Narrative:

The first flight of the day began at ZZZ with myself (the pilot) and 2 passengers departing; gas was to the tabs (36 us gal). OAT at the time of first departure (around 1100L) was 80F and although the aircraft was ~20lbs short of mgtow; climb performance was as expected in the 500-700 FPM range (I have performed several takeoffs in this aircraft before while loaded near max takeoff and performance has always been satisfactory even on hot days). The flight to the intended destination of ZZZ1 was uneventful with the engine running smoothly for the duration of the flight. Upon landing; the aircraft was secured on the ramp.several hours later; the pilot and passengers returned to the aircraft; a normal preflight was accomplished with no issues noted. There was no fuel added to the aircraft at ZZZ1. During the engine run up the lh mag showed about 100 RPM drop and the rh showed ~65 RPM. Both slick mags have 500 hours since new. At the time of the event takeoff from runway xx at ZZZ1; OAT was reported by ZZZ2 as ~88F with winds 040 at 11. During takeoff; the aircraft acceleration appeared to be slower than anticipated with the airspeed indicator bouncing +/-5 kcas around acceleration during the roll. I do not recall engine RPM during takeoff. Once liftoff speed was attained; moderate nose down pressure had to be applied to maintain vy. The climb rate while maintaining vy was 100-200 FPM; even with flaps retracted. Upon reaching ~1100 ft. MSL; a bump/jolt was felt through the aircraft and engine RPM momentarily dropped ~200 RPM and quickly recovered. Engine indications appeared normal following RPM recovery. Concerned with a potential imminent engine failure over swamp/river terrain due to the low climb performance coupled with jolt and engine RPM drop; I elected to [advise ATC] and divert to ZZZ2 which at the time was about 2 miles off the rh side of the aircraft. The diversion into the pattern and landing was non-eventful and engine power was not lost.following the landing; I inspected the exterior of the aircraft and the engine/firewall area and nothing remarkable was found. An a&P mechanic based on the field came out and also inspected the aircraft and engine and performed full power static runs and mag checks and found no major issues. The lh mag was noted to be inconsistent in the RPM drop during mag checks dropping between 20-100 RPM. Finding nothing wrong with the power plant; I elected to arrange alternate travel means for the passengers and ferry the aircraft back to the home airport solo in order to improve performance margin and reduce the evacuation risk in worst case of a ditching.during pre-flight before the ferry flight; I noticed a small paint fleck in the pitot hole of the airspeed probe. I was able to blow it out of the hole easily. Prior to takeoff; mag checks were again performed and a static full power takeoff was performed with no issues noted. The aircraft performed normally for the entire duration of the return flight with no issues.probable cause for the event is believed to be a paint fleck; from the painted airspeed mast; partially covered the pitot port and resulted in an indicated airspeed lower than actual during takeoff and climb out from ZZZ1. As a response I held the nose down to maintain the improperly indicated vy; resulting in significantly lower climb rate than the aircraft was capable of. Compounding the perceived issue was a potential momentary mis-fire of the lh mag that has not been reproduced subsequently. Both mags were sent in for overhaul following the ferry flight.

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Original NASA ASRS Text

Title: Pilot reported an air turn back after low airspeed and engine RPM drop.

Narrative: The first flight of the day began at ZZZ with myself (the pilot) and 2 passengers departing; gas was to the tabs (36 US gal). OAT at the time of first departure (around 1100L) was 80F and although the aircraft was ~20lbs short of MGTOW; climb performance was as expected in the 500-700 FPM range (I have performed several takeoffs in this aircraft before while loaded near max takeoff and performance has always been satisfactory even on hot days). The flight to the intended destination of ZZZ1 was uneventful with the engine running smoothly for the duration of the flight. Upon landing; the aircraft was secured on the ramp.Several hours later; the pilot and passengers returned to the aircraft; a normal preflight was accomplished with no issues noted. There was no fuel added to the aircraft at ZZZ1. During the engine run up the LH mag showed about 100 RPM drop and the RH showed ~65 RPM. Both Slick mags have 500 hours since new. At the time of the event takeoff from Runway XX at ZZZ1; OAT was reported by ZZZ2 as ~88F with winds 040 at 11. During takeoff; the aircraft acceleration appeared to be slower than anticipated with the airspeed indicator bouncing +/-5 KCAS around acceleration during the roll. I do not recall engine RPM during takeoff. Once liftoff speed was attained; moderate nose down pressure had to be applied to maintain Vy. The climb rate while maintaining Vy was 100-200 FPM; even with flaps retracted. Upon reaching ~1100 ft. MSL; a bump/jolt was felt through the aircraft and engine RPM momentarily dropped ~200 RPM and quickly recovered. Engine indications appeared normal following RPM recovery. Concerned with a potential imminent engine failure over swamp/river terrain due to the low climb performance coupled with jolt and engine RPM drop; I elected to [advise ATC] and divert to ZZZ2 which at the time was about 2 miles off the RH side of the aircraft. The diversion into the pattern and landing was non-eventful and engine power was not lost.Following the landing; I inspected the exterior of the aircraft and the engine/firewall area and nothing remarkable was found. An A&P mechanic based on the field came out and also inspected the aircraft and engine and performed full power static runs and mag checks and found no major issues. The LH mag was noted to be inconsistent in the RPM drop during mag checks dropping between 20-100 RPM. Finding nothing wrong with the power plant; I elected to arrange alternate travel means for the passengers and ferry the aircraft back to the home airport solo in order to improve performance margin and reduce the evacuation risk in worst case of a ditching.During pre-flight before the ferry flight; I noticed a small paint fleck in the pitot hole of the airspeed probe. I was able to blow it out of the hole easily. Prior to takeoff; mag checks were again performed and a static full power takeoff was performed with no issues noted. The aircraft performed normally for the entire duration of the return flight with no issues.Probable cause for the event is believed to be a paint fleck; from the painted airspeed mast; partially covered the pitot port and resulted in an indicated airspeed lower than actual during takeoff and climb out from ZZZ1. As a response I held the nose down to maintain the improperly indicated Vy; resulting in significantly lower climb rate than the aircraft was capable of. Compounding the perceived issue was a potential momentary mis-fire of the LH mag that has not been reproduced subsequently. Both mags were sent in for overhaul following the ferry flight.

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.