RE: I need help with a lateral balance problem.
As mentioned by others you might have problems with incidence or twist of the flying surfaces in addition to possible lateral balance issues. Do use an incidence meter at the root and tip of each wing to check for wing twist, then twist the wing by hand and use a heat gun to reshrink the covering to reset the twist.
The best way to test lateral balance is in flight. Trim your plane out for straight and level flight. Make long passes back and forth without touching the ailerons, trim it in roll as accurately as you can.
Then set up for a long pass and roll inverted. Make a long pass using only elevator, do not touch the ailerons, and check if the plane flies wings level clear across the field or if it tends to roll one way or the other. Repeat this test many times. If it rolls to the right when inverted the left wing is heavy and of course if it rolls to the left, the right wing is heavy. Add a chip of stick on lead to the light wing tip and test it again.
When the plane trims out in roll the same upright as inverted, it is laterally balanced. Any differences left in the aileron trim at that point will be mostly due to twists of the aero surfaces.
This is an important trim step with any new plane. Lateral balance affects tracking on all looping maneuvers and trim while inverted.
Dave