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96 YZ 250 broken power valve.

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I did a through cleaning on my bike today only to discover a half circularity piece of steel sitting in my expansion chamber. Apparently my power valve cracked directly down the center about a 1/4in in width. I haven't removed my exhaust in quite some time (3-4 rides) so I am unsure as to when this might have happened. Nevertheless, I also did not notice any tangible loss of performance from my bike either ( I ride trails exclusively.)

Anyone know how much a replacement would run? So far, the TT store cost $170ish and Cheapcycleparts runs in the $150 range.

Here are some pictures of the power valve...

IMG_2112.jpg

IMG_2113.jpg

IMG_2115.jpg

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I looks like someone has done some serious grinding on the pv at some time in its life..or former life.

The powervalve was probably touching the piston at some point. This is normal when the stop/cover on the left side wears and has to be dealt with. To do things right, you also meed to replace the round cover on the left side.

You cna clearly see the bolt that connects through the center of the pv. This gets loose over time and the piece loses strength when the bolt is not tight.

If you look at the drawing here:

http://www.yamahasportsplaza.com/pages/parts/viewbybrand/2/Yamaha.aspx

You should replace part 9, 19, 28 and 29 at a minimum. New o rings in the area would be a good idea as well.

It is a little expensive, but if you keep it tight from here out, it will last a long time.

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Thank you RCannon. If the bike was driven with a broken power valve, would there be any additional wear and tear on other parts that I should inspect? Like said, the power valve may have broken either during the last ride, or 3-4 rides ago so I would like to side with caution and not overlook any details.

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Since the powervalve rides so close to the piston, look at the piston to see if its damaged. You probably dont need to pull the cylinder. The damage were talking about here would be visible though the exhaust port....especially once the pv is removed. You should see a polished stripe on the piston if its been touching the powervalve. Often times this contact is what causes the powervalve ot loosen up.

The linkage and other parts are strong and dont really get damaged unless improper assembly happens. If you replace those 4 parts I talked about earlier, you should be good to go. You could also choose to replace 22, 23, 24, 25 and 27 but this woudl be to make you feel good rather than having to do it.

From here out, check it at least every 10 hours of riding. This sounds excessive but you'll be able to catch problems before they get expensive. Also, on thsi bike, the pv is easy to work on. Once you get the hang of it, you can service it in well under 15 minutes.

Its not as effective as the newer style pv's are, but its a million times easier to work on.

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Once again RCannon, thank you for your help. Upon removing the powervalve today, I noticed several things that help identify why the powervalve sheared in half.

First, one of the pan head screws holding in part# 19 had become undone. I believe that the previous owner had done some work on the motor, and didn't apply the proper torque when reassembling the powervalve. The reason I suspect this is because the other screw holding part# 19 in was a hex head (while the manual designated that both screws should be panhead screws.) On the right side, another screw (part# 29 had also come undone too.)

Lastly, the piece of metal that was found in the expansion chamber is infact not part of the powervalve, but rather a section of part# 24. Having said that, I assume that I should also replace the additional part numbers you provided in your last post too.

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19 should not have loosened. I wonder if ti were ever tight??? Is 20 in place?? That keeps 19 from rotating.

I would probably replace the parts we talked about..most of them are cheap anyway.

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Hey Roland I have a question hope it's not a stupid one but here goes. Can you work on the powervalve like disassemble it while the jug is still connected to the bottom half of the engine? This thread got me thinking about mine. I don't think I have any issues with mine but it could use a good cleaning. Sorry to the op i'm not trying to jack your thread I have the same bike and didn't wanna make a new thread. Thanks for any info.

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Hey Roland I have a question hope it's not a stupid one but here goes. Can you work on the powervalve like disassemble it while the jug is still connected to the bottom half of the engine? This thread got me thinking about mine. I don't think I have any issues with mine but it could use a good cleaning. Sorry to the op i'm not trying to jack your thread I have the same bike and didn't wanna make a new thread. Thanks for any info.

Its almost easier to work on with the jug in place. Thats why the design is so cool. You remove the pipe and the covers on each side. Usually the gaskets are so wet they dont even need to be replaced. You'll be disappointed in yourself for not checking it sooner. Figure 30 min the first time to remove everything. ...maybe not even that long. After you do it once or twice, its not much harder than an airfilter.

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Rc, what part are we looking at here in the first two photo's, it looks nothing like what's on the parts diagram from the link you provided.

It's driving me nuts

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One last question,

Do you have any pointers or warnings when working on the powervalve while the jug is still bolted to the engine?

I don't think that my valve has ever been cleaned, the engine is bone stock never touched as far as I know

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This is my best guess...but on the parts drawing for the 96 yz 250, go to the cylinder head section.

Piece 19 looks like 1 piece, bit its actually split down the middle. The two halves are held together by a hex head bolt and two locating pins.

Its easy to see his front locating pin is gone..maybe dumped into the exhaust???? The half round broken piece is from the center section of the left side of the powervalve.

Now, the odd thing with his pv is seeing it so rough looking in front. The ones I have seen, maybe 5 total, all had a cast look to them..not smooth, but not rough either.

I am thinking someone ground the front off of his pv (with a grenade) and removed the section with a locating pin. No idea why, but....This woudl also explain why it came apart so badly. Usually you can hear a knocking sound that is an obvious sign to shut the bike off.

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I have some spare new power valve parts, if you have trouble finding what you need.

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