You didn't say what motor you have. Some can be run on muffs and some can't. Runing on muffs will not tell you if your water pump is working properly, the water pressure will override the pump and cool the engine even if the pump is shot. If you run in a barrel, make sure water level is above the water pump.
You probably shouldn't exceed 1500-2000 RPM while running on muffs or in barrel.