Power Valve Borken AGAIN!

[cookiemonsterMoA]

Well this is my 3rd on in 110 hours. Well the original one lasted 8 years( 06 YZ250). Last time i did a top end I decided to put a new one in because it was looking worn. That one lasted 24.6 hours. Installed another new one and that lasted 41.2 hour's so I installed the original one back in since it lasted 8 years and was hoping to get another 8 out of it. I took the top end off over the weekend and noticed that one was broken. It was in for 44.4 hours but do not know when it broke since I did not have any symptoms this time. The last time I did the top end 110 hours  ago the only things different were new crank, had the head cut. These 2 things should not cause any power valve problems. I also had the cylinder re-plated. I cant really see that causing the problem.

 

So what do I do. Buy another one, keep one of the broken ones in or buy a different brand bike because Im tired of replacing them. I know this subject has ben posted a lot over the years and even once by me but I never got or found the real answer as for why did Yamaha change the design of the power valve in 2002-2003, what is the tab really for and what is causing it to break. I really hope someone knows a real solution to this. Maybe I will buy a 2001 valve since it does not have this problem.

 

 

[Rosehill Ryder]

Well this is my 3rd on in 110 hours. Well the original one lasted 8 years( 06 YZ250). Last time i did a top end I decided to put a new one in because it was looking worn. That one lasted 24.6 hours. Installed another new one and that lasted 41.2 hour's so I installed the original one back in since it lasted 8 years and was hoping to get another 8 out of it. I took the top end off over the weekend and noticed that one was broken. It was in for 44.4 hours but do not know when it broke since I did not have any symptoms this time. The last time I did the top end 110 hours  ago the only things different were new crank, had the head cut. These 2 things should not cause any power valve problems. I also had the cylinder re-plated. I cant really see that causing the problem.

 

So what do I do. Buy another one, keep one of the broken ones in or buy a different brand bike because Im tired of replacing them. I know this subject has ben posted a lot over the years and even once by me but I never got or found the real answer as for why did Yamaha change the design of the power valve in 2002-2003, what is the tab really for and what is causing it to break. I really hope someone knows a real solution to this. Maybe I will buy a 2001 valve since it does not have this problem.

I've been running mine that way . The only problem I had before was no low-end  , but the broken piece was not letting the PV close all the way . Plus its not a cheap part . I remember it being pricey ! More opinions please .

[mlcompound]

It is just a stopper. I would drill and tap it for a small bolt to install at the same height and locknut or locktite it in there.

[Sycamore]

I dont want to hear this kind of stuff.  

[E36M3E]

Isn't the PV cover directional because of where the pv stopper is supposed to contact. Seems like ive heard of guys installing a small bolt or screw in the cover to act as a stopper.

That being said I've had 2 different cylinders and pvs overhauled by EG and they both came back with no stopper tab on them. And I don't have any other form of stopper installed.

[cookiemonsterMoA]

From what I do know is it is not a stopper. If it was a stopper there would be markings on the top of the valve and on the back of the cover. From reading the posts on this it looks like it might be to keep the valve from fluttering but not sure.

And not to be a jerk but please no responds about how you use it broken or to use a screw in its place. Im trying to find the real problem. I might go with the 2001 power valve since it was a longer piece and did not brake.

[numroe]

I remember once looking at mine (not broken) and I had a really good theory on why it existed. But I cannot remember now. ? It'll probably come back to me when I have another look. Which wont be a for a couple of weeks sorry. Perhaps something to do with forcing the valve closed when the governor releases.

In your cyl casting, is the 8.1mm hole in the alum for the PV main shaft worn (out of round) on the right side? Shaft alignment problems wouldn't help.

Are you certain your whole PV mechanism is functioning with no binding or surplus friction? The "pulley guides" on each side valve pin are held in position on the shaft by a screw, but you can still align those parts for smooth operation before tightening those screws.

I suspect it gets broken during flutter of the main valve due to exhaust gas pulses. Perhaps more of an issue on the 295cc bore. Just speculating.

[Snider]

My 2002 YZ250 PV doesnt have that tab.. Although my first PV broke in a different place...

.

[cookiemonsterMoA]

I remember once looking at mine (not broken) and I had a really good theory on why it existed. But I cannot remember now. ? It'll probably come back to me when I have another look. Which wont be a for a couple of weeks sorry. Perhaps something to do with forcing the valve closed when the governor releases.

In your cyl casting, is the 8.1mm hole in the alum for the PV main shaft worn (out of round) on the right side? Shaft alignment problems wouldn't help.

Are you certain your whole PV mechanism is functioning with no binding or surplus friction? The "pulley guides" on each side valve pin are held in position on the shaft by a screw, but you can still align those parts for smooth operation before tightening those screws.

I suspect it gets broken during flutter of the main valve due to exhaust gas pulses. Perhaps more of an issue on the 295cc bore. Just speculating.

It might have to do with helping it close but if you slowly move the PV with you hand the does not touch or very lightly touch the valve. The hole is perfect and the PV system moved smoothly when turned by hand, no binding or sticking at all.

In 2001 the valve did not have that tab and did not have part # 29. Part # 29 was added in 2002 but I think the tab on the PV was not until 2004. From looking at all the part numbers part # 30 is the same from 2001 to 2016 and all the other PV parts are the same as well. So I might just install the 2001 valve and remove part # 29 and be done with it. I just would like to know why Yamaha went to the new setup. I have asked a dealer and they had no idea and I also called Yamaha and they were no help at all. They just told me to see a dealer.

 

 

My 2002 YZ250 PV doesnt have that tab.. Although my first PV broke in a different place...

.

I seen that kink of break before on the web but I don't think it is as common.

[numroe]

It might have to do with helping it close but if you slowly move the PV with you hand the does not touch or very lightly touch the valve. The hole is perfect and the PV system moved smoothly when turned by hand, no binding or sticking at all.

Maybe the end tab is there to do something when the valve opens too far. Perhaps the valve can be pushed open by an exhaust pulse. What stops it from going beyond the normal range of motion? Maybe this tab. With no tab, then there are only the arm springs. Those springs will then fire the valve back inward. Perhaps into the closed position. If the close operation is too violent then those half-round shoulder pieces could eventually break off.

What material is the 2001 main slide valve made of? Maybe it is steel. A heavier valve would not flutter anywhere nearly as bad.

[Snider]

Maybe the end tab is there to do something when the valve opens too far. Perhaps the valve can be pushed open by an exhaust pulse. What stops it from going beyond the normal range of motion? Maybe this tab. With no tab, then there are only the arm springs. Those springs will then fire the valve back inward. Perhaps into the closed position. If the close operation is too violent then those half-round shoulder pieces could eventually break off.

What material is the 2001 main slide valve made of? Maybe it is steel. A heavier valve would not flutter anywhere nearly as bad.

The 2001 is the same as the 2002 I think... The main slide valve is a nonferrous alloy, very light. the bushing it rides on is steel as well as the pin

[cookiemonsterMoA]

Maybe the end tab is there to do something when the valve opens too far. Perhaps the valve can be pushed open by an exhaust pulse. What stops it from going beyond the normal range of motion? Maybe this tab. With no tab, then there are only the arm springs. Those springs will then fire the valve back inward. Perhaps into the closed position. If the close operation is too violent then those half-round shoulder pieces could eventually break off.

What material is the 2001 main slide valve made of? Maybe it is steel. A heavier valve would not flutter anywhere nearly as bad.

I don't think that tab is there for that reason. If you look at my pic the plate that I have outlined in green sits in a little grove in the cylinder and is flush with it. Then #9 in the above diagram bolts to the cylinder and holds it in. When the power valve moves the part outlined in black hits that plate and stops it.

 

The 2001 is the same as the 2002 I think... The main slide valve is a nonferrous alloy, very light. the bushing it rides on is steel as well as the pin

From 1999 - 2001 are the same part number, 2002 - 2003 are a different number and then 2004 - 2016 are another number. I think 2004 is when they went to this design but not 100% sure.

The pin is not steel. The blue outlined parts are one piece. What it looks like they did was take the old design and lathe the end down to make the pin and then press the orange piece onto it. The green, red and orange pieces are metal (don't know what kind but are magnetic). The blue and black(witch are all one piece) are alloy. The valve does not ride in a bushing unless you are calling the green piece a bushing.

 

So at this point im not sure what im going to do. Either use it as is, buy another new one and see what happens, use my good spar cylinder and a new valve and see what happens(maybe when it got replayed something is not right that I cant see or feel), use the 2001 design, buy a Athena 300 kit or buy a KTM.

 

 

 

Edited November 13, 2015 by cookiemonsterMoA

[numroe]

If you remove the actuator arm cover on the right side, then reinstall the pipe, start the motor and warm it up gently then give it some higher RPMS blips, do you see any flutter in the PV shaft rotation? I'm wondering if your governor spring is sacked out.

How much up/down play do you have when you wobble the front end of the PV slide valve up and down? I'm wondering if the cavity in your cylinder is worn beyond the service limit.

When I get home in a couple if weeks and look at it again, I might remember my theory about what purpose the tab has.

[cookiemonsterMoA]

If you remove the actuator arm cover on the right side, then reinstall the pipe, start the motor and warm it up gently then give it some higher RPMS blips, do you see any flutter in the PV shaft rotation? I'm wondering if your governor spring is sacked out.

How much up/down play do you have when you wobble the front end of the PV slide valve up and down? I'm wondering if the cavity in your cylinder is worn beyond the service limit.

When I get home in a couple if weeks and look at it again, I might remember my theory about what purpose the tab has.

Great questions and good things to check. You know how it is sometimes you really think about things and you forget the other half of their system. Well right now the engine is on the bench and cylinder is off. Maybe ill just pop it back together and slap it in the frame and try it. As for wobbling the PV when I did the last top end I checked it and it was fine but I will double check it again. Ill keep you posted.

BTW I hope its not the governor spring because its under $3 and that would suck that it causes a $200 part to break.

[Kinger317]

Check the Governor Spring while it's on the bench. I would hope it was, it would be nice to know the reason for the failures.

[cookiemonsterMoA]

Check the Governor Spring while it's on the bench. I would hope it was, it would be nice to know the reason for the failures.

There is no speech for the spring( i.e., spring height or strength). when I pill on the rod that goes to the PV it movies freely but has good resistance. When I get the motor back together and running I will check it how numroe said to. Ill keep everyone posted.

 

BTW, I know it cant be done bit will the bike fun (on stand of course) with the front PV cover off so I can watch things? I don't think it can because it needs to be sealed or the engine will suck air. But I figured I would ask.

[Sycamore]

I have no idea about the open pv cover.  But curious how you made out?

[cookiemonsterMoA]

Well I ran the bike with the side cover off to watch the linkage and it looks very smooth. I did not see any flutter in the PV shaft at all. So the next time I have to do a water pump seal on it I will change the governor spring and buy a new power valve and see what happens. For now im using it with the top piece broken off. The bike is running fine. I talked to a local shop that's just moved to my area (they have been around for years) and he said they see them all the time and do not know whey they brake either.

 

If I find anything out I will post back with my findings.

[Sycamore]

Interesting.  Maybe yammie should redesign their PV?  This time make it adjustable?  You mentioned going to ktm, there PV setup is stellar for sure but after an amazing ride yesterday on my YZ I dont think I could give up that SSS.  lol

[cookiemonsterMoA]

Interesting.  Maybe yammie should redesign their PV?  This time make it adjustable?  You mentioned going to ktm, there PV setup is stellar for sure but after an amazing ride yesterday on my YZ I dont think I could give up that SSS.  lol

Im not going to KTM I was just pissed about the valve breaking for a 3rd time in 1 year. I will figure it out one day,LOL