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No low end power after changing piston

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Over the winter I put a new top end in my 2006 YZ 125, and now the bike has absolutely no low end power.  It still runs great on the top end of the rpm range though.  I did some research, and a lot of people were saying to check the power valve.  So I removed the cover and started the bike to make sure it was moving, and it was, but it seemed to move kind of slowly (not sure what it's supposed to look like though).  So I took it all apart to inspect and clean it anyway.  One side looks to be covered in oil.  I've attached some pictures.  Does everything look normal?  How do I go about cleaning it, and do I need to take it apart out of the cylinder?

 

I also checked my reeds, and they seem to be fine.  The spark plug looks pretty rich, but I never changed the jetting and the thing has run amazing the last 2 years.  I also noticed some small scratches on the top of the piston.  Not sure if this is normal?

 

Any help would be appreciated!  Thanks

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Give the powervalves a good cleaning and Make sure that both powervalves are moving freely.

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Id like to know the answer to this as well. I recently just changed my top end on my 2007 yz250 and its doing the exact same thing. Starts just fine, but has zero low end poer and then just hits insanely hard from middle up. Its my woods bike and before top end it lugged just fine on tight trails and now its pretty much all or nothing. All i did with the top end was take cylinder and head off and installed new piston. And the only part of my power valves i touched was the activating lever on the right side of the jug. Just unbolted it and bolted it back in. Some im very curious the answer to this as well.

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Give the powervalves a good cleaning and Make sure that both powervalves are moving freely.

 

They seem to be moving freely.  In the 4th picture of the cylinder where the power valve is closed...  Is this as far as it closes?  It doesn't seem like a lot.  Can I clean them while they are installed in the cylinder or do I have to completely disassemble them?

 

Id like to know the answer to this as well. I recently just changed my top end on my 2007 yz250 and its doing the exact same thing. Starts just fine, but has zero low end poer and then just hits insanely hard from middle up. Its my woods bike and before top end it lugged just fine on tight trails and now its pretty much all or nothing. All i did with the top end was take cylinder and head off and installed new piston. And the only part of my power valves i touched was the activating lever on the right side of the jug. Just unbolted it and bolted it back in. Some im very curious the answer to this as well.

 

Yep, your problem seems identical to mine.

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It would be best to take them all the way out and inspect for anything that's worn or broken. For reassembly just follow your service manual and you should be good to go!

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They seem to be moving freely.  In the 4th picture of the cylinder where the power valve is closed...  Is this as far as it closes?  It doesn't seem like a lot.  Can I clean them while they are installed in the cylinder or do I have to completely disassemble them?

 

 

Yep, your problem seems identical to mine.

Yes, that is how far they are supposed to close.

 

If the power valves are causing a no low end condition it means they are stuck *open* (If they were stuck closed you would have great low end but flat top end.)

 

First (with the linkage still unattached) make sure the PVs open and close as smooth as butter. There should be absolutely no hangups or binding, and they should move together evenly.

 

Then put the cylinder back together and reinstall the PV linkage, then look up the exhaust port. At rest, the PVs should be fully closed. If installing the linkage caused them to open slightly something is wrong with the linkage. It should actually be pulling them slightly towards the closed position at rest. Then move the linkage up and down and confirm the PVs open and then return to fully closed position at rest.

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After reading your issue the one fact is clear the engine behavior changed after only a new piston. It's pretty hard to incorrectly assemble the PV linkage, and beyond verifying that the PV's slide freely within the cyl that pretty much rules out PV issues. I'm thinking there is nothing wrong with your engine, i'm actually thinking you had a very worn out top end, and now that you have a fresh piston you have 100% full power. It's extremly hard for us to help as we are only looking at a computer screen and not actually able to ride your bike and listen/feel the power delivery. Do you have a buddy that is 250cc experienced ? Does the low end power seem slow to accelerate ? Is there a flat spot in the acceleration then hits hard immediately ? There might be some debris clogging the pilot jet, I suggest a carb clean to rule this out...

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Are you 100% positive that you installed the piston right side forward?  Maybe you installed the piston backwards and the intake window is on the exhaust side rather than the intake side.  Look through the exhaust port and turn the engine over by hand to be sure.

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Are you 100% positive that you installed the piston right side forward?  Maybe you installed the piston backwards and the intake window is on the exhaust side rather than the intake side.  Look through the exhaust port and turn the engine over by hand to be sure.

Stamped arrow on the piston crown is pointing towards the exhaust port which indicates correct orientation. However you made me realize those look like aftermarket piston p/n's , now i'm wondering if the piston is the wrong size..

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Stamped arrow on the piston crown is pointing towards the exhaust port which indicates correct orientation. However you made me realize those look like aftermarket piston p/n's , now i'm wondering if the piston is the wrong size..

 

Yes, it is an aftermarket piston.  However I don't have record of the purchase because it was purchased for me as a Christmas gift.  I'm fairly certain it was a Wiseco kit.  How can I be sure that it is the right size?  Unfortunately I reassembled everything the other night...

 

I'm pretty sure this is the kit I got:

 

http://www.motosport.com/dirtbike/Wiseco-ProLite-Piston-Kit-2Stroke

 

Looking at the part number in the picture it looks to correlate to that piston from the Wiseco website (845M05400):

 

http://www.wiseco.com/ProductDetail.aspx?ItemID=845M05400&ModelID=456&ModelYear=2006&AppID=2280

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Update:

 

I cleaned the power valve and it now is moving much better.  I went out Friday night, and the issue is fixed.  The bike is running fantastic again now!  What a night and day difference.  

 

Thanks for the help everyone. 

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Update:

 

I cleaned the power valve and it now is moving much better.  I went out Friday night, and the issue is fixed.  The bike is running fantastic again now!  What a night and day difference.  

 

Thanks for the help everyone. 

Glad to hear it's fixed. Power valves were a great invention and made a huge difference in the performance of 2 stroke engines. Just imagine were we'd be today if improvements were still being made.

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Glad to hear it's fixed. Power valves were a great invention and made a huge difference in the performance of 2 stroke engines. Just imagine were we'd be today if improvements were still being made.

Yeah no doubt.  I can't believe the difference it makes.  With the power valve it is so smooth.  Without it, it is like light switch.  

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