Narrative:

Preflight inspection completed according to aircraft specific checklist. This included verification of oil quantity; verification of fuel quantity via aircraft specific dipstick; and sumping of fuel to check for contaminants. No issues were found during preflight. [I] observed on engine startup that the oil pressure gauge took more than 30 seconds to rise into green at 1000 RPM. Run-up mag check revealed right mag plugs fouled with 100 RPM drop from 1700 RPM. Increased throttle to 2100 RPM and leaned out mixture to clear plugs. [I] re-verified each mag operation normal with less than 75 RPM drop from 1700 RPM. Other run-up items operation normal according to aircraft specific checklist fully re-enriched mixture prior to departure. Mixture leaned applicably above 3;000 ft MSL and re-enriched; during any descents enroute. When at cruise at 6;500 ft MSL; throttle set to 2300 RPM and fuel mixture leaned accordingly. After approximately 20 minutes of cruise flight; engine RPM fluctuated severely between 2300 RPM and 2000 RPM during cruise flight. Engine noise fluctuated as well and ruled out tachometer failure. Mixture fully enriched and carb heat fully applied. Verified oil pressure in green; verified oil temperature in green; verified fuel selector set to fully on; verified left and right tank gauges showed equal and high quantity; verified both sides of master switched; verified mags on both. Engine RPM stabilized at 2300 RPM; carb heat deactivated after approximately a full minute of application. One to two minutes later; engine RPM fluctuated again between approximately 2300 RPM and 2000 RPM. Fully reapplied carb heat; reduced throttle to 2000 RPM and diverted to nearest airport. RPM fluctuations ceased at this power setting and maintained altitude of 6;500 ft MSL. Declared emergency with approach while on active flight following. Reduced engine RPM to approximately 1500 RPM when well within gliding distance of airport then began descent. Carb heat remained fully applied and mixture remained fully enriched during descent until landing. Changed to tower frequency when advised and reiterated declaration of emergency on initial call. [I] completed descending 360 turns over runway threshold in case of complete engine failure. Applied full flaps; idled engine and completed forward slip on short final for normal landing. [I] taxied to transient parking on engine power at 1000 to 1500 RPM with no issue. [I] overheard during flight that the egt gauge fluctuated over any few second intervals +/- 10 degrees regardless of fuel mixture leanness/richness. [I] verified against aircraft specific checklist that proper procedures were completed for such conditions; except as engine was still producing thrust at lower RPM; did not pitch for glide speed nor turn fuel off.

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Original NASA ASRS Text

Title: C152 pilot experiences fluctuating RPM during cruise and diverts to a suitable airport.

Narrative: Preflight inspection completed according to aircraft specific checklist. This included verification of oil quantity; verification of fuel quantity via aircraft specific dipstick; and sumping of fuel to check for contaminants. No issues were found during preflight. [I] observed on engine startup that the oil pressure gauge took more than 30 seconds to rise into green at 1000 RPM. Run-up mag check revealed right mag plugs fouled with 100 RPM drop from 1700 RPM. Increased throttle to 2100 RPM and leaned out mixture to clear plugs. [I] re-verified each mag operation normal with less than 75 RPM drop from 1700 RPM. Other run-up items operation normal according to aircraft specific checklist fully re-enriched mixture prior to departure. Mixture leaned applicably above 3;000 FT MSL and re-enriched; during any descents enroute. When at cruise at 6;500 FT MSL; throttle set to 2300 RPM and fuel mixture leaned accordingly. After approximately 20 minutes of cruise flight; engine RPM fluctuated severely between 2300 RPM and 2000 RPM during cruise flight. Engine noise fluctuated as well and ruled out tachometer failure. Mixture fully enriched and carb heat fully applied. Verified oil pressure in green; verified oil temperature in green; verified fuel selector set to fully on; verified left and right tank gauges showed equal and high quantity; verified both sides of master switched; verified mags on both. Engine RPM stabilized at 2300 RPM; carb heat deactivated after approximately a full minute of application. One to two minutes later; engine RPM fluctuated again between approximately 2300 RPM and 2000 RPM. Fully reapplied carb heat; reduced throttle to 2000 RPM and diverted to nearest airport. RPM fluctuations ceased at this power setting and maintained altitude of 6;500 FT MSL. Declared emergency with Approach while on active flight following. Reduced engine RPM to approximately 1500 RPM when well within gliding distance of airport then began descent. Carb heat remained fully applied and mixture remained fully enriched during descent until landing. Changed to Tower frequency when advised and reiterated declaration of emergency on initial call. [I] completed descending 360 turns over runway threshold in case of complete engine failure. Applied full flaps; idled engine and completed forward slip on short final for normal landing. [I] taxied to transient parking on engine power at 1000 to 1500 RPM with no issue. [I] overheard during flight that the EGT gauge fluctuated over any few second intervals +/- 10 degrees regardless of fuel mixture leanness/richness. [I] verified against aircraft specific checklist that proper procedures were completed for such conditions; except as engine was still producing thrust at lower RPM; did not pitch for glide speed nor turn fuel off.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.