The aircraft was being flown at about 900 feet above ground level while the pilot spotted horses for a programmed muster. When the pilot attempted to increase engine power, he found that there was no response to throttle movement. Pumping the throttle resulted in a decrease in power to idle. The pilot was unable to correct the situation and selected the best available area for landing. This area was covered with small basalt boulders. On touchdown the left wing struck a small dead tree and the propeller struck the ground before the aircraft came to rest. It took the pilot some time to exit the cabin of the aircraft because the doors had been jammed during impact. An inspection of the aircraft revealed that the outer throttle cable had come loose from the clamp at the carburettor. This allowed power to be reduced but not increased.