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Power Valve hitting piston


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I have a 2001 YZ 125 that just got rebuilt becuase of a cracked piston. Everything ran smoothly for break in and for the first hour of our ride. On the way back the bike locked up in 4th gear. Took exhaust off and looked in the port, the powervalve is scraping the piston. It has marks all down the exhaust side of piston. What went wrong. I set powervalve right, was this just a clearence problem. I have read a few post on here about the stopper getting worn. Also there was alot of spooge in the powervalve cover on side of the engine. The seal seems to be broken. So just replace powervalve with new one and get new piston?

Tim

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Tim,I just recieved my Wiseco piston for my 250, and they have a warning about this possibly happening on cylinders that have been bored. They instruct you to relieve the power valve in the area that protrudes into the cylinder bore + an additional amount for expansion. I would suggest you talk with someone who has done it because I have not.

Mike

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Tim,I just recieved my Wiseco piston for my 250, and they have a warning about this possibly happening on cylinders that have been bored. They instruct you to relieve the power valve in the area that protrudes into the cylinder bore + an additional amount for expansion. I would suggest you talk with someone who has done it because I have not.

Mike

Yeah anytime you bore a power valve cylinder, you need to relieve the valve. Basicly its just removing some material to clear the new bore. Eric Gorr does it for $50, he also has Wiseco piston kits for $95, may be your best bet if no one local can help you!
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If you slide your piston in the cyclinder , then close powervalve you can measure the clearance between piston & pvalve . generally about 20 thousandths is necessary . if you didnt bore it then you wouldnt have to clearance it "" of course " and the bushing you talked about may of been your problem !

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I did not have it bored out. I sent it out to get welded, bored, coated and honed. It was stock bore set to A spec. I got a A spec piston as well. I have done this in the past with another bike and did not have to shave powervalve. I called for a price check on the power valve, it was like 200 a valve. WOWWWW. So that is not good news to hear. I will tear it down later this week. I know I will need at least a piston and at least a new shaft and seal. Funny part is that I did check the clearence of the power valve and piston. I did not measure it because in the manual it doesnt even talk about the powervalve.

Thanks for your help. I will post some pics once I get it tore down.

Tim

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  • 3 weeks later...

So I tore down the bike a few days ago. What I saw was not good. The cause of blowing up was most likley NOT the power valve. By looking at the piston it was clear that either it over heated or the gas mixture was off. I have three two stroke bikes in my family, 2 others besides yz125. None of the other bikes had any problems, they all use the same gas. All I did was add more oil to the 125 for the break in. So I can rule out the gas.

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it looks like either no oil was in gas...

very little oil was in gas...

or the clearance was WAYYYY too tight...which I find unlikely but possible.

Sometimes user errors DO happen when mixing fuel...I get customers who do it backwards ALL the time and accidentially do too little oil.

Maybe the gas was oil and the oil seperated?

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I have contacted Athena and Millennium Technologies about this issue. Athena is sending me a new piston as we speak. Also I am sending everything back to Millennium to get inspected to check adhesion of coating. About your ideas

Crank Free-Play = I had check it before I put it in and there was none.

Oil in gas = Like I said before I use the same gas with other bikes. I mixed all the same gas 32:1 then made 2 gallons of it 20:1 for break in.

Clearance = was set to one thousand's from Millennium

So we will see what happens. I really want to find the root of the problem before I go ahead and replace everything again.

Thanks for your input

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I have contacted Athena and Millennium Technologies about this issue. Athena is sending me a new piston as we speak. Also I am sending everything back to Millennium to get inspected to check adhesion of coating. About your ideas

Crank Free-Play = I had check it before I put it in and there was none.

Oil in gas = Like I said before I use the same gas with other bikes. I mixed all the same gas 32:1 then made 2 gallons of it 20:1 for break in.

Clearance = was set to one thousand's from Millennium

So we will see what happens. I really want to find the root of the problem before I go ahead and replace everything again.

Thanks for your input

I may be wrong here, but I would expect the clearance to be about 2 thousandths.. maybe a tad greater.

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  • 9 years later...

Looks like I’m gonna be reviving this thread 10 years later! I’m having the exact same problem. At least I think. I have attached a picture of the piston on my 99 CR125R. Any ideas if it’s the same issue and if so, would grinding the power valve help me out? Also, this is only happening on the left valve so should I grind just that one or both valves together? Thanks guys🙏

C37F4A51-15BD-4A21-9922-2D2D3D8B0017.jpeg

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5 hours ago, Sailorwithabike99 said:

Looks like I’m gonna be reviving this thread 10 years later! I’m having the exact same problem. At least I think. I have attached a picture of the piston on my 99 CR125R. Any ideas if it’s the same issue and if so, would grinding the power valve help me out? Also, this is only happening on the left valve so should I grind just that one or both valves together? Thanks guys🙏

C37F4A51-15BD-4A21-9922-2D2D3D8B0017.jpeg

My initial guess is something broke that allowed it to come down to far. Usually only need to grind pv if you done a big bore

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5 hours ago, Smoking 2's said:

My initial guess is something broke that allowed it to come down to far. Usually only need to grind pv if you done a big bore

Hmm...any idea what could’ve broken? It has to be something in the power valve assembly right? I checked the valves for play with my finger but there’s zero play, indicating the bearing is in good condition, so I’m confused as to what’s causing it to stick out. 

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1 hour ago, Sailorwithabike99 said:

Hmm...any idea what could’ve broken? It has to be something in the power valve assembly right? I checked the valves for play with my finger but there’s zero play, indicating the bearing is in good condition, so I’m confused as to what’s causing it to stick out. 

No buddy I'm not sure, I've never had one apart.

Your in the Yamaha section so might ask in Honda 2 stroke section. If motox367 isn't busy and pops on he'll for sure know.

@Motox367

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9 hours ago, Motox367 said:

Post pic of cylinder.  I wouldn't grind pv something is broken or wore out for it to do that. Pic of pv also if can

Got it mate. I’ll send pics as soon as I can in the morning. Thanks for the tip!

10 hours ago, Smoking 2's said:

No buddy I'm not sure, I've never had one apart.

Your in the Yamaha section so might ask in Honda 2 stroke section. If motox367 isn't busy and pops on he'll for sure know.

@Motox367

Thanks so much brotha!

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Alrighty @Motox367 here’s your pics. The cylinder looks pretty good to me but then again, I’m new lol. That left powervalve has some severe scraping on it, not so evident on the right one (although it has a tiny tiny bit). Also, there’s a clip on the back end of the right powervalve, but it’s missing on the left one. Could that be my problem? Thanks man💪

69DAE530-17B7-4755-8D22-5C53A39DB825.jpeg

3DEA030E-5254-4BDE-A309-145F83337C34.jpeg

10609FE0-C16D-48A9-B79D-9E86F86C8534.jpeg

376D1B4A-6746-4D2A-861C-16B1A76BE2DE.jpeg

82944BC7-54F1-41FF-80E3-3441147C5613.jpeg

93530DC3-1E8B-461B-BF05-A9E3E2493F0D.jpeg

C3752302-3E85-4E32-B9C6-6B0B54F955FA.jpeg

850436F1-6151-41CC-A51C-B43FA777462E.jpeg

30F2F98C-4AEC-463A-94B1-75B6FA17BCC6.jpeg

image.jpg

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5 minutes ago, Sailorwithabike99 said:

Alrighty @Motox367 here’s your pics. The cylinder looks pretty good to me but then again, I’m new lol. That left powervalve has some severe scraping on it, not so evident on the right one (although it has a tiny tiny bit). Also, there’s a clip on the back end of the right powervalve, but it’s missing on the left one. Could that be my problem? Thanks man💪

69DAE530-17B7-4755-8D22-5C53A39DB825.jpeg

3DEA030E-5254-4BDE-A309-145F83337C34.jpeg

10609FE0-C16D-48A9-B79D-9E86F86C8534.jpeg

376D1B4A-6746-4D2A-861C-16B1A76BE2DE.jpeg

82944BC7-54F1-41FF-80E3-3441147C5613.jpeg

93530DC3-1E8B-461B-BF05-A9E3E2493F0D.jpeg

C3752302-3E85-4E32-B9C6-6B0B54F955FA.jpeg

850436F1-6151-41CC-A51C-B43FA777462E.jpeg

30F2F98C-4AEC-463A-94B1-75B6FA17BCC6.jpeg

image.jpg

Yep clip keeps it from you in to far

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