Gerard,

I've had something similar to this about 8 - 9 years ago in my early LNC2
(Lyc O-235-L2C with helicopter pistons).

After fueling at a local airport where I often went for breakfast, I took
off and in about 15 minutes, noticed a slight drop in RPM (I was running a
fixed pitch prop at the time) and a subtle but distinct vibration. No amount
of fiddling with the throttle, mixture, or fuel boost pump had any effect,
however when I tried switching off the left mag, the engine nearly stopped.

Since I was about half way between the departute and my home airport with no
other landing sites available, I elected to continue toward home. Landing
was uneventful except that the vibration seemed to be worse once on the
ground and taxiing.

Long story short, when I pulled the plugs (which only had about 100 hours on
them), I found the top plug in the front right cylinder completely shorted
by a large glob of lead that looked like it had been melted into the firing
end with a blow-torch. I knew that the O-235-L2C had a reputation for
collecting lead in exportable quantities but had never seen anything quite
this bad. You didn't say what kind of engine you're running but I understand
that under certain conditions, plug fouling can happen to any engine.

Ever since, I've added Tri-Cresyl-Phosphate (TCP) to my fuel and have never
had a similar problem.

Dan Schaefer