Ok, found out what caused the engine failure...

[MilkJuGGz]

Thanks a lot to all who replied to my previous post. Jmics was right - the piston/rings hit the powervalve and snapped the top ring which put a large lip on the top part of the piston. (I have pics but I will post those later). Btw, this is a WR250 that smokes, 96 model (same as YZ engine).

The cylinder is perfectly fine, except for a few dings around the bottom part of the exhaust port. They do not extend into the cylinder. This can be filed down/chamfered, yes? It does not look serious.

The powervalve also has fresh ding marks on it, and those don't look serious either. These too can be filed down? I am having problems removing the powervalve. Seems to require a lot of torque with the allen key. More than I can provide.

What could have caused this failure? How can the powervalve hit the piston all of a sudden, out of all this time that I've had the bike running perfectly? The only thing I've done to the powervalve beforehand was remove the left side cover. I recall having problems getting it back on flush, but after some fidgeting it finally went on.

The piston was a Wiseco with an unknown amount of hours on it (but it looked good). I have a new standard size wiseco to put in, and I want to make sure this doesn't happen again. I don't want to end up pushing a heavy bike through desert sand for another few miles. Oh man that was a real workout... :ugg: I had blisters that would put sunny-side up eggs to shame.

Thanks in advance. I'm going to post pictures on here once I figure out how it works. :thumb:

~JuGGz

 

[Rcannon]

You need Erics book. The YZ powervalve stop wears down causing this problem. A local machine shop ground mine down to avoid this. Erics book recommended replacing the PV. I would go with what eric said. Look at the left-side powervalve cover plate and the valve. Check ot the wear marks when the valve closes...Sorry this happened to you. his sucks!

 

[Rcannon]

This link to Eric's site is a much better description than I gave....

http://ericgorr.com/techarticles/mxbooktun97.html

 

[jmics19067]

I believe you have to be a sponsoring member to post pics. Rcannon has you pointed in the right way. Yes that powervalve screw can be a real bear I suggest you go out and buy the best 4mm< I believe that is what it is> allen head socket you can find and still hope you dont twist it up. Use a block of wood in the exhaust port to hold the powervalve from spinning around. take the cylinder outside or somewheres safe where you can warm up the powervalve with a torch <careful now there can be left over gasoline /oil that will catch on fire >

There should be a little dowel on that left side cover that holds the stop plate in its proper spot. If that stop plate isn't put on correctly or the stop plate and/ or powervalve wears down the powervalve can rotate too far and hit the piston. When you get the new parts assembled in the cylinder spin the powervalve around and look inside and you will see what had happened. If the bike was completely assembled and you had that plate off usually you have to open the powervalve slightly and that plate should just pop right in. the powervalve is about 90$ and the stop plate around 40$ the last time I bought a set.

If there is no damage to the cylinder bore or its exhaust chamfer the cylinder should be ok if it isn't worn<measure it or have someone measure it for you it is an 8 year old bike> if the chamfer is nicked up the cylinder should be easily saved but take it to someone who knows what they are doing or find out the proper way to clean it up. I tried cleaning up the chamfer once and the plating chipped at that spot shortly after. I am not sure if it was my angle of chamfer or grit selection. all I know is what worked for a cast iron sleeve won't work on plated.

 

[yz250roost]

these guys have it nailed, this is definetly your problem, I've got a 98 YZ 250 (similar engine) and (as Eric's book suggests) I checked the powervalve just in time, it had a highly polished spot on it where it was hitting the piston, but no damage yet. I replaced the whole assembly and it seems to have a little more bottom end power. to do the repair correctly you must get the powervalve stop and actual powervalve as well as new o-rings and gaskets. The whole setup cost me 160.00. Sorry to hear this happened to you, all those late model Yz's are out there just waiting to do this to their owners, and the sad thing is not many people know about it.......Yamaha should have put out a notice or something.

 

[a454elk]

I just recently purchased an impact driver for just that reason, can't get enough torque on a screw head or allen head bolt. These drivers are a life saver. A little heat will also help the situation. I had to polish my powervalve on my 01 RM because it was sticking in the port. I polished the port and the powervalve, problem seems to be solved. If the powervalve hit the piston rings, it slid too far into the port. There is a dowel that holds it from sliding that far down, my guess, like everyone else has said, it's broke or popped out somehow. It could have been worse if the valve got jamed up on top of the piston. Good luck

 

[jmics19067]

the power valve on your suzuki and his yamaha are completely two different animals. On the older suzukis they are a guilotine type in that they slide down from the top to lower/close port hieght. Not sure how your new ones are but probably not that much different in action . The older yamaha has a large drum with the center of it ground flat < for lackof better description>
the drum rotates just above the exhaust port when it is rotated shut whats left off the round part in the center of the drum lowers the port hieght. As it opens the round part dissappears into the castingof the jug until the ground flat part sets flush with the roof of the exhaust port.

the problem with yamahas is that the round part can rotate too far closed when a stop tab wears out. A picture is worth a thousand words in this instance. And my description is not doing anything justice. if you could see it what I said probably makes sense but without seeing I am probablycoming up short describing.

 

[MilkJuGGz]

Great info thanks a lot everyone. I took it to an engine builder today and am having it honed and chamfered. This guy scared me when he said he's going to use a dremel tool to round off the lip that was formed on the exhaust port from the rings slamming into it. But whatever, everyone tells me this guy is the best... I just hope and hope and hope... :scream:

The a new powervalve and stop tab cap is going to set me back oh, about 130. I could probably just get away with a new tab itself for only 40 bux, but both the stop tab AND powervalve tab are out of spec. I'm going to check out the dealer tomorrow to see if they can offer a lower price than this shop. Anyone know of a good place to get such parts?

Anyways, the big question for all Yamaha owners is... how do you stop this from happening again? This bikes been running for six years before it failed. I hope I don't have to spend a few hundred every couple of years to keep my powervalve in spec because of a (shortsighted?) design flaw. But hey what can you do? I love my two smoke!

~JuGGz

 

[jmics19067]

Anyways, the big question for all Yamaha owners is... how do you stop this from happening again?

you don't brakes wear out , clutches wear out pistons wear out , and yamaha powervalve stoptabs wear out. Sorry justthe nature of the beast. The only thing you can do it to keep an eye on it and watch for wear and replace it before it becomes a problem.

I hope I don't have to spend a few hundred every couple of years to keep my powervalve in spec

well in my humble opinion if this thing lasted 6 years it aint nothin to sneeze at. I would rather spend 150$ every six years then having to clean my powervalve every ten hours as some of the early hondas where supposed to.

 

[MilkJuGGz]

I agree... it's not that much money over time to keep the YPVS running smooth.

Ok, another question. Can I grind the powervalve's face slightly to give a tad more clearance (In the fully rotated position)? I don't see how it can adversely affect performance unless I'm missing something.

~JuGGz

 

[Rcannon]

Milkjuggs...good question...I bought my new piston at a machine shop. The guy did exactly this to my powervalve...but, I dont think it is the right way to do it. My parts are worn. I did get me another thre months of riding without spending the money, though.

Honestly, after X-Mas, I am ordering new parts.

 

[yz250roost]

good discussion guys, this is something every YZ owner should know, Me and Rcannon have taken this subject and practically picked it apart in the past, between the two of us, we know our stuff, so rest assured that this is legitimate information.

 

[jmics19067]

yes you can grind it, but you will be only delaying the inevitable . The closer the fit<within reason yours was already too close obviously> between the face of the powervalve and the piston the more bottom end power you should have. How much to grind off , how much clearance do you atleast need and how much power you lose I couldn't tell you.Shouldn't bother top end power though. I assume if you know exactly what you need it is possble to get a little extra power by eliminating normal mass production sloppiness.

I suppose it could be easier quicker and cheaper to build the worn out tabs up with weld if you had the capabilities of welding aluminum. Dont know how long it would last if it would last.

Buying the stock parts and replacing them in a proper manner will give you the easiest most reliable way to achieve the performance that it was meant to have. It is a sure bet and should be very easy to spot excessive wear in the future now that you know the problem. To repair them would be a guesstimate and a gamble in an average garage . The speculation and risk factor is your decision.

 

[steve.emma]

I had the same thing happen on a old 92' wr250,made a real mess of the cyclinder and piston. The bike was a bit of a banger though so i just got a second hand barrel and piston. Removed the powervalve and cleaned it up with emery tape etc so it had piston clearence.Then i modified the left hand powervalve cover by drilling a hole in it then using a small nut and bolt in the place where the stop tab had worn off. I know it sounds rough,but hey the bike was rough and this worked fine, went a lot better to...

 

[Rcannon]

JMICS, I ordered a new pv and cover. Still, is this not just a temporayy fix? My bike was used when I bought it. I wonder how long the new parts are going to last?

 

[jmics19067]

Still, is this not just a temporayy fix?

heh I guess it is, but no more temporary than putting on chain and sprockets or new brake pads. I didn't mean to say that it couldn't be done or shouldn't be done just that it is easier not to make a mistake if you put the new parts in.

I kinda like the idea of Steve Emma. If you could make an adjustable stop on the left hand cover, all you would have to do is to see how much wear is on the powervalve. Then adjust the stop to make it up.

 

[Wanabe]

I have a 97 YZ250 and I have never heard of such a thing and am worried now. What exactly do I need to check for? Do I have to pull the cylinder off to check it or can I just take that cover off? I don't remember seeing any problems in the spring when i had it all apart due to drowning.

 

[Rcannon]

try this link....http://216.37.204.205/Yamaha_OEM/YamahaDB.asp?Type=13&A=128&B=3


Were talking about part "9" and "19" A tab on 19 stops 9, the powervalve, from rotating too far. The stop pieces smack together when the powervalve closes.

My bike showed a polished spot on the piston from light contact. Over time this would have been bad! Mostly removing the cover, part 19, and looking at the stop piece will show you how bad yours might be worn.

I am sorry if this is not understandable. Hopefully the other guys can translate what I tried to say into english!

 

[Rcannon]

Originally posted by jmics19067



I kinda like the idea of Steve Emma. If you could make an adjustable stop on the left hand cover, all you would have to do is to see how much wear is on the powervalve. Then adjust the stop to make it up.

Me too! I fyou guys come up with anything let me know. By the way, a flat piece cut to replace the stock cover helps this bike a lot. Thanks to you gys for sharing this one with me. Low end feels much better!