valve job question

[Shannen Durphey]

William T Wilson wrote:
> 
> On Sun, 2 May 1999, Shannen Durphey wrote:
> 
> > > Cleaning the carbon deposits off of the piston isn't really all that
> > > important because they just come right back when the engine runs the next
> > > time.
> >
> > Sounds like a tire shop I went to once.  "No need to clean the mud off
> > the wheel before we balance it.  It just gets muddy again anyway."
> 
> Heh.  No, that's different, because the carbon deposits don't affect the
> operation of the engine.  Really.  Unless there's so much of it that it
> causes dieseling.
> 
> My suggestion is more akin to saying "no need to clean the mud off the
> wheel before you rotate the tires."
> 
> :}
WHAT???  Carbon doesn't affect engine operation???  Where, pray tell,
did you get that information?
If you're sending an aluminum head out for a valve job, typically the
gasket surface gets planed in the process.  Now you leave the carbon
on the pistons, knowing full well it wasn't dieseling before the head
job.  The head comes back with slightly smaller chambers and nice
tight valves, and you bolt it on and go.  If the engine's a high
compression, late model type (like a Geo Metro at 13:1), there's a
good chance you'll burn the exhaust
valves the first time you head out on the highway.  But it won't
diesel.
But hey, the engine this original question was about is a small block
Chevy.  Pretty durable, probably 8.5:1 compression, iron heads.  No
problems there, right?  But maybe there's a bunch of carbon built up
on numbers 6 and 8 cylinders from worn guides.  So ya take the car out
with your nice, tight valves, and get on it 'cause you know it's got
more power.  There's some rattling, not too much, probably just too
much timing advance.  So you play with the timing, get on it some
more, play with the timing, but you end up with your total advance
pretty low and not as much power as you expected.  But it doesn't
diesel, it's only two cylinders, and they're only showing problems at
higher RPM, so the carbon can't be causing it.  Must be, oh, a
mystery.

Sure the carbon comes back when you run the engine.  But what you see
during a tear down is an accumulation over a large period of time and
after a
fair amount of wear.  Putting an engine back together like that is
asking for trouble, and I would consider it a sign of laziness knowing
how easily carbon buildup can be removed.

Shannen