YZ125K Rebuild

[pecrazy]

I'm no expert by any means but I always believed that a 4 corner seizure like that was caused by incorrect clearance, regardless of oil, oil ratio or timing.

[topari]

I'm no expert by any means but I always believed that a 4 corner seizure like that was caused by incorrect clearance, regardless of oil, oil ratio or timing.

Agree, had a similar problem on a xr400. Installed a hi-comp piston in place of the standard. Full 4 corner seizure. The hi-comp piston had different clearances specification. Noob problem.

[evo550]

I'm no expert by any means but I always believed that a 4 corner seizure like that was caused by incorrect clearance, regardless of oil, oil ratio or timing.

Me too ...

[alexbrown64]

Thanks for the responses fellas. I will look at the muffler Geoff as its DG aftermarket. I set the bigger carb up exactly as the Yamaha race report recommended, but also went up on main and jetted needle richer.
4 corner seizure is clearance issue. My cast piston also had detonation. Famous tuner named Harry Klemm saw my pics and told me i need race fuel due to me having a high compression engine. I am not going to use race fuel, so it's check squish, lower compression, check and retard ignition, etc.
I think the high compression, low octane fuel caused detonation that has heated both the cast and forged piston beyond tolerances and grabbed on the 4 corners.
So, will check piston tolerances again, compression, air leaks, jetting, timing, muffler squish etc..

I also believe tts is ok. It was used by everyone in 83 at 25:1 and no issues. I run my other 125K and other bikes on the same Jerry of premix.

Fuel tap is new and flows freely.
Dial gauge is good quality.

I am working away, but when I get home, I will install new piston, check timing, swish, compression, muffler, leak down, then give engine to tuner to inspect and measure. See what he reckons.

One other thing, another member has had issues with his 125k, which has a worked engine. His was delivered with 2 base gaskets and had previously ran race gas before this. It may of been a way to reduce compression for normal fuel.

Cheers,

[OverTheHill]

Looking back to find the seized piston. Just looks like not enough clearance initialy. Being yamaha & if a yam piston they probably only run about 2 thou at a point at the top of the curve of the cut away inlet. Maybe the ring got pinched on the exhaust side to start it all downhill slide to be seized on 4 corners. Not something like a water pump problem?, had an early water cooled KX80 where the impeller came unstuck & not turning 'at all'. In saying that son kept racing it hard for the day & never caused a problem luckily just thermosyphoning i suppose.

[Lozza]

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?)

Your process is close but you need to look at the curve closely as after the engine is running it will follow the pre-programmed curve inside the cdi not what you static set the stator/rotor to. 

PVL only have 4 curves to choose from. 1.2mm BTDC= 16* (I think your rod length is 100mm too) so when you set that to 16* every point on that curve will be +/- 16*. At the point where the curve says "0*" will you have what you first set the stator/rotor to. The digital curve at 4000 rpm is +22* so you will have 38* and at 11,000 you will be back to 16*. As you can see 38* is a wee bit more than 26* of the yamaha curve.

Have you measured the head volume? A water cooled engine will easy run 15-16:1 with 98 pump fuel. A high comp engine I would regard as +17:1. The Yamaha figure is the useless corrected compression ratio that uses the volume form top of exhaust port not full swept volume.
Squish is about right.

[alexbrown64]

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?)

PVL only have 4 curves to choose from. 1.2mm BTDC= 16* (I think your rod length is 100mm too) so when you set that to 16* every point on that curve will be +/- 16*. At the point where the curve says "0*" will you have what you first set the stator/rotor to. The digital curve at 4000 rpm is +22* so you will have 38* and at 11,000 you will be back to 16*. As you can see 38* is a wee bit more than 26* of the yamaha curve.

Took me a while to get my head around what you are saying, but I believe you mean the digital curve in the PVL diagram is relative to "0". So when PVL say 22* at 4000 rpm, that would be as if you had your bike set at top dead centre.  When you reset your piston away from ("0" , TDC), you are affectively adding degrees to PVL's curve.  So, by using Torqsoft to calculate my crankangle at 1.2mm btdc, which gives me 16*, I have to add that to all the readings on the chart.  The 16* I set the bike at plus the 22* from the chart at 4000 rpm, which is the 38*.
When I use Torqsoft to calculate the angle at .8mm btdc I get 13*, which would take the affective advance at 4000 rpm to 35*.

So, the Yamaha curve (I will have to put a pic up at some stage), shows the degree readings with the bike already set at the Yamaha recommended 1.88* btdc for their standard system, which is indeed about 26* at 4000 rpm.

At wide open throttle though (11000 rpm) Yamaha's chart shows 13*.  PVL chart shows that the timing in the cdi has been retarded again back to "0", but add the 16* because of our 1.2mm movement of the piston and we have 16* again. 

Yep, got all that now, but they are relatively the same at wide open.  Dont ride the bike much at 4000 and it ran for 45 mins at these lower throttle openings when I was running it in. So, the PVL is not far off. And if i retard the timing to .8 mm btdc, i will be running 13* at 11000 and that would be the same as Yamaha.  In fact, i re-read the PVL documentation and it does state i should be running between 1 and 1.2 mm and even lower if i have a modified engine.

I will be taking the cylinder and head up to Ray Easson and he is going to measure the head volume. He also wants one of my standard heads to measure and compare.

Overthehill, at idle i checked the new rad fluid by opening the cap and i could see it pumping around the bike. The cylinder is fine, it only had aluminium smeared on it, no marks at all from a ring snagging.

On another less complicated note (my head hurts).. i have ordered a new Mikuni 34 mm carb, and have compared and ordered the necessary jets to run the bike as per Yamaha 1983. I will also adjust the needle and run a bigger main anyway to run the bike richer.
Also ordered the new ProX piston.

Used Oemcycle.com this time.  Anyone else starting to use them.  I got a bunch of parts really cheap and they gave me 3 postal options.. One of the options was free post!!  I have ordered nick nacks through their Ebay site, but their regular website is pretty good too.

Cheers,

[OverTheHill]

Not sure what i said now but probably meant the ring getting pinched in the groove from overheating on exhaust side rather than snagged in the port but no worries. Just get the clearance checked with a new piston & if it's not a recent rebore then get it checked for wear around the exhaust port. Ran mine on methanol back in the day, was damn fast too. Had my 125G on meths too. Purple shell racing special it was called. Double the petrol price & used twice as much--but was fun times. Nothing wrong with my long term lol--what was the question again!!.

[Lozza]

Over 30* is too much advance, then there is the little problem of the curve going from -10 to -19* (for ease of starting) between about 0-1000 rpm which means you would need static advance to be more than +20*. Where did you get the PVL from?
Using 1-1.2mm BTDC as a base would be OK with the non digital ignitions but I don't see any real advantage of any PVL curve over the standard curve.

[alexbrown64]

The pvl digital kit comes from penton racing in USA. 80115. I am using the digital chart you posted up Loz. Not sure if it's the curve in my Cdi. Pvl advertises these for the yz125. Do you think the ignition could be the problem?

I am putting a standard ignition on anyway for a while, but I am not sure about these mapping issues.

[justanothercgrader]

Although you probably didn't buy it from him and he is no longer a dealer, i would try to contact Newmann on Vital mx for advice on setting up the PVL, try the "old school " section.

He should be able to clarify the set up.

[ralph311]

Still guessing re the PVL if you don't put a timing light on it. 

[Lozza]

The pvl digital kit comes from penton racing in USA. 80115. I am using the digital chart you posted up Loz. Not sure if it's the curve in my Cdi. Pvl advertises these for the yz125. Do you think the ignition could be the problem?

I am putting a standard ignition on anyway for a while, but I am not sure about these mapping issues.

I don't think the ignition itself is the problem. Curve maybe , the piston crown will tell

[alexbrown64]

I have the Yamaha map which shows peak curve at 28* for piston set at 1.88 btdc.
I have hpi map which shows peak curve at 30* for piston set at 2mm btdc.
I have contacted pvl. They want to know all my part numbers. I wil have to get back to them in a few weeks.

I did get it through Newmann as he had an account with Penton.

Perhaps curve could be wrong. Lots to measure up and a puzzle to solve. I will run lots of tests, take lots of notes and give everything over to an expert for analysis.

[alexbrown64]

I stripped the bike down today and took pics of the PVL. I do have the digital cdi 537202.



So, perhaps my ignition is advancing too much. I will have to re-read the last few pages of this thread to get my head all around it again, but it looks like at 4000rpm i am running 38* advance compared to the Yamaha 26*.
I also have the engine on the bench and am ready to assemble it with a lightly honed cylinder, new gaskets and new piston and ring.
I also have the plugs and gauges ready to do a leak down test and compression test.  I will also check my ignition advance settings using my dial gauge, as the motor is on the bench.

It will be going up to Ray Easson in the morning for further evaluation. I will be taking the other cylinder, a few more heads and the old piston.

I got my genuine Mikuni 34mm carb in the mail from oemcycles and will be fitting the jets that the YZ125K came with as standard. 6f21 needle etc..

Cheers,