When we used to run Cessna 150s, we often found them hard to start.
Then we noticed that just after a flight, or lengthy runup before
inspection, that oil would dribble out of the bottom plug holes when
we pulled the plugs. These engines tend to foul the bottom plugs at
idle for some reason, and we instituted a 1700 RPM runup prior to
shutdown to clear them. No more hard starts or fouled plugs.
We have few fouling problems with the Lycs, other than with too
much circuit work.

Dan