what burned valve looks like

["Watry, Andrew (LNG-SFR)" <Andrew.Watry@domain.elided>]

Per Kevin's question about burned valves, I have a burned exhaust valve
out of my old 1969 SPICA 1750 Berlina in front of me.  First a caveat: I
think burned valves can look different, from very burned, to slightly
burned depending on how long the condition has persisted.  I believe it
is most common for exhaust valves to burn, because the hot gases go out
that way over the valve head.  I think it's rare, but I supposed it's
possible, to have a burned intake valve.  Usually on disassembly of a
head the intake valves look new, and the exhaust valves look like they've
lived in a blast furance.

The valve head (and about 1/3 of the stem) is coated with black crud, and
shows evidence that it was seating pretty much solidly on the valve seat
about half way around.  The other half, it's sort of hit and miss because
material is slightly burned away the whole way around. The actual fully
burned part that leaked significantly is a trough, kind of like a tiny
pie wedge cut out of the seating surface, about 4 mm wide, and reaching
fully across the valve seat like a passage.  The surrounding area of the
valve seat on both sides of the missing wedge are burned progressively
less as you move away from the burn, tapering up to the better, but by no
means good, part of the valve seat.

This car failed its 1995 Calif. smog check because of this valve.  After
a valve job it ran well, and cleanly.  Symptoms were very low compression
on this cylinder plus a chuffy exhaust note that wasn't what Alfas
normally sound like. At speed the car felt OK, but at idle and low RPM it
was rough and lopey.  I hear from Norman Racing that it is common for
slightly burned valves to feel and work basically OK at high RPM, but not
at low, when things are happening slowly and the cylinder is filled and
pressurized slowly.

In this engine, all 8 guides, and all 4 exhaust valves, needed replacing,
along with the valve grind.

If I had a digital camera I could take a picture.  Sorry I don't.

Andrew Watry
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