I wouldn't recommend running a motor without a load either. It can be very bad... They will get hot and just don't like it. If you get lucky, the motor will greneade on you too.
i really dont think its the joints, i only soldered new 5mm on the c-50 but still both motors did the same thing, should i try to drive it maybe easy to see what happens, last time i drove it i didnt unload the motor it just drove so maybe it works fine, i will see