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Burning valves


rpcarlson

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I think the most common cause is the exhaust valve clearances being too tight.

When the motor gets to operating temperature, every thing starts to expand.

If the exhaust valves are too tight then they don't fully close & overheat as they never come in contact with the seat.

This contact with the seat normally allows the valve get away from the hot exhaust & reject its heat.

 

Looks like a trip to BMW Motorcycles of Ventura County may be in your future tongue.gif

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valve clearances being too tight.
Exactly. People think there is something wrong with the valvetrain when it is 'ticky' so they adjust the valves to where they are quiet. Then they don't fully seat and burn.
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People think there is something wrong with the valvetrain when it is 'ticky' so they adjust the valves to where they are quiet.
Gawd, I hope no one would really do that! eek.gif

 

But yes, I concur that valve misadjustment is far and away the most common cause of a burned valve (sorry Ed.)

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I would make the argument that carbon buildup from oil consumption and not putting adequate load on the engine to burn it off is a more common cause. A bit of the carbon breaks off, allowing the cutting torch effect of combustion to burn a hole. The valve can look like it broke, rather than burnt. JMHO. Good Luck!

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ShovelStrokeEd

No need for apology, Seth, I just don't buy it.

Yes, it would be possible to be so ham fisted as to adjust the valve(s) so one or more of them never gets down onto the seat but, one would also have to be nearly brain dead not to notice the marked decrease in performance that accompanies such a condition. The cold setting clearance is 0.012", yeah, I'm a dinosaur, call it o.3 mm if you want.

 

Considering the position of the cam in the head and the relatively short length of the pushrod, the clearance is not going to change all that much with the motor hot. Remember, the distance an object expands with temperature is a function of the lenght of the object and the coefficient of expansion of the material.

 

Every picture I have ever seen of a burnt valve on a BMW motor has not, in fact, shown that of a burnt valve but rather gas cutting of the valve face. This symptom is far more likely to show up if a piece of debris (carbon) becomes lodged in the valve seat area. It doesn't take much, a couple of thousandths of an inch is more than enough to start the process when you consider the combustion gases are, at that point, more or less a plasma having a temperature, at high throttle openings, of around 2000 deg C and the velocity of same, through a partially open valve face is huge. At those temperatures, the valve head is pretty plastic and will flex enough to conform to the seat except in a region where it is prevented from doing so by the afore mentioned chunk of carbon.

 

Just my $0.02.

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No need for apology, Seth, I just don't buy it.

Yes, it would be possible to be so ham fisted as to adjust the valve(s) so one or more of them never gets down onto the seat but, one would also have to be nearly brain dead not to notice the marked decrease in performance that accompanies such a condition. The cold setting clearance is 0.012", yeah, I'm a dinosaur, call it o.3 mm if you want.

 

Considering the position of the cam in the head and the relatively short length of the pushrod, the clearance is not going to change all that much with the motor hot. Remember, the distance an object expands with temperature is a function of the lenght of the object and the coefficient of expansion of the material.

 

Every picture I have ever seen of a burnt valve on a BMW motor has not, in fact, shown that of a burnt valve but rather gas cutting of the valve face. This symptom is far more likely to show up if a piece of debris (carbon) becomes lodged in the valve seat area. It doesn't take much, a couple of thousandths of an inch is more than enough to start the process when you consider the combustion gases are, at that point, more or less a plasma having a temperature, at high throttle openings, of around 2000 deg C and the velocity of same, through a partially open valve face is huge. At those temperatures, the valve head is pretty plastic and will flex enough to conform to the seat except in a region where it is prevented from doing so by the afore mentioned chunk of carbon.

 

Just my $0.02.

 

So Ed, would that mean when using additives like Techron (sp?) that is supposed to clean away valve deposits we are running a chance of a piece flake off and burn a valve as described above?

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Gas cutting of the valve face also occurs when a valve fails due to misadjustement. When the valve operates in an overheated condition someplace along the circumference of the valve has to fail first, and when it does it leaks, and when it leaks the erosion you describe occurs. That kind of damage doesn't necessarily have to be the result of a foreign object.

 

Just my .02 as well as I can't be certain, but in my life's experience I've seen a number of exhaust valves fail on engines with manual valve adjusters, but only once on an engine with hydraulic adjusters (and that was an old and abused truck engine which probably just wore out its valve seats.) Excessive engine load and general valve/seat wear on older engines are probably the most common cause of exhaust valve failure in the real world, but beyond that (and on BMW motorcycles) I think that more exhaust valves have been (eventually) destroyed by careless or hurried adjustment than bits of errant carbon.

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ShovelStrokeEd

Alan, yes, there is that danger if you suddenly put a whole bunch in after years of not doing so.

 

Most of those additives work best if used on a regular basis over the life of the engine.

 

Seth,

I didn't say it couldn't happen that way but the ones I have seen don't really show much evidence of overheating over the whole face of the valve as would be the case if the valve were that badly adjusted.

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