I'd have to agree with you there. As an engine wears, the valve
clearances will increase....not decrease. Also if the valves are too
tight, they won't close against the seats. The cause of burnt valves
(related to valve clearances), is because the lash adjustment is too
sloppy, thus not allowing the valves to open completely, and therefore
causing the valves to stay closed and hot for too long and then burn
(usually the exhaust valve because it is hotter than the intake). There
is combustion occurring in the cylinder, but the exhaust is staying in
the cylinder too long, and thus you get burning.

Regarding the rough idle.....when is the last time you had your
transmission cooler flushed ? Over time, the same crud that pollutes
your transmission also takes up residence in the small confines of the
cooler, which in turn creates a back pressure in the cooler hose coming
from your transmission, which in turn causes the oil pump in the trans
to work harder, which in turn puts extra, unnecessary load on the
engine, and so on and so forth. It's worth a shot as nobody ever thinks
of doing this (usually only done when a trans is replaced at the
dealer), and it does cause rough idle. I know a lot of people have trans
flushes done, but I don't think that the cooler is included in the
flushing circuitry.....just hooked up to the trans cooler lines.

D.Mills
Licensed Automotive Technician.