on coil air flow rate

I assembled a small unit of ac. Now I get the evaporator coil temp. 7 deg. C , with the blower off; i.e. almost no load.

If I start the blower, ie when I apply the heat load, the off air temp. falls down to 26 deg. C within 2-3 minutes.

I want this temp. (off coil air temp.) to be 12 deg. C .

What should I do ? Increase the mass flow of the refrigerant ? If yes, should I achieve this by cutting the capillary short or should i replace the capillary with a larges bore one ?

Is this happening because of an oversized evaporator ?