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RPM-drop for some seconds at climb-out

Joined
Aug 26, 2017
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I currently have an issue for the first time I would like to get some thoughts from you:

Yesterday at first flight of the day during a moderate climb I recognized a slight RPM drop for maybe 2-3 seconds. Flight time had been maybe 30-45 seconds till then. Afterwards I was flying without any further issues on three additional flights including take-offs and landings.

Engine is a Lycoming O320 E3D with engine TT by 3000 hours, last complete overhaul had been in 2009 at 2000 hours.

Magnetos passed 70 hours ago a 1000-hour inspection including replacing used parts. SB 388 and borescope inspection have been also conducted 70 hours ago with no issues. All spark plugs were replaced at last annual. Oil analysis showed no high nickel data ever, I am also using camguard + Philips X/C oil btw. The aircraft has no engine-monitor. The plane is always hangared, draining at pre-flight check showed nothing wrong like water, sediments etc.. Magneto check was also fine, no rough running even with leaned power settings.

The night before we had a rapid weather change means temperature coming down from 40 °F to 21 °F. The relative humidity has also dropped from 99 % to 78 %. I pre-heated the engine but no longer than 30 minutes and only with mild warm air supplied to the cowling inlets, engine´s carburator is located at the bottom.

Carb-ice e.g. means ice trapped in the carburator housing or somewhere else from the night before? How would you proceed? Leave it like it is? If I would tell this story to a maintenance shop I am afraid to start with a long-lasting investigation but with no real outcome…. And an engine monitor is a top list item now....

Thank you for any reply.
 

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