if you dont already have the mario kart patch from eshop, you would need to download that first to be able to play online.....which means running b2 (which has the brick code) to get the update, then just switch back to b1 .....having emunand on 7.1 but using b1 causes region free and system settings to not work....so its a trade off.
if you already downloaded the patch before 5.x was released though you can play online even on 4.5 with no issues