RE: GWS Tiger Moth 400 - build, esc, servo, batt questions
As you can see from my pictures I added an ounce of weight to the stick mount for the motor... even with the motor extended out. I never would have done this had patnchris not given me the advice. Building the CG machine was simple and it helped me figure out how much weight to add.
I don't have the problem you're experiencing. My plane flies straight and true anywhere from 1/4 to 3/4 throttle with elevator and rudder trimmed even with the stabilizer. The only trim I had to add was to the ailerons.
I have the motor mounted forward, 1 ounce of weight, and the 3S battery is always pushed as far forward as possible. Also, I was very careful about glue when building this. I used epoxy very sparingly so as not to add weight to the tail.