Hum... are you using the sensored cable with the MMX? Or is it off? (you need a separeted "gizmo" for sensored use um X's)

Castle before didn't have sensored capability on the Mamba Monsters ESC's...only on the little one Mamba Max Pro.

Also i strong believe that the Mamba Monster X or 2 or 1 are ESC's for more then 3s.... using them on 2s seens useless.

For 2s use a competetion ESC like Hobbywing or Trackstar or even SkyRC....120A ones sensored.

You will like the "turbo" and "smoothness" that come out of them!

There is nothing wrong with your ESC, it's just not made for sensored or 2s use.

Try to put some timing on the ESC (little) and trim forward on radio throttle to!

My bet is the timing software on the Castle does not meet the motor's criteria... and of course 2s if you are using that lipos on a Mamba Monster! Check what manufacter's website state about that speficic motor timing....