i have a similar issue...
my tris are great, inner bi head fine, outer looks small so i have a narrow bicep
i did variations of different things, like close n wide curls n altered widths n grips.
the best 2 exercises for me are slightly different than what i wouldve assumed.
i use db's so i keep pressure off my wrists, the ez bar is a nice change at times but it puts pressure on my shoulders with a closer grip.
i use db's w shoulders up and pulled back, i keep my elbows tight to my sides and the entire time try to raise with the inner portion of my palm near the pinky to focus on the outer head. you can try keeping the db's closer which generally does work the outer head better but i get a better feel keepin a slight angle outward (which is not how its supposed to be but it is for me). i also do cable curls but i face away from the machine, whereas some face the machine. facing the machine helps my peak facing away helps stress the stretch of the movement from bottom to top of the movement.
db curls 3x6-8
cable curls 2-3x6-8
reverse ez curl or db curls 2x8