Concerning Yamaha Power Valve failures

[MilkJuGGz]

So most of you are aware that when the stop tabs on the powervalve system start to wear, the piston rings will scrape against the actual powervalve itself. This in turn causes a nasty failure.

Now I don't know if this has anything to do with the failure I experienced because of this, but when I first got my bike, I noticed clicking noises within the engine. This would happen if I was in too high of a gear (well below the powerband) and giving it a lot of gas. It sounded like a smooth rattling until I finally got near the powerband, then it would disappear.

Is that noise the piston rings kissing the powervalve because it's not fully open at low rpms? Or could it be something else. I noticed this the other day when riding some wide open sections. What could cause a rattling noise like that, obviously RPM and load related? I replaced the powervalve stop, but not the actual powervalve itself. I'm guessing the stop put it back within spec, but maybe not.

I'm putting on new rings soon so I'll inspect for any damages.

~JuGGz

 

[bclapham]

could be a worn piston? the year and size of bike might help

 

[marcusgunby]

Definatly detonation.Too advanced ignition, lean jetting, airleak, too high compression /too low octane rating in the fuel.

 

[Rcannon]

Milk, at first the noise is not much of a noise at all. If your powervalve was clicking, it went way to far. Mine started off by a rattling noise. It rattled because the bolt holding the two pieces together backed out. This SHOULD have been my clue, but being a great mechanic (???) I just put them back together!

I pulled my stock 97 YZ 250 apart last summer. There was some contact, but very light. This looked like a polished part on the powervalve and a matching mark on the piston.

I know the contact would eventually get serious. I had amachine shop relieve the powervalve just a bit to offer more clearence. This worked well enough to let me finish the year. This winter I replaced the PV and cover. There was no performance difference between my old ground down P Valve and the new one.

 

[MilkJuGGz]

Ok thanks for the replies. I had posted about this before but hadn't ever thought of it being related to the power valve. Btw, it's a 96 WR250. I'm going to try some higher octane fuel to see if that helps the problem. Jetting is spot on though. Timing seems to be set correctly but I'll check again.

 

[Rcannon]

Is is not very hard to check the powevalve. The covers on the cylinder can be removed without taking off the cylinder. The valve splits down the middle and is removed from the sides..

If your sitting on the bike, remove both the inner and outer cover on the left side. Make sure the bolt is still tight that is supposed to hold the PV together. I cought mine before anything bad happened, but I know it could have.

 

[yz250roost]

I also did the PV fix in the summer of last year, As I've said before, it would be in your best interest to replace the actual PV and the PV stop. This is because when the PV strikes the stop both are being worn down, this could be because similar metals are used to construct both, if the stop was made out of brass or something softer than what the PV is made out of then it wouldn't be a problem. Take my case for example, the stop tabs were worn down.001mm on both sides, and the PV itself was also worn down .001mm which amounts to a total wear of .002 at each stop. and .003 is the limit I believe. That is why you should replace both the stop and PV hope this makes sense.
Jeff

 

[Rcannon]

Jeff, good post. Did you have shiny spots on your piston, or did you catch it soon enough.

The best mod for cheap on this bike is the flat powervalve cover. That really made a difference on my bike.

 

[whyzee]

RC, did you ever post pictures of that mod? I remember the thread but never saw pictures of the cover you made.

 

[Rcannon]

I have never been able to post pictures. I ended up making a copy of the cover on the work copy machine. I used this as a pattern to cut a new cover. It works very well and gives the bike more midrange. The bike is much more fun to ride with a flat cover.

 

[bclapham]

Roland, if you want to post the pics, you have to buck up the $12 mate!:) send me a photo and i will put it up

bclapham@scripps.edu

Jeff: are you sure about those measurements? it seems like the "." is in the wrong place, 0.001mm is very hard to measure, heck, even 0.01mm is hard toi measure without some really precise tools! (i am not trying to be an arse btw):)

 

[MilkJuGGz]

Is this a common mod, the flat powervalve cover, or is this something that you invented on your machine? Sounds promising but very difficult to make!

 

[Lorin]

The flat cover is easily accomplished. I just traced the original cover on top of a flat sheet of 3/16 aluminum, cut it out with a jigsaw, and drilled the holes out. Took all of 20 minutes, and even polished up nicely.

 

[yz250roost]

Bclapham, you're right that is very small but I can't remember what the measurment was off the top of my head, maybe it was in Inches. shoot I don't know I don't have EG's book in front on me but I'll find out. RC, yeah I had some cylinder polish when I took apart the PV, sounds like your PV cover is nice hopefully some pics will be up soon, I was thinking about doing the same thing but Havn't gotten around to it yet.
Jeff

 

[Rcannon]

I did not invent it. One of the other guys sent me a link to a comapny that was selling them. I e mailed them but did not get a response.

I decided to make one myself. I used 1/8 inch aluminum and roughed it up with scotchbrite.

The bike runs a lot harder in the midrange. Top and bottom feel about the same. The midrange improvemant is amazing.

 

[whyzee]

RC, Lorin,
The cover we are talking about is #43 correct?
This cover on mine is only 3/16" as it is. :think:

 

[Rcannon]

Different model bike...On the older 250's the ocver is on the left hand side of the bike. Similar idea though. It is piece #38 in this picture.

http://216.37.204.205/Yamaha_OEM/YamahaDB.asp?Type=13&A=130&B=3

 

[whyzee]

Aaaahhhh! Ok, makes sense now! How dare they change parts. :confused:

 

[bclapham]

PV cover

 

[Lorin]

Just dont make the mistake of drilling the drain hole in your cover. I actually ended up making a second cover due to that oversight.

 

[Rcannon]

Lorin, I guess neither you or I worry abotu draining the "tar" out of our covers after every ride. The owners manual says to do this. I am telling on you.

By the way, do you think the cover made a power difference?

 

[yz250roost]

nicely done RC! you just motivated me, I'm on my way to the local metal shop for some aluminum scrap! Any more tips before I go about the project? thanks
Jeff

 

[Rcannon]

Thank you. I used a jigsaw and a cordless makita. For the sake of arguement, there was some handfitting involved and the blisters healed just fine.

This cover was well worth the effort. It made a larger difference than th eRad Valve and Pipe/silencer.

I wont say "easier" to ride, but way more fun.

I think my next experiment will be a spacer under the stock giant cover. I think it might really mellow the power for those "follow the 5 year old nephew on his XR 50 days"...

 

[Lokair]

Ok I noticed this is an old subject, but I am working on my 97YZ250 power valve this week. I am gonna make the flat plate, I have a few ?s first. One how does this increase midrange? Next My power valve(I am new to Bike stuff so bear with me) seems to not be connected to the push rod and my book a Cylmer's manual really does not explain much about how it connects, so is it just the light pressure from the part with the spring on it(pushrod arm) with the boss up against the exhaust valve lever? I torqued the pushrod bolt to 35 inch lbs as the book says is this correct?(this bolt was only finger tight when I opened the cover)

I still have the bike apart due to my exhaust getting worked on any recommendations at this point would make my week.

Thanks Lok

 

[Rcannon]

The rod does tighten to the powervalve. 35 lb/fin sounds about right . They can come loose from the powervalve hitting on the piston. The easiest fix is to file a small amount off of the powervalve...no big deal.

The flat plate reduces volume in the chamber on the side of the cylinder. It acts somewhat like a shorter exhaust pipe.

I like the flat plate on my ported cylinder. Its not as good on the stock cylinder.

The 98 YZ 250 ignition module will snap right in to the 97 wire harness (black box). This gives much better low and midrange power. There is not quite as much top, but its a good mod.