Hello RickP, you stated that the elevators are at zero but need down trim to stay level, but you state that it dives while inverted? hmm.... I would try and move the GC back a little at a time. A technique I use when testing the CG, I would trim the plane for straight and level flight. Then I would put it iverted and if it dives I would need to move the CG aft. Not too much aft because it makes it very unstable. You probably already know this technique but that's what I would do. I'm not familiar with your plane but does it have any dihedral, that might cause it to canopy up on a down-line. Also I read in your post that it elevates upward when on full power. You could check it again but it might be that the thrust line is at an upward angle. A combination of up thrust and forward CG could cause your plane to behave the way it does. Hope this helps.