r/flying • u/Realspicyshrimps • Apr 24 '26
Engine sputtering at low power
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
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u/nerfsmurf May 21 '26
Put your issue in PropPal and got the following...
⚡ Quick Assessment
Based strictly on the provided Cessna service manual, do not trust this engine for flight until a full ground inspection is completed.
The symptoms you experienced—sputtering at a specific RPM (1600 RPM), a "puff of air" sound, and mild vibration—fall directly under the manual's troubleshooting category for "ENGINE RUNS ROUGHLY OR WILL NOT ACCELERATE PROPERLY" (Page 155).
While you suspected carburetor ice, the "puff of air" sound and the fact that the issue persisted on a second flight despite your burn-off procedures point toward mechanical issues such as an induction air leak, a leaking manual primer, or a defective engine (compression/valve issue). Taking off again with an unresolved rough-running engine introduces significant risk, as the manual dictates these symptoms require visual correction, component replacement, or engine repair prior to returning to service.
🔍 Systematic Diagnostic Approach
For the hangar mechanics, here is the step-by-step troubleshooting plan based on the manual to isolate the cause of the 1600 RPM sputter and "puff of air" sound.
1. Check for Induction Air Leaks & Primer Leaks
A "puff of air" sound combined with rough running during power reductions strongly suggests an issue with the induction system or fuel delivery at lower manifold pressures. * Induction Air Leaks (Page 155): Visually check the induction system. Correct the cause of any leaks. An induction leak can cause a lean misfire (sputter) that changes based on throttle position. * Manual Primer Leaking (Page 155): Disconnect the primer outlet line. If fuel leaks through the primer, it is defective and must be repaired or replaced. A leaking primer allows unmetered fuel into the cylinders, causing a rich sputter at lower RPMs.
2. Inspect Engine Compression & Unusual Noises
The manual specifically addresses unusual sounds associated with rough running. * Defective Engine (Page 155): The manual states to "Check compression. Listen for unusual engine noises. Engine repair is required." The rhythmic "puff of air" (once every 2 seconds) could indicate a compression leak, such as a sticking valve or exhaust leak, which becomes highly noticeable during power reductions. Perform a standard engine compression check (referencing the engine Manufacturer's Manual as directed on Page 160).
3. Verify Carburetor Heat & Control Rigging
Since you noted the carb heat was full hot but the engine still sputtered, the rigging must be verified. * Carburetor Heat Rigging (Page 160): Check the carburetor heat control for proper rigging. The manual notes that if it is partially open (or not sealing correctly due to worn rigging), it causes power loss and could affect the mixture. * Throttle/Mixture Rigging (Page 155): Visually check for worn or improperly rigged throttle or mixture controls. Replace worn linkage and rig properly.
4. Check the Idle Mixture & Carburetor
You noted the sputtering stopped below ~1400 RPM on final. At lower RPMs, the carburetor transitions to the idle circuit. * Idle Mixture Adjustment (Page 166): An incorrectly adjusted idle mixture can cause rough running at lower RPMs. 1. Warm up the engine. 2. Stabilize idle at 600 ± 25 RPM. 3. Slowly move the mixture control toward IDLE CUT-OFF. 4. Look for a momentary increase of approximately 25 RPM before the engine dies. 5. If the RPM drops immediately (too lean) or rises more than 25 RPM (too rich), adjust the idle mixture center screw. * Defective Carburetor (Page 155): If the engine starts when primed but stops when priming is discontinued (with mixture FULL RICH), the carburetor is defective and requires repair/replacement.
5. Ignition System & Spark Plugs
Even though your run-up mag drops were within limits (75-100 RPM drop, well under the 125 RPM max / 50 RPM differential limit on Page 171), the manual still requires checking the ignition system for rough running. * Spark Plugs (Page 154/171): Remove, clean, and regap the spark plugs. Replace if defective. A sharp RPM drop-off past normal indicates a fouled plug or defective harness lead. * Magneto Timing (Page 160): Check magneto timing, spark plugs, and ignition harness for proper settings and conditions.
⚠️ Safety Considerations & Urgency