run at 20 to 1 than keep buying new pistons.
Also check your cylinder for roundnes if you can. Newly bored 2stroke cylinder can have high spots since there is a lots of holes(ports) and these high spots will grow when motor is hot.  It takes lot of skill to hone cylinder and get it round and straight. Thats why you should run your cylinder in for a little longer. Even modern cars should not be driven hard for first 1500 or so km i was told

[alexbrown64]

Ok, so let me get this straight.
I set the bike to tdc
I use a dial gauge to make a mark on my rotor and stator at, for example 20 degrees btdc.
I then strobe the bike to see if the marks align at a certain given rpm that would be given by pvl for this ignition.

Make adjustments as nesessary.

Does that sound correct.

Questions:
Do I need a 2 T timing light.
What rpm gauge is used.
Is there an all in one unit that is cheap.
Do you just download and print out a dial gauge.
What's the software for.

Justanothercgrader why are you weary of my setup? I did it as per pvl instructions. I do think the timing light is a great idea to check the advance though.

Tomas, bike shop did test for round, but will do again.

Great input everyone

[Tomas]

Would not be easier to use ignition on your stock bike as a reference to set your PVL ignition. All you need to get right is when the magnet in rotor pases coil core that is bolted to stator . Use dial clock and maybe timing wheel($10-15) to harvest these values and use them to set your PVL. So you have PVL ignition set to stock yamahaha setting

[Lozza]

Would not be easier to use ignition on your stock bike as a reference to set your PVL ignition. All you need to get right is when the magnet in rotor pases coil core that is bolted to stator . Use dial clock and maybe timing wheel($10-15) to harvest these values and use them to set your PVL. So you have PVL ignition set to stock yamahaha setting

might not be possible and you would have no use for the PVL.


Yes you are almost there. Just use dial gauge to give you mm BTDC that corresponds to the amount of advance. Distance from tdc is more accurate than a degree wheel.
http://www.torqsoft.net/piston-position.html
http://www.ebay.com.au/itm/152070562497
here is a cheap and cheerful tacho.
A cheap timing light is OK that will do 2T and 4T, there are more expensive lights that can tell you the amount of advance/rpm most will only go up to 5000 rpm on 2T mode.
You will have either of the top 2 curves

[Tomas]

And why it might not be possible?
Would you like to explain.
Anything special about PVL ignitions? Dont they use magnets and coils like all other ignitions?

[alexbrown64]

Would not be easier to use ignition on your stock bike as a reference to set your PVL ignition. All you need to get right is when the magnet in rotor pases coil core that is bolted to stator . Use dial clock and maybe timing wheel($10-15) to harvest these values and use them to set your PVL. So you have PVL ignition set to stock yamahaha setting

Hi Tomas, I know you set the stock unit for 1.88mm btdc and align the rotor to the stator.  The rotor has a keyway so it is fixed.
The PVL has no keyway. For my bike and pvl, you set the piston to 1.2 btdc and put the rotor on to roughly align the marks then adjust the stator to get them perfectly aligned. The PVL and stock Yamaha use different timing settings.  In fact, I went back to the instructions and it said I should be using between 1-1.2mm, so even to start with I could be 20% too advanced (1mm compared to 1.2mm), and it said you may have to even retard more for modified engines.  So maybe it should be .8mm.  The stock Yamaha setting would probably be way to advanced for the PVL ignition.

I found out my stroke is 50mm
My rod is 98mm centre to centre

When I use the torque soft app at 20* I get a value of 1.88mm

I have the Digital PVL, not an analogue one, and an internal rotor.  Not sure about windings etc.. I dont know my pvl cdi number yet, as I am working away. So, not sure what PVL ignition curve I have.  Would the curve not be closest to the curve on the bottom right?

My Yamaha curve in my manual shows about 26* btdc  at 4000 rpm

When I find out the correct value from pvl and then plug it into torqsoft, I assume I set the piston with my dial gauge at the given mm btdc, then make a mark on my outer stator and inner rotor that can be anywhere, but they must align.  Then start the bike and using the rpm gauge and timing light, bring the rpm to the correct range and check to see if the marks line up, and then adjust the stator a whisker to align the marks.  Does that process now sound correct?

Would this just test whether the ignition was working properly and set it to the recommended PVL baseline?  As even PVL states that the measurements they give are only a starting point.  I may need to go a lot more retarded for a higher compression engine no?
Also have to look at a leakdown test, a compression test, compression ratio test (Yamaha state 7.8-9.9:1) and squish band test. Yamaha recommends the squish for this bike at .8mm.  Does anyone know what is considered a normal range for compression, and then what is considered a high compression engine?

I will have to find the correct curve for this bike. I know the kit is PVL Digital Ignition 80115. (anyone have an ignition curve for this kit?)

Tomas, cylinder was checked for roundness by Eric Gorr in the US with the Woosner piston and then by a tuner in Perth (Ray Easson) with the ProX piston.  I am hearing you and maybe something was missed. When I get the new piston, there should be plenty of clearance but I will check again.
As for the oil, I may change brands and ratios.  I will speak to the tuner about it.
I am also thinking about taking it all back to stock to start with, and then adding the parts slowly and checking everything again as I go.

Thanks for your input and patience guys. Some of this is new to me, and just have to slowly work through it.

Cheers,

[ralph311]

Full marks for perseverance, I'd have gone completely postal by now. I got a bit lost in the thread but a couple of thoughts that might be of some use.....

If detonation is killing the motor seems to me that its at least possible for the problem to be caused by/resolvable by ignition timing alone, especially if you're confident it isn't running lean and you haven't put a timing light on it yet. So sorting that as a priority sounds sensible.

Also seems to me that a good place to start is to know what your ignition has been doing when the bike keeps seizing. If it were me... when I put it back together I'd set it up exactly how it was when it died and use a timing light and the torqsoft calculator to convert to degrees. Until you put a timing light on it you just don't know what its doing, you're guessing!

Then you can compare to the stock Yamaha advance. At least you will have a starting point that you can discuss with your tuner, which is another must do in my mind. He may be able to recommend something more appropriate for the compression/fuel you are running.

Also... afaik the best lubrication is castor oil. And whatever the initial cause, in the end you could say your problem is a lubrication failure. (I don't understand the fetish that some seem to have, putting the least lubrication into a two stroke that they think they can get away with) After all you've been thru.... geeze again if it were me I'd run that maxima 927 at 20-1, cross my fingers, kill a chicken and anything else I could think of to help keep it alive. 

ps if you've been borrowing your buddy's super bargain special-for-you ebay dial gauge, it is metric and not imperial??

[GMC]

My 2c worth;

Oil Brand
Seems you are running TTS, you will get many recommendations about oil brands but I doubt there is anything fundamentally wrong with TTS otherwise bikes would be dropping like flies all over the place.
I have swapped oil brands a few times over the years and I currently run TTS without drama and I also used it in the eighties without drama as well.
I doubt that the oil brand is the problem

Oil ratio
You will also get many recommendations about what oil ratio to use but if you’re running anything between 20 and 40 to one then you should be fine.
People run ratios inside this window all the time without drama. Changing the ratio will cause more jetting issues due to changing the viscosity of the fuel so if you’re running inside this ratio I would also leave well enough alone.
There may be decent argument for decreasing wear with more oil but I doubt it would straight up cause a seizure otherwise bikes would be dropping like flies all over the place.
I assume you run your other bikes on the same oil and ratio without problems!

Fuel flow
This one is often overlooked, if the gauze filter in the fuel tap is blocked or if you’re running a fuel filter you may be restricting fuel flow which will throw your jetting specs out the window.
You should take the fuel hose from the carby and hold it in the same shape as if it was on the carby and turn the tap on to see if you have decent fuel flow.

Piston marks
Pistons are made oval and tapered, the theory is due to more material around the crown and gudgeon is that they will expand to become round when at operating temperature, this same theory says that if they become hotter than intended then they will expand more to become out of round thus crating seize marks at certain points on the piston.
I doubt that Eric Gorr stuffed up the machining however some brands have misjudged the expansion rates for their pistons and rubbing the high spots is an old school trick to get them to spec.
Most modern pistons I thought were pretty much on the mark though.

Exhaust
People use all manner of brands without drama and I can’t see this as causing any problems unless the stinger/muffler core is the wrong size. A 125 should be approx. 23mm ID plus/minus 1mm
I have seen guys fit 125 mufflers on a 250 and wonder why it doesn’t run properly.
Even had a guy fit a 4T muffler to his 2T and wonder why it didn’t rev properly!

Carby size
Increasing the carby size will mean more air enters the motor at full throttle so you will need a bigger main jet than normal to compensate to the correct fuel/air ratio. What main jet people are running on smaller carbies will be irrelevant
You may have miscalculated this one!

Timing
Being an aftermarket ignition it will be a bit more complicated to set compared to stock.
It may even have an odd advance curve so checking this out thoroughly would be my first thing to check.

Multiple issues
Often problems can be caused by more than one issue, jetting alone may not cause it to seize and timing alone may not cause it to seize but both being a little bit out may push your specs over the edge.

Good luck, when you feel it’s doing your head in then sometimes it can be best to sleep on it

PS. Yes beware the European dial gauge!