C172 pilot reports two consecutive precautionary landings while performing test and break-in of rebuilt engine. Carburetor malfunction causes the first and fuel starvation causes the second when fuel that is lost and wasted during the carburetor malfunction is not considered.

Narrative

This report describes 2 related incidents during flight testing following the installation of an overhauled engine. 2 separate flights were conducted in the immediate vicinity of the airport. The initial test flight was terminated after 15 minutes due to a loss of engine power at an altitude of 6;000 FT. A precautionary landing was conducted without incident but the engine quit on exiting the runway. At this time; fuel was observed draining from the cowling. The fuel was shut off and the aircraft was towed to a maintenance hangar. Inspection by mechanics indicated a stuck carburetor float; most probably due to foreign matter. Following repair; reassembly; and an extensive run-up; a second flight was conducted in order to continue engine test and break-in; including a climb to 8;500 FT. I then descended to 6;500 FT and orbited above the airport. During this time; higher than normal cruise power settings were used in accordance with the engine shop's recommended break-in procedure. After approximately 1 hour of flight; I began a descent in order to enter the traffic pattern. Shortly afterwards; the engine began to run rough and I expedited descent and a return to the field while attempting to determine the cause. Upon entering a high downwind leg; fuel pressure dropped to zero and the engine quit completely. Restart was unsuccessful. The aircraft was landed on the in-use runway without incident and coasted to a stop on a taxiway. Inspection by mechanics indicated that although the gauges indicated fuel was remaining in both tanks (1/8 on both sides); the tanks were below usable levels and the engine had died of fuel starvation. For both flights that day; the amount of fuel on board had been estimated based on flight times; not by direct measurement. By that estimate; 1 hour 40 minutes of fuel was on board for the planned 1 hour second test flight. Extended ground operations combined with fuel lost during the episode with the stuck float resulted in less fuel available than expected; leading to running out of fuel after approximately 1 hour 10 minutes of flight.