You say - it turns to the left...
Full throttle it turns to the right...
Please post picture of the motor mount - picture from the side and from the back - as right in line as possible.
Easystar does only have a side rudder - no ailerons if not modified.
The standard side rudder is very small and does need some time to get the plane to change direction.
If i recall the standard prop is only a "Gunter" 5" or possibly 6" soft quality.
A "normal" upgrade is with a 2800 or 3000 kv inrunner with an APC style 5" harder prop.
Your motor is probably positioned higher and gives a lot more torque - specially with full throttle - making the plane twist to the right.
I skilled pilot will start slowly and compensate with left rudder until the plane gets up in speed.
As soon as the plane starts to turn in either direction you need to give a little up elevator to compensate for the down thrust.
Also starting an easystar with full throttle pushes the nose down until the plane has built up speed - need to give full up elevator to compensate if you don't start gently with throttle until the speed is up.
Try to make the rudder at least twice the size
or add ailerons to help faster respons to compensate for the left or right the plane want to make