Well my friend who is a pro detailer and also owns Detailing store tested it with same batch 15 Mk2 and they were identical and both were better than mk1. So i thought the machine is ok and works as it should, but i have seen many videos online where 15mk2 is just so much more powerful than mine were so i sold my old unit and bought a new one and the difference is huge! My new 15 mk2 feels a lot like my 21mk2 and not stalling problems whatsoever. Also it sounds bit different, more powerfull i would say and its not as silky smooth, i mean it is very smooth but my old was kinda like too smooth. My new 15 mk2 is stronger on speed 3 than my old was with speed 6!!!
I am not the only one who have seen so big difference between two 15 mk2 units:
Do you use a Rupes Bigfoot mk2 15? Is your machine faulty? - Detailing World
I wonder why this is happening? Anyway im now happy that i have great 15 and 21 Rupes.