Lean Times

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leaning the mixture

Lean times
I have been trying to come to grips with proper leaning of my Lycoming IO-360-A1A. It has the stock Bendix injection system and 10:1 pistons so it’s not a standard setup (due to the high compressions pistons).

Unlike many of you, I have only recently learned that I should be aggressively leaning on the ground. This procedure wasn’t utilized back when I learned to fly pistons, except at high altitude airports.
Lycoming issued a Service Bulletin on the topic while I was busy flying jets and had quit keeping up with GA. So I’m starting to implement that procedure to keep my engine happier.

But what about the rest of the flight? My old standard was to keep the red knob full forward until cruise and then pull it back until it stumbled and then richen it up bit – simple. But is this the best way to do it on my machine with all the fancy engine sensors?

The two predominate authors on the subject are Mike Busch and John Deakin. Both of these guys preach lean-of-peak (LOP) operations and both of them seem to center their discussions around injected 6 cylinder Continentals, or so it seems.


using the mixture

Not that their principles or techniques are flawed, but I’m struggling converting their words into a consistent procedure for my machine, nor do I want to run LOP all the time.

The old standard that I grew up with (lean it until it stumbled and then richen it up a bit) is making me nervous since these guys warn about a “red box.” This box is where your engine is under too much stress if you run it too close to peak EGT at high power settings, like climb or high-speed cruise.

I have been able to run LOP a couple of times, but most of the time it runs too rough. I have blamed this on non-balanced injectors, but after discovering I had a couple of bad spark plugs I need to do further testing. Honestly, I would love a more simplified approach.

In re-reading one of Mike’s articles he talks about using cylinder head temperature (CHT) as a key limiting factor. To me that feels more sensible, since in theory you aren’t harming your engine if you are limiting your CHT to some a number well below redline. Mike recommends 380F, which is below Lycoming’s economy cruise CHT number of 400F for my engine, thus confirming his practice (redline is 500F).

So what have I learned this far from reading volumes on the Internet? A lot really and that I have much more to learn. I need to do some more testing on my machine to be sure I’m comfortable taking theory into practice.
I may find a consistent setting that yields reasonable results or I might find that I need to work on fine tuning my injection system or something else.

So I am still learning and I’ll report back my findings.


AvWeb Pelican’s Perch – John Deakin 

AvWeb Savvy Aviator – Mike Busch

EAA Leaning Basics webinar – Mike Busch

If you have a technique that works well or thoughts on the matter, please share them here.

by Brent

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