Try it! If the plane zooms up when you advance the throttle from a level flight, then it needs downthrust. Add downthrust until there is no zooming with throttle. If the engine is bolted to an engine mount, add the thrust washers behind the mount rather than under the engine lugs. It is best to have the engine lugs contact the mount all along their length and for the mounting surfaces to be flat, parallel and in the same plane.

Regards,

Richard