With working biceps and brachioradialis, if your pinky is above the thumb on the curls (made easiest with the EZ bar curl), you will better target your biceps. The more you have your thumb over your pinky (or when your hand is straight up in down like it is when you're jerking off your uncle), the less it targets biceps and the more it targets the bracioradialis.
The lesson is to use the EZ bar for curls. No momentum, back straight against the wall.