Tuesday August 19, 2025
Engine Idling Solved and Transponder/Static Checks (Firewall Forward)

Last night, I was reading through the Airflow Performance installation manual again looking for inspiration on my rough idling problems and ran across this statement:

The idle mixture adjustment is sensitive. Do not adjust more than one to two flats at a time. Normal adjustment should not require more than one full turn in either direction.

Well, I had missed this before I adjusted the idle mixture yesterday and made very large adjustments (two times in fact) toward the lean direction. It suddenly occurred to me that my current symptoms would be completely explained by running far to lean at idle. In fact, the first run of the plane before I adjusted the idle mixture had been smoother than any of the others. This would explain everything! I could hardly wait to get out to the airport and test my theory.

This morning, I adjusted the idle mixture about a turn and a half back toward rich and then restarted the engine. Bingo! It immediately ran much better, without any of the big cylinder misses I had been having. I ran it up again to 1800 RPM and the mag check is still about ~100 RPM drop.

I think the idle mixture is a bit rich now, because I had to lean out to get a smooth idle, but it will idle smoothly when appropriately leaned now. Once I shut down the engine, I turned the adjustment one flat back toward idle. I'm so happy this is solved.

I had scheduled my transponder and static system check today, so we got that done. As soon as the tech saw my static/pitot/AOA fittings (the slide on type), he mentioned that he has seen a lot of leaking issues with them, so I was a bit worried. However, everything checked out nearly perfectly.

The altimeters were bang on. In fact, the GSU 25 (ADAHRS) was almost perfect, except for the 18,000 and 20,000 feet checks where it was 5 feet low. The G5 altimeter is about 10 feet low at every test altitude. He suggested we could recalibrate it at the next check in two years if it is still off. Apparently the G5 sensors sometimes relax a bit on their own.

The transponder tested just fine--no issues there. Since this is a new build plane, he also tested the airspeed indications for me--they are very close as well.

So as of today I think the engine is ready to go and I have one more piece of paper toward my airworthiness!

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