Hi Jim,
I am guessing this is a Moki 1.8, it doesn't sound like it is broken in yet they seem to take quite a while.
I am running Bryons 5% with a 20*8 schimitar and it turns right at 6800 and seems happy enough.
since speed is not an issue with this plane I was thinking of a 20*6( think Zinger makes one), lots of static thrust.
Set the low speed where ever it needs to be to run right. mine is between 2&3 notches lean.

If it is not to much trouble let me know where you have it balanced, just looking at the wing I would think
the cg needs to be further back than the 1/2 in. from the spar that the plans call for.

Mike,