Excessive valve clearance cause low power?

How old are your sparkplugs? We've had a lot of trouble over the
years with Champions, and used to buy the Auburns before Champion
bought them up and shut them down. They were an excellent plug and
almost always eroded to limits before the internal resistor failed.
With Champs, we've had the internal resistor misfiring at 50 hours.
One didn't work right out of the box. The Unison plugs are no better.
A poorly-firing plug will cause a slight RPM drop. They need
to be tested under pressure to see that they continue to fire up to at
least 120 psi.

Dan