You have to first eliminate things slowly. Get a fresh plug and then tune the engine up properly. Check the low end settings and the high end settings ... as usual do the pinch test for both settings, I am sure you can get a lot of websites telling you how to do the pinch test.

DO NOT test for maximum RPM on the ground only. Hold the plane at 65 degree or almost vertical and then do the high end tuning. Take it up to full throttle and then RICHEN the HS needle till the RPM drops, then LEAN the HS needle till the engine hits max revs and then start to loose RPM. Once you know where the max is then RICHEN the engine out about say ... 5 clicks.

That is only your almost perfect setting. You will have to fly the plane to see how she handles as the prop loads up. Pull some gradual loops on full throttle to see how the engine performs, if you notice that the engine looses power near the top of the loop then RICHEN the low end a bit.

One thing I noticed about and engine ... I may be wrong but this is something I observed. When you RICHEN the low end, you can then LEAN out the high end more and get more RPM. Anyone else noticed this, of course you need a tach to test this out.