I have a North Star (kit from Balsa USA: http://www.balsausa.com). I like to fly it but I have notice that when I perform a low pass at low speed and then I go to full throttle, the airplane nose goes down and I have to pull Up the elevator. This is appening only when I go from Idle to full Throttle. I guest this is appening because of the special configuration of that aircraft. I haven't install an elevator like the original plan I have preferred using delta wing mixing. Should I add DOWN or UP thrust to the engine mount to correct the situation?

I'd go for down thrust.
This would lessen the moment arm of the motor thrust over the c.g..

many moons a go a guy was taxi testing a similar setup ---
then he went for take off and the thing just stayed planted on the runway.
So he made a looong full throttle attempt and it still stayed planted -past the point of stop /go further and had exceeded the calculated take off speed! So he shut the engine down and it THEN lifted off - with disasterous results.
I heard this tale years back - don't know if it is folklore or a real event.

I Will test it at the field tomorrow to see if "down thrust" solves the problem or helps.

Giving the engine MORE down thrust will make the problem worse, think about it. The thrust angle is trying to pull what ever it is attached to in that direction, the more power you add, the stronger this force gets. If your aerodynamic forces are fighting the thrust angle you will be wasting power and turning what should be a fast plane into a speed sensitive mess. If the thrust line and C/L of the wing are in rough allignment, the engine should be pointed straight ahead [with maybe a touch of right thrust]. As a rule of thumb, high wing model designs with lifting airfoils need down thrust.

This is the way I see the dynamics...

that's right

I can see now after zooming in on the photo that the engine is mounted quite high, and yes it would raise the aft end of the plane when you gun it. Why not lower the engine and allow the prop to pass through a slit in the wing? The engine would be better off supported from a short pylon and this idea would make the plane behave better at all speeds.

Hi Guys, thanks for replying to my topic, It's an interesting way for me to solve problems and upgrade my skill level witch I call intermediate.

On my North Star, I haven't install an elevator, I use elevon mixing. Of course, ailerons are a lot bigger to plans and the aircraft performs fast rolls and loops, etc.. with authority. Actually, It flys very well without down thrust except the problem I have notice here in that topic.

I'm not sure I understand well the situation!?!. Anyway, Tall Paul's picture shows well Down Thrust dynamic of the North Star. If I understand well, down trust will force the airplane to fight the engine thrust angle, it will waste some power but may help to keep the nose up when going at full throttle. I don't really like the idea. So guys, what do you think about moving back the CG point (closer to the tail) instead of adding down thrust to the engine? will it solves the problem this way?

yes!- the plane will likely flatspin in first chance it gets and you won't have to worry about it anymore.
IF you shift the CG- do it a little at a time --