Stupid power valves.

[Nathan S]

Took my 'only ridden at CD4 since rebuild' YZ125L for a blat on the weekend, to make sure I remembered how to ride it before CRC (I didn't).

Did about three laps and then it ran out of compression. 

Pulled it down tonight, and this is what I found:





The two segements of the power valve came loose, then they hit the piston, forking both bits.
It occoured to me that I've seen a couple of other motors expire for the same reason...
Now, I know to do the allen bolt up tight, and it was f%^king tight - but it's come loose anyhow.

So... How do you keep them tight? Or is it just a matter of very regular checks and re-tightening?

Edit: Head damge is old - look at the top edge of the exhaust port....

[Rosco400]

Ouch, loctite thread locker might of helped, definitely lot cheaper than whats now in front of you

[cyclegod]

Use loctite RED its the one designed for high temperature applications.

[DJRacing]

What size was the piston Nathan? Once ya go over 1mm the powervalve has to be machined. I might have a spare barrel for ya if ya get stuck.

[Lozza]

Go to the RZ/RD forums they should have the answers.IIRC the pins should have no movement to.Looks standard Yamaha stuff.I'd be swapping to the 90-93 pro lite piston to
Hmmm, answered some questions on a up coming RZ 500  job . Geez that could be a fast engine with those piston/head and port combo.
What year model is an L and can you make a rubbing of the cylinder bolt holes please Nathan???Would love to find out if they cylinder/head fits on the 500,case or barrel reed?

[Nathan S]

Lozza, Seeing as The Institute won the repair contract, I was planning on bringing the barrel and new sleeve to Buladelah anyhow... Along with a CR125 barrel and sleeve and CR125 crank and rod kit, and AT1 barrel and... /
L = 1984, barrel reed.

DJR, it's +2.00mm - so I figure the PV had already been machined.

[Lozza]

Look forward to mid-low 30's and about 15.5:1 comp , might be able to hook up some electronic gizmos to that PV to,moves from a servo motor I assume.I know your Kangi might not be the best but I assume you can read a graph.Nice TZ too

http://perso.orange.fr/arzhdata/ss_07.pdf

[DJRacing]

If you venture over here Nathan I'll introduce ya to a guy who can make the 'L' motor rev out like it should have from the factory. He rode an L in the series and won. He had to race against an '85 CR125 mugen. He'll give ya some great tips on the motor.

[fatboyracing]

Hi Nathan,
Sadly that is a commen fault I had a 125 that did this not once not twice but three times , it didn't matter what lock tight I used it just seemed to come undone and shag the piston, I even brought a brand new piston and power valve at the cost of $600.00 only for it to happen again. I sold the stupid bike in the end because of this problem.

Im a great help ah

Cheers
Fatboy

[yzhilly]

Just wondering if anyone has tried modifying the powervalve to run it fixed in position, or limiting it's travel . I've got 2 i'm doing up both have had the same failure so i am looking for a low budget solution . I have a engine that i am willing to experiment with as someone has already butchered the powervalve.   Any Ideas ??. 

[Lozza]

Hilly the powervalve is there for a very good reason,that is they work and work very well.Considering the same system runs perfectly in many RZ 250's 350's and 500's not to mention the Banshee variant the Cheetah.All of which suffer from none of the same fete as the YZ.Question is what are/are not you blokes doing that the RZ hordes are.I have installed a few aftermarket Zeeltronic RZ PV controllers with fantastic results.Believe me the PV aint the problem.You could run a small battery and the PPV and your world will never be the same again.

My guess is they may not be set correctly to begin with,the pins have developed movement and or lack of regular maintenence.
Lookng at them I reckon an RZ 250 PV would got very close to fitting in there.Take your down to a wrecker and compare.

[Nathan S]

I own four PV'd YZ motors. They're all damaged in some way now...

In two of them, the halves of the PV came loose and destroyed themselves on the piston.
Two of them sheared off the drive bit off the PV barrel.
The last one has been run with the acutator loose on the PV barrel, which has rounded off the rectangular drive - it's still useable, but I'm worried that it's on its way to the sheaing it off completely.

(And yes, that is five PV faults across four engines...).

They're all mechanically driven, unlike the much-preferable servo motor system on DTs and RZs.

[Lozza]

Ah mechanicaly driven eh well I take some of that back and say the drive mechanism must be shit, any chance of a pic/fische of the driver? Must easily go out of alignment.Ideal solution is rigging a servo of a DT/RZ with a PPV controller.

[yzhilly]

 I agree they will only crunch the piston when something comes adrift, thats what has happened to my ones .Im going to try a few things with my spare engine as it is only a dunger the powervalve is held together ok but the end where the link attaches is rooted. I was going to machine the stud & drill and tap it to reattach the link or i could use my favourite a 2.5mm weldall to keep it on the valve and loctite the nut.

[Nathan S]

Let's see if this works...

Drive mech:


PV mech:


Looks like a pretty generic set-up to me. The YZ-Js seem to have a weak PV barrel and when they get jammed up with carbon in the hands of paddock racers, they shear off the end of the barrel.