03 yz125 all rebuilt no power runs like a 60

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Oct 15, 2006
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#1
We just got this 03 yz125 it was real down on power but everything else is in very nice shape.It has a pro circuit pipe and silencer and some carbon fiber reeds.EVrything else looks stock.We pulled the head off and the piston would rock in the cylinder so We thought just low compression no power.We ordered a brand new cylinder ,piston and ring from yamaha.Its a b version.Im not sure what these letters mean but I think they have A through D versions.
We put it all together today and it still has no power,it starts easily and seems to run fine just no power.It will barely lift the front wheel in first and it wont lift at all in any other gear.I could outrun it on a stock 300ex.The plug looks medium tan on the insulator and we are running 32 to 1 super techniplate with premium pump gas.WOuld anyone have a clue what else would kill the power on this thing.Thanx
 
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Oct 15, 2006
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#3
I just swapped all the power valave assembly over from the old cylinder and its all free and hooked up to the linkage
 
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Oct 19, 2006
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#6
Just because the power valve is connected to the linkage doesn't mean it's operating properly. If it were stuck shut, the bike would certainly be down on power output. If it were stuck open, the bike would be slow in the lower rpms and have a hard hit in the power in the transition to the top end.
 
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Oct 15, 2006
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#7
The reeds look new and they closeall the way up,they are carbon fiber ones.

The power valve is for sure free and not gummed up ,I just swapped it into the new cylinder yesterday.The part in the cylinder is for sure free but Im not sure how to tell if the linkage coming out of the case is moving .I installed it in the cylinder just like it was in the old cylinder.Will the power valve fit in there upside down.We just got this thing and this is the first bike I have rebuilt and had to swap the power vlave assembly over.This thing has a small hit in the midrange and thats about it,its all you can do to lift the wheel 2 inches.Im wondering if this power valve assembly is in upside down,or the linkage is stuck in the case somehow
 
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Jul 8, 2006
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#10
dufrain said:
I havent it blows smoke out pretty good but I can pull it and check
I had a silencer on a kx250 that broke inside and let some of the packing get into the center core tube. It would start easy and seemingly run fine but just would'nt make any power. Just something to check.
 
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Jul 18, 2007
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#12
yes, on some models maybe all??? the actual valve of the power valve assembly can be slid in upside down, i think...
Man, this probly is not it, but i rebuilt a cr one time and it was real late, we had to put a new crank in it and I was finishing up and I put the piston in backwords. lol, it would run believe it or not but just barely, and then die. But always restartand run real doggy....anyway it came to me the next day what I did...just a thought.
 
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#13
I forgot to tell you guys what I found wrong.The power valve wasnt opening.I pulled the cover off the arm and put a nut under there and it hit like an early 80s 125 with the power valve stuck open.That showed me the power valve wasnt opening.I had to pull the side cover off,the pin that hold the powervalve governer together had came out so it wasnt opening the valve.I cut off a drill bit and made a pin and it runs pretty good now.Thanx for the help