The white soot is from the Lead in the fuel. If you run too rich or
are burning oil you will not see the white deposits only black soot.
In this case a white deposit is a GOOD thing.

A long long time ago you looked at the exhaust pipe of a car to see if
it was burning oil. If it was white you had a good engine. Then came
unleaded fuel and now all look black.

On 17 Apr 2004 20:28:34 -0700, (R. Wubben) wrote:

While taking my new to me'62 Cessna 172 with an O-300 (with about 75
hours on it SMOH with Millenium cylinders), I decided to start leaning
as I should on my last flight.
During the next preflight I noticed a whitish soot around the exhaust
and on the cowl just adjacent to the stack during the preflight.
Looking up on r.a.o for "white exhaust soot" I came across posts from
several years ago about the white soot being a side effect of leaning.

Does this sound right?
Am I leaning too much? (I'm still trying to figure out the lean and
rich of peak thing, especially for this particular airplane that I
have about 18 hours in).

Thanks,
Ryan Wubben
Madison, WI