pre-flight mag check showed a drop of 100 rpm on the L mag, R mag dropped 50 rpm as both have been doing previously. The change was very noticeable on the L mag.
In flight LOP at 6000' I did another mag check with this result. L mag first then R mag.Any reason for concern? Why the sudden change in rpm drop on pre=flight check? Spark plugs are Tempest massive about 500 hours old.
Ideas?
Suggestions?