The pilot reported he arrived at Cooktown airport at 0615 EST to carry out a flight to Weipa. After carrying out a pre-flight inspection, and submitting a flight plan, he departed Cooktown at 0650. The flight was being conducted below cloud at 2,500 ft until about 130 NM from Cooktown when the cloud could be seen on top of the McIlwraith Range ahead. The pilot said he commenced a turn through 180 degrees and descended to 2,000 ft to clear the southern end of the ranges. Development Road was identified and the pilot was about to commence tracking towards Cohen when the engine suddenly began to run roughly and vibrate. Power was immediately reduced and trouble checks were carried out but to no avail. The pilot said he had the impression that the engine was running on three cylinders. Due to the rising ground ahead the pilot made the decision to land on the road. During the landing roll the left wing impacted some small trees on the road verge which slewed the aircraft to the left. The nosewheel then got caught on the left side of the rill in the middle of the road and the pilot was unable to get it back to the right before the left wing impacted the side of a cutting. The aircraft was slewed through 90 degrees and the propeller struck the embankment. The aircraft then slid sideways for 20 metres into a table drain, and the right main and nose landing gear assemblies were torn off. The cause of the engine malfunction was later found to be a failed exhaust valve.