Fadi,

No you shouldn't always expect changes in pitch trim like this but don't be alarmed, most planes need minor adjustments after construction.

It sounds like your suspicion of needing additional down thrust is correct. Add a washer or two on the upper motor mount bolts between the motor mount and firewall. Alternately you can put washers between the engine and motor mount on the rear engine bolts.

Once that's done, take off and trim the plane out at full speed. Make a level pass and when the plane is in front of you, chop the throttle. What you're looking for is for the trim to be uneffected by the throttle position. If it climbs when you chop throttle, you need more up thrust. If it dives when you chop throttle, you need down thrust. Then continue adjusting as necessary. When you get the proper amount of thrust offset it should hold trim at any throttle setting.

To help with landings on your test flights, slow down and readjust your pitch trim before landing.

Dave