I am trying to use a mushroom powerup in manual mode which is the B button, but instead when i press B I drift instead.
According to the instruction manual
Depending on which control scheme you're using, "Use Items" is mapped to either L, Z, or the D-pad.
D-Pad up (launches item forward if possible) or down (launches item behind if possible).
Wiimote & Nunchuk
Classic Controller or NGC controller