I'm not sure, but .70 4 Stroke sounds a bit small to hold 7+ pounds vert.

The specs on this plane (this is the 40 size?) say that it should only be 6.5 pounds max.
The OS 70 4 Stroke only weighs 6 oz. more than the .46 2 Stroke, so I don't think that the engine is to blame, but it seems you do have a pound to much in there somewhere.

Some things to look at:
Spinner, can you find a lighter one?
Do you have ballast weights? Get rid of them...move stuff to compensate.
Lighter wheels.