If you can't get it to go above 7000 RPM, you are either too rich, or your prop is too big, or your carb is not fully opening (Or you have crappy fuel).

First I would try running a 14x6 or 14x8 prop (Or my personal pref, a Graupner 13.5 x 8)

VISUALLY check to see that the carb is opening FULLY.

Use a good (As in fairly NEW) 15% Fuel.

Crank it up, bring it to WOT and turn the needle valve RICHER at first. This should cause the engine to run worse and start spitting fuel out of the muffler (if it isn't already)

So now that you KNOW it's rich, slowly turn thr needle valve in. The engine should run up to peak and then start to sound a little sick (You don't need a tach, you'll hear it). Don't go too far! 4-strokes don't like to run lean and will backfire spitting the prop across the field. Just return the needle to where it sounded best, then open it a little more but not so far that the the RPM go down again.