yz125 eating pistons

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#1
1997 yz125, purchased used, after a few months, top end job needed, piston was so loose in cylinder, shop sent cylinder off for re-finish, turns out it was over bored, 50?. special wysco piston needed. Ring gap filed to be in spec. New bearing on piston, no play in bottome end of connecting rod. after a couple of hours use after careful break in, sounded funny, pull head, piston and cylinder head looks like asfault. shop says something got in or was detonation, second piston installed,
carb richen'd, timing light on running engine shows 25 deg BTDC, spec is 32. After 20min of low to medium riding, smoking heavy, indicating pleanty of gas and oil, engine stops, compression is down to 60, and piston top is again rubble. what could it be? I am wondering if head has been shaved and compression is too high? Is there a way to tell? Can a compresion test when i get another piston in be too high?
Could iginition moduel be doing funny things to timing that didn't show when I checked it?

At my wits end how to help my son.
 

bclapham

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#2
Originally posted by perrytime
timing light on running engine shows 25 deg BTDC, spec is 32.
so is it advanced???? if so that will cause detonation. what plug??? and check compression when new???? is it off the scale? check the squish clearance and check for airleaks causing it to be lean also
 

Uchytil

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#3
Get a new jug. The plated cylinders last alot longer than most people believe. I have a 97 YZ125 and just had the cylinder replated by EGorr (and it was just starting to show wear). The std set-up may eliminate your problems by getting the engine back to spec the way it was intended to run. Just my two-cents.
 
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#5
Just found the crankshaft is loose, bearings on waterpump side allowing a lot of movement, taking motor to shop for tear down, I guess that could explain things, movement letting piston travel more toward head, creating little squash space, or to high compression at high rpm? Guess I better hope its nothing more than bearings!
 
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#6
I have a 2003 YZ125. It is a stock bike and I went through 3 pistons and 2 cylinders and heads this summer because of detonation. I'm not too happy with Yamaha right now. The mechanic didn't figure this out until this fall.
 

Faded

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#7
Originally posted by mx416
I have a 2003 YZ125. It is a stock bike and I went through 3 pistons and 2 cylinders and heads this summer because of detonation. I'm not too happy with Yamaha right now. The mechanic didn't figure this out until this fall.
mx416,

Curious if they were able to give you a good diagnosis of the problem? A person I ran into locally that said he was having the same problems (same bike as you) and now he rides yellow because of it. FWIW, this kid and his Dad were die-hard Yami fans, raced YZs for the past 6-7 years...they wouldn't just up and switch colors unless they had a reason.
 

marcusgunby

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#8
As manufactures try to get the max out of the 125s they do run close to detonation and sometimes into det city.All it takes is poor fuel-lean jetting and your over the edge.
 

dkortje

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#9
I've heard reports of 03 YZ125's being on the edge. In fact Yamaha decreased compression for '04 appartently because of it. I have never had a problem with my 03, but I'm starting to think 50/50 race gas might be worth the investment. :(
 
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#11
My quad did the same. It was from a leak in the gasket at the point where the variable exhaust mechanism is to close to the cyl wall on the lower end of the jug.It was sucking air.
 

Rich Rohrich

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#12
Originally posted by mx416
I have a 2003 YZ125. It is a stock bike and I went through 3 pistons and 2 cylinders and heads this summer because of detonation. I'm not too happy with Yamaha right now.
You break multiple parts because of YOUR inability to tune or your unwillingness to put decent fuel in it and you are mad at YAMAHA? :silly:

You purchased a RACE BIKE and it needs to be treated with the appropriate care. The lowest common denominator approach to tuning doesn't work with these engines. It's not an XR80.
 
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#13
Hey Rich how do you know it was inability, a good friend blew two cylinders and several pistons due to faulty cdi box on his yz another friend could not get his to stay running due to stator. both were very well cared for bikes with more than a few pro mechanics trying to figure out what was wrong. saying that do not think this is this persons problem, there are to many possibilties.
 

Rich Rohrich

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#14
You'll see the see the signs of detonation on the plug LONG BEFORE you'll hurt the engine. If you see those signs you really need to deal with it immediately, not after you turn two cylinders into paper weights.

Losing ONE cylinder to a faulty CDI during a race is something that you can't always avoid. Losing a second one to the same cause is just a turning a blind eye to a problem. You can't blame the OEM for that. ;) That's just piss-poor judgement on the mechanics part.

Anytime you break parts it's worth taking the time to determine WHY. It's really crazy to put the engine back together without understanding why it failed in the first place because chances are good it will fail again.

You don't have to be a mechanic to look at the plug at the end of practice. Doing something as simple as that can catch problems before they turn into major issues.
 
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#15
Hey Rich tell that to the two yamaha tech reps that could not figure out the problem, they switched the cdi just because it was the only thing left. the problem was intermitent and with no positive test procedure. no brand is without fault or perfect. the stator would start working as soon as the motor died, now tell me how to figure that one out! by the way the cdi would run fine for months and then blow in just a few seconds of eratic running before you could even shut it down.