I was recently having a discussion about valves with a friend who is as equally ignorant of the finer points of valve operations as I am.  I thought where better to get our confusion cleared up than on this forum.

The basic question concerns how a valve becomes burnt.  I understand that it is caused when a valve fails to seat properly for any of a variety of reasons.  It has to do with the ability of the valve, particularly an exhaust valve, to transfer heat to the valve seat.  So far, so good… I think.  But here is where the confusion begins.  Using just valve adjustment as the potential cause of a valve becoming burnt, under which condition is it most likely to occur? When the valve adjustment is too tight as measured between the cam and bucket, meaning the valve opens further than intended; or, when the valve adjustment is too loose, meaning the valve opens less than intended?  And, how much more likely is it for an exhaust valve to burn than it is for an intake valve to burn given both are equally out of adjustment?

I knew I should have taken shop in high school.

When it's too tight, because eventually the clearance will be zero, which means it doesn't close when it should (during combustion).

and what are the symptoms of a burned valve?

Loss of compression, rough idle, loss of power output, back firing or misfiring.  If bad enough it may not even start.   All of this even after correctly adjusting the clearances.  

and what are the symptoms of a burned valve?

Exhaust pipes can become red and then whitish see through after that in severe cases.

What is the lowest limit of clearance for a valve before it start to burn or give problems ??

When the valve no longer has any gap you are at the critical tipping point.  Once there is *any* wear after the zero gap point the valve won't fully close letting burning combustion gas leak around the valve flute and that's going to be the end of the valve very quickly.   One tip-off the exhaust valve clearance is getting very tight is improved high rpm performance  because the exhaust valve is open longer with higher lift.  Hopefully there is enough valve/piston clearance that the early opening, high lift valve won't kiss the piston.

^ this.   But...  

We make our valve clearance checks with stone cold (assume 60-70 degree F) engine.  That is not necessarily representative of what happens at full-on engine operating temperatures, with the metals in the combustion chamber reaching into the hundreds of degrees.  

Engine blocks are made from alumin(i)um alloys that have a higher expansion coefficient than the steel alloy valves.  This means the blocks get bigger faster when heated than the valve stems do, resulting in closing valve clearances as an engine gets hotter.  

Lele's question is a good one, but I don't think any of us have a definitive answer as to exactly when engine doom occurs, except to say that it is sometime (well) after one exceeds the factory valve tolerance specs (when cold).  

The more salient point is that if you check your valves at the proscribed 25k(ish) mile intervals, and the valves check at or above the low limit when you measure them, you will be just fine.  Which is the entire point of doing these check.

PS - ionbeam...  you need a better back drop for your new sexy Gray Ghost's avatar than the boring old Sandown station.   Find a nice lookin' mountain or sumptin'  

Mike Nixon, at the motorcycle project, has a lot of truly great information -- valve clearance among the mix:

http://www.motorcycl...clearances.html

It's a good read, and nothing I see that contradicts my experiences and other references

k.

PS - ionbeam...  you need a better back drop for your new sexy Gray Ghost's avatar than the boring old Sandown station.   Find a nice lookin' mountain or sumptin'  

It's a place holder until I can find a classic NE/NH setting.  If it ever quits raining I'll be on it 

A little too loose just results in a tad more mid-range power. Really. Common practice back in the day with screw adjusted Kawasaki in-line fours was to run at (or slightly above) the loose limit.

That being said, you have to ask why is the clearance loose? Next shim down just a little thinner than it should be? Or has the cam lobe or follower worn? Never a good thing.

So what can result from a valve being too loose? Seems that is never an issue that pops up.

It can lead to pitting of the cam and/or the cam follower, the cam follower is "hitting" the cam at a steeper angle than the hardening was designed for.

I've seen the result, though this was 50-odd years ago, when materials and oil weren't as good as they are now.

If the clearances are set too wide then the valve lift and duration will be (slightly) reduced.  This will slightly reduce the peak power available at high rpm, but as noted earlier can make the engine run smoother and stronger at lower rpms and mid range.  

Too loose is a far safer situation, and valves on modern engines with shim under (hardened) bucket followers, like the FJR, tend to always wear in a way that reduces the clearances, which is one of the reasons why many folks will set their clearances to the wider end of the spec range.  It also prolongs the need to re-shim, perhaps never requiring it again during the lifetime of the engine.    

I think my engine is completely unbalanced due to different clearance between cilynders. (Within spec)

If the clearances are set too wide then the valve lift and duration will be (slightly) reduced.  This will slightly reduce the peak power available at high rpm, but as noted earlier can make the engine run smoother and stronger at lower rpms and mid range.  

 

Too loose is a far safer situation, and valves on modern engines with shim under (hardened) bucket followers, like the FJR, tend to always wear in a way that reduces the clearances, which is one of the reasons why many folks will set their clearances to the wider end of the spec range.  It also prolongs the need to re-shim, perhaps never requiring it again during the lifetime of the engine.    

This is what I did last winter.  We'll see if I ever have to replace a shim again.

Thanks for the info to those who have responded.