Do you idle at idle?

[Aluminum]

Back in a (newer) Cessna, I'm reminded of the "high idle" practice to keep the battery from discharging while awaiting clearance, and to keep the vacuum gyros spinning. This seems like the kind of silly systems design blunder that should have been solved in the past millennium and gone the way of dodo and carb heat--why do we still have to worry about it in this day and age!? Idle should be idle, without catastrophic consequences.

Do you idle your Lycoming at idle, or do you crack it open to a thousand RPM while stationary?

[N941WR]

1,000 RPM for me

I was always told it helps prevent fouling and helps keep the engine cool due to air movement.

As for carb heat, I have that too, along with a carburetor.

[rzbill]

Originally Posted by N941WR

As for carb heat, I have that too, along with a carburetor.

Didn't the archaeologists dig up a Marvel and a carb heat lever in the Neander valley some time ago, Bill?
Just kidding of course. Hope to see you at GMU some weekend. My SWMBO has kept me too busy to go.

[N941WR]

Originally Posted by rzbill

Didn't the archaeologists dig up a Marvel and a carb heat lever in the Neander valley some time ago, Bill?
Just kidding of course. Hope to see you at GMU some weekend. My SWMBO has kept me too busy to go.

Getting the centuries of dust out of it was a real pain but the thing starts every time with no hot start issues.

We need to get together some time Bill

[Jesse]

Was the high idle not a practice because of the generator, which doesn?t put out much power at idle, versus the alternator, which does?

[vic syracuse]

Our Lycomings are all about splash lubrication as well. 1000 or higher is a great idel speed, for lubrication & ignition.

Vic

[Carl Froehlich]

Verify the engine can idle at full rich down at 750 RPM or so with the throttle on the idle stop. Then never leave it that way as this is how spark plugs get fouled.

For non-taxi stuff where you are not riding the brakes, Idle at 1000+ RPM and mixture almost to cutoff (if the engine bogs when you add throttle you are just lean enough). This will reduce lead fouling of the plugs.

Carl

[Brantel]

Originally Posted by Carl Froehlich

For non-taxi stuff where you are riding the brakes, Idle at 1000+ RPM and mixture almost to cutoff (if the engine bogs when you add throttle you are just lean enough). This will reduce lead fouling of the plugs.

Carl

That's good advice right there!

[lr172]

Originally Posted by Carl Froehlich

Verify the engine can idle at full rich down at 750 RPM or so with the throttle on the idle stop. Then never leave it that way as this is how spark plugs get fouled.

Carl

My idle speed is set around 700. My idle mixture is properly set (lowest MAP method). My spider is properly calibrated. I run an idle advance of 35*. I run up to 1500 for 15 seconds before every shutdown. I have never attempted to lean on the ground, as I don't think it is realistic to lean with the red knob for ground operations.

I have 600 hours on my IO-320 and have never had a fouled plug. I may find a couple of very small lead balls when I clean and re-gap the plugs at each condition inspection, but have never had a plug not fire.

Just offering a counter-point to this commonly held belief. A properly set up FI servo should not require efforts such as this. I only had my carb for 80 hours so can't offer the same guidance. However, I have tuned numerous 4 barrel carbs on hot rods and they are very effective at their role when setup properly.


To the OP, I don't run an idle higher than 750 unless the circumstances warrant, such as rough idle due to fuel boiling, Egnine warm up, heavy electrical load, etc.

Higher idle speeds will reduce fouling (due to higher combustion temps) and increase alternator output. However, that in and of itself is not a reason to make it an SOP. There is plenty of oil splash at 700 RPM. However, running 1000 for the first minute of two after start is a good idea to accelerate the coating of parts after sitting for more than a day.

If you do some research, you will find that the typical fouling problems we see are lead fouling and not carbon fouling. Lead fouling happens when combustion temps are not high enough to convert the lead to lead bromide (which doesn't build up). The answer to this is more heat, not necessarily less fuel. Idling at 1000 rpm will reduce fouling (at any mixture), but there are other ways to accomplish this. In order to get carbon fouling, you need to be in the filthy rich region and the answer is proper calibration, not manual leaning.

I would add MMO to my gas well before leaning on the ground, if I had this problem. MMO was designed to ease the conversion of lead to lead bromide and will reduce lead fouling by allowing the conversion at a lower temp.

In defense of those who hold this belief, leaning to best power or peak EGT does provide an increase in combustion heat. However, this really only applies if the heat is artificially low due to being too rich. This can usually be addressed with proper idle mixture adjustment. At RPMs below 1200, the overall mixture is heavily influenced by the idle mixture adjustment.

Larry

[Carl Froehlich]

Sorry Larry - but I disagree with most of your thoughts.

A couple of Mike Busch articles that cover this topic:

Start on page 4, https://www.savvyaviation.com/wp-con...exh-valve).pdf

https://www.aopa.org/news-and-media/...re-in-the-hole