Justin Bieber -- I WON'T Turn My Back on Lil Twist, Lil Za

Justin Bieber refuses to throw Lil Za and Lil Twist out on the street ... even after all of the trouble they've caused ... because he considers them "true friends" ... TMZ has learned.

Sources close to Bieber tell TMZ ... members of Justin's camp have been pleading with him to boot the Lils out of his Calabasas home and forbid them from driving his fleet of fancy cars after a string of embarrassing incidents including Twist's DUI arrest in JB's car.

We're told Bieber's camp has been very concerned about the way he acts when he's with the Lil crew -- and have been vocal that the guys are a bad influence.

Why? Weed ... sizzurp ... neighborhood wars ... pissing in public ... anti-Clintonism ... and of course, Twist's DUI arrest. The Lils had a role in all of that stuff ... and people close to Beiber want it to stop.

But despite the warnings, Justin is pushing back HARD ... claiming he would never turn his back on his boys, because that's not what friends do.

Plus, one source says Justin's been surrounded by adults for most of his life -- and now that he finally has a group of close friends that are his age, the last thing he wants to do is push them away.

Bottom line -- get used to Za and Twist ... they're not going anywhere.