Fouling plugs?

d)
and
e) you are leaving the mixture too rich while taxing..
this is especially true at higher DA airports.

B

"Chris Colohan" <colohan+@cs.cmu.edu> wrote in message
news:ucly75uc4yh.fsf_-_@tretiak.stampede.cs.cmu.edu...
> With all this talk about runups...
>
> I'm currently doing my IFR training in a Cessna 172SP. I have noticed
> that every single time I start the engine and taxi to the runup area,
> the plugs are fouled when I do the mag checks. As a result I always
> have to do an extended runup to burn off the lead. Leaning the engine
> (no matter how much) before taxi doesn't seem to matter...
>
> Does this mean that:
>
> a) something is wrong with the engine
>
> b) some engines are just finicky this way
>
> c) the other renters are forgetting to lean before taxiing back to
> parking
>
> d) I'm doing something wrong
>
> ...?
>
> Chris

On May 29, 8:51 am, Gezellig <nokon...@> wrote:
> On Wed, 28 May 2008 19:33:33 -0700 (PDT), Dan_Thomas_nos...m
> wrote:
>
> > Could be. It's a Lycoming, which doesn't usually have the oil-
> > fouling problems that the smaller Continentals often display, but if
> > the rings are worn some there might be enough oil sucking past them
> > that the bottom plugs get wet and will misfire until a full-throttle
> > runup clears them.
>
> Dan, would you see this in the color of the exhaust?

Maybe just the tiniest bit of blue when the throttle is opened. At
idle the cylinder pressures are so low on the intake stroke that oil
gets pulled past worn rings into the combustion chamber and will lay
in the bottom and run into the bottom plug and drown it. Opening the
throttle wide makes more fire and turbulence and heat and will burn
that stuff out. It's much more of a problem in small Continentals than
it is in the Lycomings. That 172P has a Lyc in it. I would bet that
your problem is lead-fouled plugs that aren't getting changed out
often enough.

Dan