I tach my g90 today and with 15% cool power
on a 15x6 zinger prop I got 8700rpm
not bad I think

Try a MA or APC 12 x 7 or 13 x 6 prop, as I think the 15 x 6 is too much prop for this engine as it is happiest in the 11,500 -12,000 rev range. Check the engine manufacturers section on this forum for advice about ST motors. You might be in for a pleasant surprise.