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Idle RPM surges

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Interesting thing this morning. On cold start and taxi out, RPM at about 900, the engine would occasionally slowly spool down to about 700 on its own. I’d bump the throttle a bit back to 900 and it would be fine for a bit then surge up 1200 or so on its own. I’d bring throttle back down, etc. Seemed to be running just fine otherwise, so I did a normal run up and decided to go ahead and fly. No problems. Perfect. Flew an hour and landed and taxied back in normally with no symptoms. I was leaned back like normal and it was probably 50 degrees outside. It was a clear morning, but a tiny bit of haze still out. It was maybe 8am, before the sun really got to drying things out.

Posted this elsewhere and some thought maybe carb icing. I’d be surprised if that is what it was.

Any thoughts or recommendations on where to start? It’s an O320-E3D.

My next step is to fly it again and see what it does and determine if I can establish any kind of trend or repeatable pattern. Right now it’s a bit of a one off.
 

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