Jump to content
Sign in to follow this  

power valve hit piston

Recommended Posts

piston.jpg

So I'm pretty sure this is from the power valve hitting the piston, can any body tell me why this happened, and what parts am i gonna need to replace. I'm guessing that im going to need a new piston, rings, powervalve, and stopper. also im told the stock powervalve for the 98 yz 250 is terrible, is there an upgraded one i can buy and where could i get it?

Share this post


Link to post
Share on other sites

May be that it hit the opening into the power valve and not the power valve itself. Did you check the cylinder head? It may need to be sleeved. Try Q & E in Placentia; they do great work for reasonable cost.

Share this post


Link to post
Share on other sites
piston.jpg

So I'm pretty sure this is from the power valve hitting the piston, can any body tell me why this happened, and what parts am i gonna need to replace. I'm guessing that im going to need a new piston, rings, powervalve, and stopper. also im told the stock powervalve for the 98 yz 250 is terrible, is there an upgraded one i can buy and where could i get it?

Ignore the suggestion to get the cylinder re-sleeved unless you actually have a sleeve installed. The 1998 YZ250 did not come with one. It uses nikasil like every other mx'er and there's no need to sleeve the cylinder unless there is MAJOR gouging in it that cannot be fixed with plating.

The stock PV for the 98 is not terrible. It does its job just fine if it is properly maintained.

Just check all bore measurements. Work your power valve by hand to see if it sticks out into the bore.

You can see in the pic below (ignore the blue circled areas, they are for another post explaining something else) above the oval-ish blue highlight that the PV comes pretty close to coming into the cylinder area and, if you cylinder was bored out a lot without milling off a little of the PV, it may stick out far enough to contact the piston.

RIGHT-CLICK, VIEW IMAGE to see the high-resolution image

img0098zc9.jpg

Check your bore diameter. If the bike has been bored out a lot, and the power valve was not re-shaped to account for the larger diameter bore, the power valve could very well hit the piston.

Share this post


Link to post
Share on other sites

As to what you'll need to fix the problem...

Piston Kit (Piston, Rings, Circlips, Wristpin, Wristpin bearings, Gaskets)

Replate Cylinder (Depending on condition of cylinder)

Power Valve assembly (if it was hitting the piston, it's damaged too)

Gasket Kit (I ordered the complete gasket kit because it gives you ALL the gaskets you'll need in one kit, and you can store the ones that come with the piston kit for a later top-end job)

Share this post


Link to post
Share on other sites
piston.jpg

So I'm pretty sure this is from the power valve hitting the piston, can any body tell me why this happened, and what parts am i gonna need to replace. I'm guessing that im going to need a new piston, rings, powervalve, and stopper. also im told the stock powervalve for the 98 yz 250 is terrible, is there an upgraded one i can buy and where could i get it?

You may also want to correct the punctuation error in your Signature to use an apostrophe ' instead of a comma , in the contraction, "I,d" (sic)

Share this post


Link to post
Share on other sites

The powervalve in this bike rotates too far once the stops wear down. Its a good design, but needs to have the powervalve ground down where it hits before it can do any damage. You can also replace the parts on a regular basis, but the grinding accomplishes the same thing for cheap. I would agree with replacing yours now, but if this is caught early, grinding the pvv works just fine. Its aluminum so a person can use a hand file to open it up some.

Did you actually hurt your cylinder? Often times it will rub the piston without doing any cylinder damage.

Share this post


Link to post
Share on other sites

All that and a grammar lesson too...

The suggestion was to get a sleeve not re-sleeved & only if it hit it hard enough and chunked the nikasil coating (that is why I suggested to check the cylinder...) By the looks of the piston and rings it looks like it hit the wall of the cylinder pretty hard.

I,m sure if you look at the pic again it,d be hard for you to not concur.

Share this post


Link to post
Share on other sites

R-Cannon-Your the Man-This guy knows his stuff --helped me with my 94 yz 250.

Mine was a combination of Crap!!.#1Cylinder was wore alittle to far.My rings hit the top of the exhaust port.#2.The power-valve linkage was sloppy-and the screw that retains or steadys the pv was loose.#3.PV stop was bent This combined with the the rest of the slop let the pv open too far. Now far as the Cylinder-Since these are getting way hard to get -Take it in .I have heard pro's and con's about each method.I nikasil mine Sweet''.My KTM buddy at work sleeved his ride. He said #1.Std sleeve can rebuilt many times.#2.More clearance from PV -vs-Pistion. And last but least--The design in my opinion is crap! If i had it my way--I would re-design it.This seems to be one of the weakest links in YZ Top-end falure ---Good Luck.

Share this post


Link to post
Share on other sites
R-Cannon-Your the Man-This guy knows his stuff --helped me with my 94 yz 250.

Mine was a combination of Crap!!.#1Cylinder was wore alittle to far.My rings hit the top of the exhaust port.#2.The power-valve linkage was sloppy-and the screw that retains or steadys the pv was loose.#3.PV stop was bent This combined with the the rest of the slop let the pv open too far. Now far as the Cylinder-Since these are getting way hard to get -Take it in .I have heard pro's and con's about each method.I nikasil mine Sweet''.My KTM buddy at work sleeved his ride. He said #1.Std sleeve can rebuilt many times.#2.More clearance from PV -vs-Pistion. And last but least--The design in my opinion is crap! If i had it my way--I would re-design it.This seems to be one of the weakest links in YZ Top-end falure ---Good Luck.

I understand why you dont like it, and you are right, but I liked the design because it was so easy to deal with. The stopper on the left side of the cylinder and the pv were to soft, but other than that, the design rocked! Well, not for this poor guy, but still. I found out about this in Eric Gorrs book. I went out to the garage the day I got it and looked at mine. It had been rubbing, but only enough to leave a polished spot on the piston. I cut it back and its still running today. If a person knows about it and cuts the pv back 1-2mm, it will never be a problem.

Share this post


Link to post
Share on other sites

STD piston doesn't require reshaping of the power valve. Usually anything +1mm will require mod. So 4 oversizes above STD is where they say to reshape.

I bored my 87 out to 70mm and had to round the powervalve maybe 1mm. But if I hadn't it would have destroyed my piston/cylinder. -BIG DAN:thumbsup:

Share this post


Link to post
Share on other sites

It actually did no damage to the cylinder at all surprisingly, i found out what caused it and it wasn't a mechanical error, it was the mechanic (me). I learned the hard way to not tear stuff appart when your in a hurry and leaving for a trip, i left a bolt out of the power valve stopper and loosened up wile riding, the rest is obvious. im going to take the power valve out of the cylinder later on today hopefully and see if it is salvagable ill post some pictures as soon as i get time

Share this post


Link to post
Share on other sites
Hello there

I wonder if I still have to do a cut back the 1 to 2mm on the PV on a bike like mine YZ250 year 2009.

regards

arnego2

No, this was for the powervalve on the earlier engine. The newer engine uses something totally different.

Share this post


Link to post
Share on other sites

Post revisited.....that piston is almost exactly what my old piston looked like before I rebuilt the motor. Today I learned my powervalve is still having issues with hitting the piston rings...will probably have to put a new set of rings in????? Will need to tear apart (again) and see what can be saved.

Here is what the new sleeve and piston look like after about 10 minutes of running. I also am curious about those 2 identical score lines in vertical lines on opposite sides of the exhaust port....and the scorn area on top of the powervalve

http://i182.photobuc...e/cylinder2.jpg

Edited by e36graduate

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

  • Similar Content

    • By Lone Danger
      Ok so I'm looking for opinions from those who have been on yz125s and 150 exc/xcw's and maybe the ktm 200's exc/xcw's. You gotta be out there.
      Here's the deal...I'm 5'7" 165lbs. A couple years back I was looking for a small bike for my wife to learn on and for me to play around with and race. Could only afford one bike. I got a '09 ktm 105 sx. Once I got it sprung for my weight it was great. I have been top five consistently in my local harescrambles B class, believe it or not. Ya, you were not the only ones shocked. And did pretty good in the local mx races as well, junior class up here in Canada. Lack of power was sometimes an issue but I learned to deal with it and just scream, scream, scream!
      However my wife is a complete beginner and she is scared of it. I have since moved her to a 4 stroke.
      Just got a mint '01 yz15 mid summer. Like less than 200 hrs! I have absolutely loved it on the mx track. Suspension is perfect and I love the extra power. I had planned to set it up for woods/mx with a suspension revalve. However, did a harescramble race a month ago that was really technical. Granted I have not done anything to the suspension minus clickers, had too much psi in the tires (13 front and 15 rear) (yes I know that's wonkey, I only had time to get a heavy duty tube in the front and had a stock tube in back. Less psi in the back would have been instant pinch flat because its so rocky) and it was running rich. I had to scream the thing to keep it running and even changed a plug mid race. Well I'm sure you can guess but it was the worst experience I have ever had on a motorcycle. The more open sections I passed a lot of people but the tech sections required way more effort than I liked and the suspension beat the hell out of me!!! Like my knees, wrists, and shoulders were just done. (I was out there for three hours).
      So keep in mind that I just came off a 105sx with suspension mods (set up for woods/mx). I have no problem slipping clutch and shifting precisely, yatta yatta. (I don't think the 105 would have done well here either due to even less power, however...) I've been second guessing fixing this 125 up for woods ever since I rode a friends '09 ktm 250xcw. I rode it on a local mx track. The power was just so effortless and I was absolutely floored by how the suspension worked. I never thought that I could hit the woops that hard and clear every jump on the track on an xcw and feel THAT good! It felt a lot heavier though, and I think the price tag on these things are going to be a stretch.
      So here's my question. I want the lightness of a 125, but want to be able to race both woods and mx. How does a 150 exc/xcw ENGINE compare to an mx 125 engine? I love the 125 engine, but more low end would be usefull in tech areas. But if there is not a huge difference, I will probably just revalve the yz for woods and go tubless, etc. The 150exc's are so expensive and I hardly ever even see one for sale unless its brand new. I don't want the weight of a 200/250/300. I barely touch the ground on a 125....so it has to at least be light. Is it worth waiting for a 150 engine? Thanks guys.
       
       
    • By doolan
      Hi guys I have a 2018 yz250 looking at changing springs front and rear I'm 200 pound ride b class moto.
      Having problems choosing spring weights using the calculators as mx tech say I need .45 fork springs and 5.2 rear but racetech say .48kg forks and 5.8kg rear.
      What are other bigger guys
    • By YamahaYz1
      Hey TT, I know there's endless topics on YZ250/ two stroke tuning in general but I have not been able to find one that helps me get my 09 YZ250 set up for my riding style and where I ride so I'm here asking for help. I got this bike around 1.5 years ago and have not known how to set it up properly. I know no one can give me a perfect set up but I'd like a good starting setup to work from and tweak to perfection. Throughout this time I've had this bike, its always been covering the bottom of the entire silencer and dripping all over the swing arm and rims with oil. Also, the joint between silencer and expansion pipe has oil around it too but I've heard I have to close the gap a little to squeeze the silencer a little tighter and stop the leakage there. I don't have too much experience messing with carbs but I've messed around with previous two strokes and tuned them a little better. I change the oil on my bike after 15-20 hours on it and clean and oil the air filter after almost every ride. The sparkplugs never last me more than a few months but recently have been lasting me no more than a few weeks. With the excess oil from the pipe and spark plug fouling you can see why I need to set my bike up to stop this. 
      The bike:
      Completely stock with V-Force 3 Reeds and a FMF Shorty silencer. 
      Usually running around 32:1 premix with 91 pump fuel because I prefer to keep it well lubricated instead of running rich.
      Riding:
      I ride enduro in Ontario, Canada. So mostly single track and some fire roads but nothing close to MX and not a lot of time wide open. Therefore, probably mostly 1/4-1/2 or 1/4-3/4 throttle range.
      Where I ride:
      Usually 0-500' above sea level. With temperatures ranging from around 15-35 degrees Celsius (55-95 Fahrenheit).
       
      If anyone can give me a setup to go from, that would be very much appreciated. Even multiple set ups for different temperature ranges or any suggestions would be much appreciated. Thanks and sorry for the long read, I just didn't want to leave anything out. Ask any questions if you have any, too.
    • By Goon Rides
      Hey guys... so I have a problem... I took the bike at Cal City, broke it in, and while I was doing so, the bike was bogging in first gear (Yes, first gear). Also it doesn't idle at all. I have to keep blipping it otherwise it'll stall. So I adjusted the screw circled in image 1. Well I was listening to the sound of the of the bike, yet the rpm's weren't increasing, and it was still stalling. So then, I unscrewed the screw in image 2, well I found out that it was fully twisted to the right. So then, I twisted it to the left, still nothing; it was still stalling... where's the idle screw? I am sure that the screw in image 1 is for the air/fuel mixture screw, and the screw in image 2 is the idle screw, just broken? I don't know, any and every help would be appreciated.


×