Where to start - slight intermittent 'shudder' at cruise power

[smash]

Would appreciate thoughts to assist in troubleshooting an ongoing issue. IA is involved and will continue to be but we haven't been able to isolate yet.

Plane

• Archer w/ O-360A4M
• 400 SMOH (4 yrs ago) - at that time 1 new slick mag/1 OH slick installed, new Lycoming cylinders, OH carb, prop OH, engine mounts.
• Just completed annual - plugs cleaned, rotated. Tempest massives w/ ~450 hours.
• Do not have engine monitor or EGT/CHT gauges.

Situation

• At cruise power setting (~65%-75% 2200-2400 range) very slight, intermittent shudder/hesitation manifested by a slight vibration felt in the seat and floor board via feet (not in yoke).
• Doesn't feel as much like a structural vibration but rather power plant related.
• Symptom is intermittent and very short in duration (<1sec) but nonetheless it can be felt and isn't normal. Occurrence doesn't seem cyclical from a timing perspective (doesn't occur at consistent intervals).
• Not seeing an RPM drop nor any visual clues (i.e. gauges are normal)
• With headset off, not hearing an RPM change or miss.
• No issues on start up (i.e. no morning sickness) and no issues detected in climb outs.
• Tried lower RPMs inflight and didn't seem to have the symptoms in the 1500-1900 range. Idle and run ups on ground are normal.
• Mags checks inflight and ground within spec.
• Leaning inflight or leaving rich doesn't have seem to have an impact on symptom.
• Carb heat on or off no impact on symptom.
• Seems to be more pronounced once engine has operated for >20-30 mins and op temps have been reached/stabilized.
• Symptom has been occurring intermittently for a while now and not getting worse/better. More pronounced in summer flying than in winter.

Would appreciate thoughts on troubleshooting and isolating the issue. Glad to answer questions, provide more info. Thanks.

 

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[sierrakilo99]

 

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[smash]

 

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