96 yz125

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#1
I'm wondering if anyone knows about the difference btwn piston sizes in the 96 yz125. I just replaced my piston with a wiseco piston and I LOST power. There is a noticeable difference in power esp in the higher gears...Any answers???
Thanks
Vanessa
 
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#2
Why did you change your piston?
Did you change your ring and top end gaskets at the same time?

You might need to change your main jet which is the culprit for lost power on the top end of the rpms.
 
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#3
OK I changed the piston because it had a LOT of hours on it...and my ring and the O-ring gaskets were also changed...I have changed my jets (was very rich before now I've leaned it out to where its supposed to be) but I still am missing that balls to the wall hit that I want...Its a very steady bike very easy to control...but I want more power!!!!! Do you have any answers?? I thouhgt maybe the wiseco was not the right size for my cylinder but I'm not sure. THere are supposedly 4 different cylinder and piston sizes on my model bike and when I got the wiseco it was a "one-size fits all deal". I think this is my problem but I'm not sure????????
THanks
VAnessa
 
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#4
Surely you bought the standard size piston? If you did buy one of the other 3 sizes which are oversized ones, there is no way you would have made them fit unless you had the cylinder bored to fit them. First thing you need to do is check compression. Should be in the range of 125-130 psi. If that checks out, it sure sounds like your jetting is to rich up top and you need to play around with it some more.

MY 98 I have the carb setup as follows with thse mods:

Mods:
FMF Fatty pipe and silencer
FMF Rad Valve with FMF torque reeds
Std. size Weisco top end kit

Carb settings:
Std size main jet
#40 pilot(one size smaller than stock)
needle jet clip is on 2nd from top
air screw is set a 1 1/2 turns out

Should give you a starting point.
 
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#5
I'm running nearly the same jetting...340 main, needle clip 2nd from top and 45 pilot...its running pretty lean I'm putting a 50 pilot in this weekend ...I'm gonna do a compression check today hopefully...I'll see what comes up .

3 sizes which are oversized ones, there is no way you would have made them fit unless you had the cylinder bored to fit them.
I don't get this part. My manual says there are 4 different size STOCK...ie there are small variations from factory and that the sizes vary with bike??? I talked to some friends and they said the same thing as you but why does the manual say there are different piston and cylinder sizes??? Sorry for my ignorance...and Thanks for your help.


Vanessa

PS I'm beginning to think the power loss is in my imagination but I really think this bike had a lot more balls..:think
 
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#6
NO!!! There are never any piston made smaller than stock. IF there si four over sizes made for your bike, than you can just bore it over that many more times to get use out of the cylinder.
 
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#7
The YZ 125 cylinders come stock in four different sizes, which are microscopic in size difference. If you look at the back of the cylinder there will be an A, B, C, or D stamped on it, unless it's been worn off. If you're buying an aftermarket piston (such as Wiseco) there's only one size that fits all four cylinders. However, if you're buying a stock piston make sure and specify which letter cylinder you have.
 
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#8
ok Thanks Pat...I just went down to the yamaha dealer b-4 reading this post and they set me straight...they said the exact same thing you did. Appreciate your help and God Bless
Vanessa
 
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#9
Originally posted by cr250
PS I'm beginning to think the power loss is in my imagination
I think you nailed it. When the compression gets low on a two-stroke, especially a small-bore, it loses it's power when off the pipe most, making it hit harder when it comes on the pipe. Your new top-end has restored that lost power outside of the powerband, so the difference in power when it is on the pipe isn't as abrupt or dramatic.
 
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#10
i got a vertex race piston for my yz125. it gave a noticeable amount of power across the powerband. i'm not sure if they make one for the '96 though. my bike has a '97 engine in it. if you can find one that will fit your bike its definately worth the extra $10 compared to a stock compression piston. the only downside is that you'll need to run race gas instead of pump. if you dont want to pay a lot extra for race gas ou could just get octane booster...but its inconsistant (sp?) and not as high quality as race gas.
 
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#12
its a 96 chassis w/ a 97 motor. the 96 yz had problems w/ first gear exploding...so it went and the 97 went in. it really is a great bike. excellent power everywhere. it has enzo suspension, pro circuit pipe and silencer and a vertex race piston...its faster than my friends 98 yz w/ the same mods. the bike has unbelieveable low end, but still pulls hard on top. another friend of mine has a cr125 w/ porting, head work, piston, everything...and i think my bike has more power everywhere except top end.

quote:
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Originally posted by KC_BigDog_51
I thought all sponsored racers ran a new bike every year?
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i have sponsors because i'm quick on a bike and because they see potential in me...not because i'm rich. ive had this bike for a little over 3 years. i was going to get a new 2002 YZ250F...but my mom is pissed at me and wont co-sign for the yamaha card (i'm only 17) so if my grandpa wont loan me the money it looks like im gonna spend another season on the 125. if i stick with the 125 i'll toss in a reed valve...im not sure what else there really is to do...as old as the engine is, i dont really know what good motor work would do. any suggestions?