kx125412

Member
Mar 30, 2006
341
0
I tore my top end down because I was getting piston slap, but I noticed the tiniest bit of rubbing on both the powervalve and piston. Not really anything on the skirt but up around the rings the piston has grooves all the around, its hard to explain theyre factory not from dirt and stuff, and theyre worn a little flat. There's also a rubbing mark on the powervalve. Does anyone have pictures of their powervalves rubbing their pistons? I'll get some pictures up tomorrow if I can.
 

kx125412

Member
Mar 30, 2006
341
0
Thats kinda what mine looks like. But theres wear on the middle of the powervalve right on the edge. Did your's have that? I still havent taken pictures but i'll try and take some and post them tonight. Both our bikes have the same drum valve.
Ok here are the pictures
Picture162.jpg
You can see where the wear mark is
Picture158.jpg
Its kinda of hard to see but you can see those ridges are worn a little flat and theres a little bit of scraping going on but not much. This piston was seized but not by me. The retarded last owner just put new rings in I guess.
Picture160.jpg
And here you can see where the platings worn through.
 
Last edited:

steve.emma

Member
Oct 21, 2002
285
0
hi, i used to have a 89 yz250, last year with the steel bore great bikes.... anyway this problem used to be fairly common and Eric Gorr talks about it in his books, basically the valve has a stop plate on the l/h side of the cylinder that limits the valve from turning too far when it moves. however the stop plate is made of alloy and over time it wears and allows the valve to spin further than it should. after a while it can turn so far that it starts to hit the piston rings and makes a ticking sound, when its real bad it can cause the rings to snap and make a real mess....
easy to repair however, replace the stop plate and file the valve to give a little more clearence. check this by turning the valve to full travel while the head is off looking down the bore as the piston passes the exhaust port.
hope this helps. :cool:
 

kx125412

Member
Mar 30, 2006
341
0
To me it looks like my cylinders plated, like in the picture you can see that whole wall is starting to wear through and a couple spots there's bare aluminum. The stop plate hardly looks worn at all really but the tab on the powervalve itself is pretty worn. I just don't have the money to replace the whole powervalve on top of the replate, piston kit etc, its already going to cost me over 500 and I'll be into the bike for $2200. Could I get away with just grinding off the powervalve a bit? I wouldn't notice any decrease in power.
 

damac

Member
Nov 14, 2006
7
0
where is this stop plate people are referencing to in the picture below? this happened to me on my last ride on an 89 yz125 and although i have a clymer manual, im new to bikes and being a mechanic and i don't see this plate that can be replaced on the oem parts list?




something happened during my last ride, the power valve cover and left side piece actually shot out at my leg a few minutes after takeoff halway into the day! the thing was hot as hell even after minutes on the ground and with gloves on. no spooge or oil on that sucker at all and it was smoking on the side of the engine.

sadly i lost the power valve cover, havent had luck with **** darnit :( i lost the screw that attaches the power valves as well. i looked hard, damned grass :)

anyways my piston has a chip up top, surface scratches, and the power valve is grinded away like yours.

my bike was running like crap, like there was a hit still but you had to rev the heck out of it to get past some abnormal bogging. cant say i noticed the difference in the sounds while riding, but something i did caused it to blow like that? i dont know what happened with mine but to lose both cover screws then the valve screw is kind of wierd.
 

steve.emma

Member
Oct 21, 2002
285
0
To me it looks like my cylinders plated, like in the picture you can see that whole wall is starting to wear through and a couple spots there's bare aluminum.

lol, no this is a steel bore. you can see the thick sleeve where the ports come through to the cylinder, you can see where the alloy barrell meets the steel sleeve. this is great on these bikes because if you seize or scratch the bore you can just have the cylinder bored out to the next oversize and buy a wiseco piston to suit. the factory sleeves are very thick and can be bored out many times with no problems AS LONG AS YOU CHECK THE POWERVALVE CLEARANCE.


Could I get away with just grinding off the powervalve a bit? I wouldn't notice any decrease in power.

yes you could but if the tab is very worn then you could also have someone build up the powervlave tab by tig welding it and then filling it to the correct size.


where is this stop plate people are referencing to in the picture below? this happened to me on my last ride on an 89 yz125 and although i have a clymer manual, im new to bikes and being a mechanic and i don't see this plate that can be replaced on the oem parts list?

sorry mate i dont see any picture. but on the yz250 its a small round plate about 50mm diameter with 2 bolts holding it to the cylinder on the l/h side.
dont know why yours would have blown apart like that, sounds like your bike is possessed!! :yikes:
 

kx125412

Member
Mar 30, 2006
341
0
I went outside and stuck a magnet to the bore and it stuck. So it is an iron bore, and I'm happy I don't have to shell out 300 for a replate.
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
Well, since your bike is sleeved, you should move up to the next oversize that will restore the condition of the bore, either .25mm or .5mm. You will have to get the cylinder bored, and during the proccess, have the power valve clearance set. Don't forget to chamfer the edges of the ports before reassembling.
 

damac

Member
Nov 14, 2006
7
0
im still glad i came across this thread since i have an 89 yz 250 to service. i got this thing from a guy on craigslist for cheap but im trying to maintain it. i had not even looked over this bike much because it has ran like a tank, and im having a major issue every damn time i run the 125 and have learned not to touch things if you don't have to :)


anyways for those that care the 89 125 and 250 do not share the same power valve parts and if you look at the oem parts list they aren't even designed the same way!

on the 125 there is no pin in the cover and no tab on the power valve. the cover just seats on the left side into the circular groove on the power valve end.


this makes me wonder though, on each bike, what is actually allowing the power valve assembly to be rotated too far to begin with? during normal operation should that ever happen even if you have the throttle pinned?

i dont get why the 250 would have that safety stop and not the 125. on mine i guess if something happened on the governor side like slack or breakage, there would be nothing at that point from stopping the power valve from freely spinning.
 

kx125412

Member
Mar 30, 2006
341
0
On the powervalve itself, on the left side, theres one single tab. On the left hand cover theres two tabs; the powervavle tab goes in between the two tabs on the cover. When the powervalve opens the tab on the cover stops it from rotating too far and when it shuts it hits the the other tab and doesnt allow the powervalve to go into the cylinder. Over time the powervalve tab and the tabs on the cover wear out allowing the powervalve to move too much either way and it will eventually peek out into the exhaust port and snag your piston or rings. In my case the powervalve tab is fairly worn but the stopper plate looks ok. I'll be able to get away with just grinding a tiny bit off the powervalve since the rub on it was very minimal. I will try and take some pictures of it soon. By the way how much did you pay for your 250?
 

damac

Member
Nov 14, 2006
7
0
it was 700 in san jose,ca. ok tires, the pipe was kind of bent and punctured around a mount, but i was able to get that welded for pretty cheap. has power still, seems to run fine, haven't dared to open anything up because of this :) but i dont see any harm in me taking the power valve cover off and peeking at things now that i came across your thread :)

on our 250 there is not one hint of spooge where the power valve is, seems kind of odd based on what i have been reading?


i think im just going to reinstall my setup as is on the 125, loctite the bolt holding the power valve assembly together and running it again. this is sad to say but i think the setup was actually installed backwords when it was ran last :( there were no power valve scores when i first got the thing, just tons of buildup which 10 coats of oven cleaner seemed to help with. i must have done something really stupid when i did the top end in the dark :(
 

kx125412

Member
Mar 30, 2006
341
0
I paid 1500 for mine, it needed brakes, sprockets and now the top end. Mine had a little bit of splooge not tons though. If you run it take the right hand cover off not the left cause the left is the powervalve stop the right hand is the linkage. Then you can if its operating normally. Then take the left and look at the stop and powervalve tab and see how much its squished. These powervalves are so incredibly simple to take apart and maintain its nice. Take the left hand cover off your 250 and thats what the 125 should have. The stops on the cover and the tab on the powervalve.
 

damac

Member
Nov 14, 2006
7
0
a friend found a power valve cover for me, and i will get it next week so i took apart the 125.

i think im lucky since i dont ride hard, but there are some slight scratches on the piston, i cant see or feel any on the clyinder wall.

the clyinder head on this thing looks beat up though, tons of dings and it just isn't straight across, you can see that just by looking. i have a good flat one coming and was going to throw it on their anyway.

on that note i was wondering if that could be the cause of some slight black buildup that i was able to clean off, and the topmost edge of the piston being wore away a bit?

i have atleast 50 hours on this piston since i brought this bike back to life, mostly just putting around.

i took a dremel to the whole surface of the power valve today with one of those sanding bits. didn't take long and i took my time and didn't go too deep but i got it smooth and now it doesn't hit whether its resting, or pushed around the other way.

but again just to note, the 89 yz125 and 250 have the same kind of power valve layout, but there is no stopped on the 125. i had everything apart with the yamaha schematics list and the clymer. so if my assembly broke free from the governor, it could spin whichever way and possibly rub?


i actually got mine in the right position this time, there is a little hole on the right side of the power valve that should go up when installed to the governor arm, lol. i used the grease like it said to and everything moves freely.

one question i have is i used loctite on the main bolt even though it said to just use some grease and torque. is this going to cause any problems? im so scared about what happened last time.
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
Are they little pits? That could be detonation. If they look like imprints of bearing needles, it could be bearing needles from either con-rod bearing. Inspect them boat carfully.
 

kx125412

Member
Mar 30, 2006
341
0
Little pits sounds like your main or rod bearings are toast. It happened to me on my ol RM125 before she gave up. I got one last drag race out of it and then it blew, I smoked this kid on his 250 mind you the 250 was a 78.
Anyways, the powervalve should only rotate a quarter of a turn. That's all these YZ's need to open the valve fully. For the valve to open to far it has to do almost a 360 degree turn to come back into the exhaust port but when it closes it can come into contact with the piston quick if its coming to far in. Unhook the governer and just turn the valve by hand it should only move a quarter of a turn. I'll get pictures up later today on what the tabs look like. Its much easier to see how the exhaust valve works with the jug off.
 

kx125412

Member
Mar 30, 2006
341
0
Here are the pictures:

Left side of powervalve w/tab
Picture163.jpg


Left side stopper/cover, the powervalve tab goes in between these two stops.
Picture164.jpg


Powervalve in closed position. Notice the ridge above the rubbing mark, the powervalves closing too much and causing the rub. I will need to fix the tab on the powervalve because its worn the most. You should only see the flat part of the powervalve.
Picture165.jpg
 

steve.emma

Member
Oct 21, 2002
285
0
hey those are great photos! makes it very clear for people who are not sure what this problem is all about.
one other thing i found on my old yz that contributed to powervalve problems in general is that the bronze bush on the stop plate and the other bronze bush on the right hand side carrier plate wear over time and cause the valve to slop around a bit.
the right side bush play also causes the oil seal to leak and fill the powervalve linkage cover up with thick black spooge!!
 

kx125412

Member
Mar 30, 2006
341
0
Quite a bit would have to be ground away to make the proper clearance. I work for a mechanic and he's friends with the guys at the machine shop. Im going to get them to weld a little piece onto the powervalve tab and file it down as need to get the powervalve to close properly and not go to far. I might end up grinding a little bit off but not much, there's not alot of meat on it.
 

damac

Member
Nov 14, 2006
7
0
took me a while but i got the cover for my 89 yz125 and went to put everything together today and im thinking there is something wrong still.

if i have read correctly with everything installed except the right side power valve cover, i should be able to rev the bike on the stand and watch the power valve assembly turn clockwise a quarter turn?

i have everything cleaned up on mine and i can spin the power valve assembly with no notches by hand, i can even move the governor arm by hand and it snaps back to stock position.

now i had cleaned up my power valve and assumed before i installed everything backwords but now im wondering if i actually did that because i noticed what i used to think was some sort of normal "pinging" at idle/low throttle, and what is actually happening to the governor arm with the cover off. the thing is rattling back and forth with every ping and if you put your hand on it you can feel the thing jolt with every ping, kind of like its hitting something? if i manually then push the arm to the right just a bit those direct bangs that you can feel go away and its less violent to the hand.

i wonder if the governor assembly itself and the arm could possibly have worn and mishaped as well over all these years causing the power valve to sit too much turned to the left?


anyways i thought i was ok because when i kicked the bike over by hand i seemed to just be clearing, didnt feel any resistance. perhaps something changed when the bike spins fast and gets up to temperature?

if so can i just grind away a bit more without losing any performance?

i ground out the old line when i cleaned it, i guess i did not do a good enough job.


kind of wierd, i have run this bike fine as far as the engine and the hit were concerned, then it started acting up after i cleaned it all up, lol.
 

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