What brand of batteries are you using? A motor will pull as much current as it can to get the motor to the speed the throttle is set to and if the batteries aren't up to the task, it can cog, stutter, or be slow.
sounds to me like the problems that some have been reporting on the MGMs. As soon as you give it to much throttle it goes into overcurrent safety and limits the rpm. an 8xl on 4s should be a fairly powerfull setup. More powerfull than any nitro thats for sure.
sounds to me like the problems that some have been reporting on the MGMs. As soon as you give it to much throttle it goes into overcurrent safety and limits the rpm. an 8xl on 4s should be a fairly powerfull setup. More powerfull than any nitro thats for sure.