The Biebs just showed up at MB2 Raceway in the Valley with some of his church pals -- including pastor Carl Lentz -- and was welcomed by a warm hug from his mama, Pattie Mallette.

JB's plan to celebrate his birthday is apparently zipping around on go-karts on an indoor track ... which beats bottoming out a Lambo.

No sign of Selena yet, but based on how close the 2 have been lately ... it'll be no surprise if she shows up soon to join in on the fun.

As we reported ... Justin and Pattie reconciled recently and even went on vacay together, and it looks like things are still going well.