Sounds like the engine incidence is off. That plane engine combo should not need weight on the front, what is a ton?

Trim it for straight and level flight at 3/4 throttle. Do a fly by and when it is in front of you (several mistakes high) chop the throttle and see what it does. If it piches nose down you have too much up thrust built in to the engine mounting and need to correct it.

After that is worked out do the same thing but roll it inverted at 1/2 throttle. It should require very slight elevator stick pressure down to maintain level flight.

My guess is your incidence and CG are both off.