ROB KARDASHIAN I'M NO CHEATER AND IT'S PAYING OFF ... Down a Few Dozen Lbs

Rob Kardashian is sticking to a strict diet -- no fast food, regular workouts and lots of Blac Chyna ... and friends think it's already helped him shed about 40 lbs.

We're told Rob's also cut out red meats and carbs while hitting the gym 5 times a week with a trainer ... plus additional workouts with Chyna. 

We're told the couple made a "no cheat days" vow, and they're motivating each other to stick to it. 

It's hard to say the program isn't working, based on recent pics. Some home cooking from Blac Chyna doesn't hurt either ...