Excess valve clearance

[philward]

Hi guys, I just did my first valve check over chrissy - all went pretty smoothly.

All my valves are in spec, except one.  The left-side exhaust valve allows a 0.1mm feeler in but not a 0.13mm - so clearly I'm outside the 0.03-0.08mm range.

Is this going to be a huge issue for the meantime?  How anal should I be about replacing it?   

[beRto]

Since it's clearly out of spec, I'd replace it. That being said, it shouldn't be a huge issue to keep riding while you wait for the new shim to arrive.

[Kerry]

Hmmm.  "Too loose" is marginally better than "too tight" ... especially for exhaust valves ... but if it were me I'd replace it.

When the valve is too loose, it 1) opens later than usual in the exhaust cycle and 2) doesn't stay open as long as usual.

To  me, fact #1 means that during the first part of the exhaust stroke on that cylinder, both valves are closed and some of the engine's momentum is spent trying to compress exhaust gases.  There's not much point in doing that....

Fact #2 means that less than the usual amount of exhaust is pushed out of the cylinder AND that the last bit of the exhaust stroke is again spent trying to compress exhaust gases.  It also means that there will be less unfilled space in the cylinder during the intake stroke, so the fuel charge that gets ignited will be smaller than usual, and will contain some already burnt fuel.

It all adds up to decreased power.

You could probably run the bike this way for a good while without causing problems to the engine ... no risk of burned valves anyway ... so don't worry too much on that account.  If I didn't have a suitable valve compression tool I might go ahead and "button it back up" until I could get my hands on one.  But that could mean opening things up two more times before everything is back to spec:  once to pull the shim so you can find out which replacement size to buy, and again to install the new shim.  (Unless you have the patience to leave things apart while you go about procuring the new shim.)

In the end it's all up to you, of course.  But I wouldn't let it go too long....

[gsJack]

It's hard to picture a little extra valve clearance being much of a problem with the amount of valve overlap there is on the GS engine.  The exhaust valve opens 63* before the end of the powerstroke and closes 71* after the intake valve opens to remain open for about 280* total.



A little extra exhaust valve clearance might be a concern in regards to the possible throwing of a shim if you're running redline all day.  I've let them go at .001" to .002" over the .001-.003" standard clearance.

http://img.photobucket.com/albums/v443/jcp8832/valveclearances.jpg












 

[philward]

Hmmm.  "Too loose" is marginally better than "too tight" ... especially for exhaust valves ... but if it were me I'd replace it.

...

It all adds up to decreased power.

You could probably run the bike this way for a good while without causing problems to the engine ... no risk of burned valves anyway ... so don't worry too much on that account.

That's pretty much what I figured, Kerry - thanks.  I will replace it - just need to get some bits together.  Hey, more tools is never a bad thing! 

[Kerry]

Thanks for the diagram, gsJack.  It just goes to show you how much real life can differ from simplistic models that one holds in one's head. 

It took me a few minutes to think through everything the diagram was telling me.  For example:

• You have to go all the way around the diagram twice to cover all four strokes (I thought I was going crazy at first; it looked like I was being told that both valves are never closed at the same time!)
  
• The "open" and "close" positions must be the points at which the valve just begins to open, and where the valve is entirely closed, with transition periods to match.  That made sense as I thought through the timing of events.
  
• The exhaust valve starts to open 63* before the end of the power stroke?  Whoa!  That must mean that either the impulse from the flame front is nearly spent by then, or there is some back pressure benefit from the exhaust system to be had at this point, or ... both?
  
• The exhaust valve doesn't fully close until 71* after the intake valve starts to open?  Hmmm, perhaps the influx of the fresh fuel/air combination helps to push some of the remaining exhaust out of the cylinder?
  
  

Still, these overlaps were all designed into the engine.  While I agree that "a little extra valve clearance [shouldn't be] much of a problem", it must take away a little something ... mustn't it? 

[gsJack]

The whole picture gets a bit more complicated when you remember that at the 11k rpm redline the camshaft is turning at 5.5k rpm so the valves are opening and closing 5500/60 = 92 times a second.  Actually with valve float I doubt they ever close at redline except for Ducati (desmo design). 

Besides adding a bit more space for a shim to escape, the extra clearance does give you noisy tappets if it is great enough.  Valve float leaves a lot more space for throwing a shim than a bit more clearance does since the cam lobe turns away from the shim faster than the valve can spring back at high rpm. 

I left one exhaust valve with .005" = .13 mm clearance and never heard it tapping but perhaps your younger ears would have heard it.     I doubt there is any noticeable problem caused by the extra clearance except for valve noise in everyday use.

My Clymers shows the camshaft exhaust lobe height to be 1.4288-1.4300" and the service limit to be 1.417" so that's allowing up to about .013" wear from the camlobe to be serviceable, a lot more than the extra .001-.002" clearance I've left.  The fact that my intake valve clearance has never changed would indicate near zero cam lobe wear if you change your oil occasionally.   

[gsJack]

Kerry, that valve timing diagram has been in the Racer's Corner linked to on the gsTwins front page for years.

[Kerry]

Kerry, that valve timing diagram has been in the Racer's Corner linked to on the gsTwins front page for years.

Yep - it even looked a little familiar.    But I don't think I've been to the Racer's Corner pages since shortly after I discovered GStwin.com ... back in 2002 or so?  The diagram wouldn't have "spoken to me" back then, I'm afraid.