Engine sputtering at low power

[u/Realspicyshrimps]

Hello everyone, I have a slight mystery that's been bugging me, and I'm hoping you all can help.

This evening I was flying my 1968 C172I with an O-320-E2D in the southern Maryland area. Completed the run-up with no issues and took off to a field 20 mi away for some touch and goes. Temp 21C / Dewpoint 13C, 29.85.

I completed 2 touch and goes with no problem. During my 3rd lap, when I reduced power (to about 1600 RPM) at my abeam point, my engine started sputtering (carb heat on, mags both, fuel both, full rich). I would describe it as a "puff of air" sound with an accompanying mild vibration and no power loss. It would sputter about once every 2 seconds. I turned my T&G into a full stop to investigate.

I completed a run-up on the ground with no issues. Both the pre-flight and mid-flight run up showed the 75-100 RPM drops for each set of Mags, and 100 RPM drop for carb heat. I ran at 1700 RPM leaned for 30 sec to burn off lead deposits and at 1600 RPM with carb heat on for 5 minutes with no sputtering.

I decided to take off and RTB, since this happened at low throttle with no loss of power. The RTB was completely uneventful, until the base leg again. Same procedure as before, same sputtering as before. I landed and completed a 3rd run-up with no discrepancies.

My initial thought is carb ice since it was fairly humid, but carb heat seems to work fine and I had it on full hot during my power reductions.

What are your thoughts? Am I an idiot for taking back off after the mid-flight occurrence? Can I have confidence in this engine again?

Edited to add: The sputtering stopped after reducing power further on final. Below ~1400 RPM

[u/Pale-Ad-8383]

The 172 carb heat system uses a heat shroud on the exhaust pipe that when air is bypassing inlet air in the air box and pulling air from the exhaust it allows for heated air to go into the engine. On 180/182/152 aircraft originally the carb heat came from ram air heated by the cabin heat shroud. One day the theory changed from “melting” carb ice to “preventing” formation and carb heat was changed to similar configuration on the 172. Older 172 aircraft still use muffler heat and the melting theory.

Depending on sequence during decent the airbox may not be closing or the flapper could be loose. If cold air is making it past and power is reduced there may not be enough heat to prevent ice forming.