YZ exhaust cam timing = bent valves!?


30 replies to this topic
  • TallBoysWRF

Posted June 06, 2004 - 09:59 AM

#1

Has any body had a problem with bending their exhaust valves on a WR400 with the exhaust cam timing set to YZ spec? I lost compressoion in my 99 WR and tore the top end appart to find out that one of the exhause valves is bent. I had it set to YZ spec and I'm wondering if this could be partly to blame. I think I probably over-reved it once or twice when I missed a gear so I think this also has something to do with the problem. Would this be less likely to happen again if I go back to stock WR spec? Any thoughts?

  • Rich_Rohrich

Posted June 06, 2004 - 02:31 PM

#2

Would this be less likely to happen again if I go back to stock WR spec? Any thoughts?


Yes it would be less likely because with WR timing you are closing the exhaust valve nearly 23 degrees sooner, so the valve insn't open as far when the piston is near TDC during overlap.

It's pretty easy to understand if you think about where the valves are in relationship to the piston, and why it's most likely to happen to the exhaust valves.

The piston sort of chases the exhaust valve near Top Dead Center (TDC) on it's closing cycle.
The intake is just starting to open and the exhaust is closing when the piston is near TDC. The piston is moving up as it approaches TDC while the exhaust valve is trying to get back on it's seat . On cams with reasonable amounts of overlap the exhaust valve is still pretty far open as the piston is hitting TDC. If you over-rev the engine the cam bucket can lose contact with the camshaft. When this happens it is no longer following the cam profile so the exhaust valve can easily be open too much when the piston is near TDC. If it's open more than the available valve relief clearance in the piston, then the two make contact and parts get bent. This is usually what people are referring to when they describe valve float or loft. As you reduce the overlap by advancing the exhaust cam timing (aka WR timing) you lessen the possibility of this happening but trade a lot of performance as a result.

Valve float normally happens at high rpm, but as valve springs wear and lose some of their open pressure the rpm at which this "float" can occur much earlier. As your valve springs get old they don't have a lot of extra valve spring pressure to work with in a big over-rev situation so there tends to be less of a margin for error. Rev limiters will usually protect you from this situation but down shifting under load at high rpm will mechanically drive the rpm well past the normal rev limit and can easily get those valves floating around.

Valve clearances that are too loose can render the cam's closing ramp ineffective and cause seat bounce which can also damage the valves.

The fix is probably pretty simple. Change the valve springs
when you change the valves and leave the YZ timing. If you don't mechanically drive it past the rev limiter there should be no reason to ever bend a valve. One other thing to look for is valve stem to guide clearance when you have the cylinder head apart. As bikes get older oil deposits can form that will hang the valves open every once in a while. If it hangs far enough the valve can easily contact the piston crown. Running a proper sized ream through the valve guide and then checking the clearance with a small hole guage and a micrometer will give you a good read on this.

:thumbsup:

  • TallBoysWRF

Posted June 06, 2004 - 04:23 PM

#3

Thanks. That helps a ton. :thumbsup:

  • Hamish

Posted June 07, 2004 - 02:14 AM

#4

when I YZ timed my WR I checked piston to valve clearance. It was over 40 thou (1mm) ..which is ample.

Hamish

  • Rich_Rohrich

Posted June 07, 2004 - 02:00 PM

#5

when I YZ timed my WR I checked piston to valve clearance. It was over 40 thou (1mm) ..which is ample.

Hamish




General rule of thumb is to run double the intake piston to valve on the exhaust side. In others words 1mm clearance can be adequate on the intake but it would require 2mm on the exhaust.

The compressed width of the head gasket is .035" and most WRs have a squish clearance between .060" - .080" , that along with the valve cutouts should provide more than enough piston to valve clearance with a stock height barrel and OEM piston.

  • jwriott

Posted June 08, 2004 - 04:38 AM

#6

So are you guys leading up to the fact that YZ timing requires more accurate valve adjustment than WR timing? By this I mean there is less room for out of spec valves if you are YZ timed?

  • Indy_WR450

Posted June 08, 2004 - 05:20 AM

#7

I believe this to be true. :thumbsup:

  • jwriott

Posted June 08, 2004 - 06:24 AM

#8

The reason I asked is that it seems your valves would have to be out of spec for damage to occur. I checked mine after I changed to YZ timing.

  • Rich_Rohrich

Posted June 08, 2004 - 07:09 AM

#9

By this I mean there is less room for out of spec valves if you are YZ timed?




I haven't found that to be the case unless they are wildly out of spec. If that's the case you're going to beat up the valvetrain regardless of how it is timed.

If you are using OEM compoinents valve contact with the piston will usually only occur from over-rev, or worn springs, or worn/malfunctioning cam chain tensioner.

When you start changing pistons and cams you need to measure and verify that you have correct piston to valve clearance as well as valve to piston crown cutout side clearance. The cutout to valve side clearance on aftermarket pistons can get people in trouble because they rarely verify it when checking piston to valve clearance.

  • Hamish

Posted June 08, 2004 - 10:43 AM

#10

So are you guys leading up to the fact that YZ timing requires more accurate valve adjustment than WR timing? By this I mean there is less room for out of spec valves if you are YZ timed?



no, thats not the case. If you have too much valve clearance the valve will actually lift LESS and give you MORE piston to valve clearance. You would have to have almost 1mm of NEGATIVE clearance to cause your problem. I suspect that either
a) you stuffed up the valve timing. (check it with a piston stop and degree wheel - YZ timing is 110 on the inlet and 101.5 on the Exhaust)
:thumbsup: a shim wasnt seated properly and came dislodged during camshaft installation. If you checked the specs after installing the cams forget about this possibility.
c) a big end or little end bearing has partially collapsed

hope this helps
Hamish

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  • TallBoysWRF

Posted June 08, 2004 - 10:59 AM

#11

From what you guys are saying, I think my problem is just due to over-rev. I have had a couple times on the trail where I was shifting from 1st to 2nd and did not get it completely into gear, so it was partially in but slipped back into neutral when I rolled on the throttle. I have run the bike in YZ spec for about 6 months with out a hitch. I have been keeping up on my valve clearance too.
Thanks for all the great info, keep it up :thumbsup: :awww: :lol:

  • Rich_Rohrich

Posted June 08, 2004 - 05:49 PM

#12


no, thats not the case. If you have too much valve clearance the valve will actually lift LESS and give you MORE piston to valve clearance. You would have to have almost 1mm of NEGATIVE clearance to cause your problem. I suspect that either




While you will change the net lift with a loose clearance the bigger issue is what it can do to the overall cam dynamics. If you set either valve loose enough can skip the gentle opening and closing ramps built into the cam which will radically change the valve acceleration and resultant force that the valve sees. Valve bounce on the exhaust side is a very real possibility and the close proximity of the piston makes it a likely candidate for contact. Add in an over rev situation and the whole thing just gets more complicated and likely to cause a failure.
By way of example here is what the change in resultant force looks like on a cam far less radical than the YZF with an exhaust valve that was just loose enough to skip the closing ramp.

Posted Image

and the same setup below with the proper lash set so it will open and close the valve more gently with the open/closing ramps. If you look closely you'll notice the tiny blips in force at the begining and end where the ramp eases the valve open and closed, where the graph above slams it open and just dumps it back on the seat. These type of forces seem to be common in 250Fs that break the valves heads off.

Hopefully this will help clarify some of what I was talking about earlier. I have a bad habit of over-explaining and making the whole thing muddy. :thumbsup:

Posted Image

  • F6dood

Posted June 08, 2004 - 06:06 PM

#13

Rich, Outstanding explanation and illustration. Could not have been said better.

  • Hamish

Posted June 09, 2004 - 01:34 AM

#14

As always, Rich is right.... but I do disagree on the valve float point.I'm no engineer so I can only use the experience I have gained in the 10 odd years I've been building engines for myself. In my opinion, valve float is a determined by valve spring pressure at full lift(less clearance) , valve/collet/retainer/spring weight and RPM. During the last 20 thou of lift I doubt the valve spring pressure would change very much.
Not trying to turn this into a big technical forum, the fact of the matter is that if valves were bent after doing YZ timing something was probably done incorrectly. If anyone is intresed in learning how to dial cams in properly, send me a PM and I'll write a page on the subject.

Hamish

  • jwriott

Posted June 09, 2004 - 03:11 AM

#15

I was actually referring to there being too little clearance. My valves tightened up and needed adjustment.

The reason I was asking these questions is that I didn't understand how YZ timing would cause this. Over rev with valve float or out of spec valves made more sense to me.

  • Rich_Rohrich

Posted June 09, 2004 - 04:38 AM

#16

Not trying to turn this into a big technical forum,


Why not? That's half the fun :lol:

Along with the points Hamish mentioned you also have to consider the cam profile (aka dynamics) when determining if or when valve float will occur. There are values for valve acceleration and valve velocity that are considered in conjunction with the mass of the moving parts and in some cases the rocker ratio (like the CRFs). Make the parts lighter and you can increase the valve acceleration before float, same thing goes for spring pressure. The cam desigener has to take all this and countless other complex relationships into consideration when designing a cam. The trend as of late is to use light parts and increase the valve acceleration to the highest values possible to get the valve off the seat and out of the way ASAP. If you can safely get the valve opened and closed quick you can run less cam duration and still get the air flow you need. Good airflow with shorter duration cams makes for powerful engines with nice wide powerbands. The holy grail for engine builders. :thumbsup:
The problem I've seen with the YZF/WR engines is the springs lose tension over time so the rpm that the valves float at gets lower over time. Valves springs have a finite life, but they don't seem to get changed regularly by the weekend racer. This can lead to other issues as we have seen. The current crop of engines like the YZF450 and the CRFs barely have enough spring force when new to adequately control the valve at high rpm, so it's worthwhile to consider that as you service these engines, especially with the 250Fs. :awww:

  • TallBoysWRF

Posted June 09, 2004 - 08:03 AM

#17

The problem I've seen with the YZF/WR engines is the springs loose tension over time so the rpm that the valves float at gets lower over time. Valves springs have a finite life, but they don't seem to get changed regularly by the weekend racer.



How is the spring tension tested? Will they measure out to be a tiny bit shorter, or can they still be the proper length but too weak? What experience I have with springs tells me they will be a shade shorter but I would like to confirm this with you guys. :thumbsup:

  • TallBoysWRF

Posted June 09, 2004 - 08:05 AM

#18

Rich
Great stuff :thumbsup:

  • Rich_Rohrich

Posted June 09, 2004 - 08:11 AM

#19

How is the spring tension tested? Will they measure out to be a tiny bit shorter, or can they still be the proper length but too weak? What experience I have with springs tells me they will be a shade shorter but I would like to confirm this with you guys. :thumbsup:


Yamaha lists a spec for free length that has proven pretty reliable. You can measure the spring pressures but if you ask 20 engine builders you'll get at least 19 different answers as to what the proper seat and open pressures should be. :lol: Usually if you are installing an aftermarket cam, or aftermarket valves and springs you'll be provided with an installed height, open and seat pressure, but for the OEM cam the free length spec from Yamaha should serve you well. :awww:

  • Taffy

Posted June 09, 2004 - 10:19 AM

#20

[@#$%&*!]!

how i miss it. i can honestly say i miss the pure piffle that comes out here!

no diagrams, no graphs, it's dead simple!

if you delay the cam timing you make the exhaust valve open after the piston has come and gone and it won't hit the piston on the overlap stroke either as the timing is too mild. at the very most the piston can only chase the valves home.

YZ timing the exhaust cam therefore makes the cam timing safer!!!!

one must therefore stop blaming resonances, the price of fish, the iraqi conflict and say "old mate, you got your cam timing wrong now go and try again!"

Taffy




 
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