This is why we do runups

"Roger Long" om wrote in message ...
snip
I flew the plane the day before yesterday, 15 hours out of annual with new
plugs installed. ==========

The next morning, I started it up and taxied to the run up area. When I did
the mag check, the left was rough. The right was so rough that the airframe
was shaking visibly. There was no question about flying the plane.

When the plugs were removed, all were found fouled and one was completely
bridged. The engine ran acceptably on runup but was not as smooth as
before. It gradually returned to normal on a 4 hour flight. We're still
trying to figure out what happened. (Wild ass guesses appreciated)

OK, here's my WAG. You replaced all of the plugs and they all
started fouling. Did you replace the plugs with the same brand and
heat range? Most O-320 installations have a list of acceptable heat
ranges for the plugs. I've found over the years that there is usually
one specific heat range which will prevent fouling and work perfectly
in a specific installation. It's usually found by trial and error
and when you do find it, you stick with it.

John Galban=====N4BQ (PA28-180)