Help! plane flying upside down

[akimbo]

A while ago I designed an edf f4d-1 skyray and decided to test fly it but ran into a weird issue. Right after launch, the plane would immediately pull up until it was inverted and would continue to fly like that. The motor was not mounted perfectly as I hadn't designed a mount or a thrust tube yet but, I feel like any imperfections in regards to thrust angle would not result in this kind of behavior but rather a continuous turn. I figured that the wing, battery, or motor position caused this but everything seemed fine. does anyone know why this would happen?
reference pics:

 

[Piotrsko]

Well on my KFM wing it's motor thrust being way off but it might be a side issue with the step style wing planform like a drag pocket aft of the CG. It doesn't care which way it flies, upright or inverted
are you sure your wing is flat? Might be a camera lense error but it looks droopy

 

[Merv]

It would be easy to explain if you had some kind of stabilization.
Your stabilization board is upside down.

 

[akimbo]

It would be easy to explain if you had some kind of stabilization.
Your stabilization board is upside down.

if you are referring to a gyro, i am not using one in this plane.

 

[Foamforce]

Do you have the battery mounted up high?

 

[quorneng]

In principle a delta can be stable either way up. With a non symmetrical wing section and no dihedral as you have it will only require the CofG to be too far back for it to be only stable when flying inverted.
With the correct CofG and not too much weight F4Ds fly very nicely.