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Do you idle at idle?

Aluminum

Well Known Member
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?
 
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.
 
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? :rolleyes:
Just kidding of course. Hope to see you at GMU some weekend. My SWMBO has kept me too busy to go.
 
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Didn't the archaeologists dig up a Marvel and a carb heat lever in the Neander valley some time ago, Bill? :rolleyes:
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
 
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?
 
Our Lycomings are all about splash lubrication as well. 1000 or higher is a great idel speed, for lubrication & ignition.

Vic
 
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
 
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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!
 
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
 
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An archive search will show that Don Rivera of Airflow Performance does not believe leaning on the ground is necessary for a properly set up fuel injection system. Does no harm either however.

erich
 
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.

SOP for all of our company airplanes during taxi is to position the mixture as lean as the engine will allow and still have normal throttle response (for the amount of throttle needed for taxi operations).

Keeping the idle at 1000+ is also helpful in preventing lead fouling as well, but if aggressive leaning is used, it is never a problem.

I can't remember the last time we have had an engine operation problem because of a fouled plug.
 
SOP for all of our company airplanes during taxi is to position the mixture as lean as the engine will allow and still have normal throttle response (for the amount of throttle needed for taxi operations).

Keeping the idle at 1000+ is also helpful in preventing lead fouling as well, but if aggressive leaning is used, it is never a problem.

I can't remember the last time we have had an engine operation problem because of a fouled plug.

The scary part of leaning on the ground like that is the time someone forgets they leaned, then taking off with now an overly lean condition. I lean all my planes, even FI, until they cough when advancing the throttle. Thus a friendly reminder and assured no plug issues.
 
Ground leaning

I’ve always leaned lycomings at idle to the point they will barely run. If you open the throttle and the engine wants to die from fuel starvation you have it leaned about right for taxi. There is no advantage to running any extra fuel thru the engine. A well set up fuel delivery system may not foul plugs without ground leaning, but most lycomings I’ve worked on will especially in hot weather. You cannot screw this up and take off with the mixture set too Lean if the engine won’t run much above idle. When ready to take the runway go to full rich.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
1,000 RPM for me

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

This is the urban legend that baffles me, pardon the pun. You are carrying several pounds of extra metal in the injection system (or carb for that matter) that are there only to ensure the correct MAP and mixture at idle. I see no purpose in defeating this setting by cracking open the butterfly.

And I can't see how cooling could be made better by burning more fuel to spin the prop faster and so put more energy into the system just so you could remove it faster?
 
Idle

I have fuel injection, P-Mags and my engine will idle nicely @ 720 and a little rough at 680. So I keep it set for 620. With this setup I keep it idling about 800 during ground ops.
I want it to stall when on the stops. This way when landing my idle in forward motion is about 800-900. This allows a much shorter landing for a fixed pitch RV-9.
I also lean aggressively on the ground.
 
SOP for all of our company airplanes during taxi is to position the mixture as lean as the engine will allow and still have normal throttle response (for the amount of throttle needed for taxi operations).

Keeping the idle at 1000+ is also helpful in preventing lead fouling as well, but if aggressive leaning is used, it is never a problem.

I can't remember the last time we have had an engine operation problem because of a fouled plug.

I agree that proper leaning is critical at idle in order to prevent lead fouling. I just believe that proper leaning at idle can be achieved with the idle mixture screw, eliminating the need for manual leaning. The manufacturer's design a separate idle circuit with it's own adjustment for just this reason. That said, if you don't keep that idle mixture where it needs to be, manual leaning will help to get it properly leaned, but only at a specific RPM/MAP setting.

Something to consider. Normal idle is in the neighborhood of 1.5 GPH and normal taxi around 2.5-3 GPH. If you set your mixture (the red knob only sets max fuel flow into the servo, nothing else - it uses a simple valve and the restriction/flow set with that valve does not change with air flow) for 3 GPH to support taxi, how is this helping to lean your mixture at idle.

Larry
 
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I agree that proper leaning is critical at idle in order to prevent lead fouling. I just believe that proper leaning at idle can be achieved with the idle mixture screw, eliminating the need for manual leaning. The manufacturer's design a separate idle circuit with it's own adjustment for just this reason. That said, if you don't keep that idle mixture where it needs to be, manual leaning will help to get it properly leaned, but only at a specific RPM/MAP setting.

Something to consider. Normal idle is in the neighborhood of 1.5 GPH and normal taxi around 2.5-3 GPH. If you set your mixture (the red knob only sets max fuel flow into the servo, nothing else - it uses a simple valve and the restriction/flow set with that valve does not change with air flow) for 3 GPH to support taxi, how is this helping to lean your mixture at idle.

Larry

I'm with Larry on this one, set the idle mixture correctly, and there is no need to lean on the ground.
 
I'm with Larry on this one, set the idle mixture correctly, and there is no need to lean on the ground.

Except that you set the mixture at sea level... What happens when your breakfast destination is at 8000 MSL and home base is 4000? That red knob comes in real handy for taxi and runup then.
 
Except that you set the mixture at sea level... What happens when your breakfast destination is at 8000 MSL and home base is 4000? That red knob comes in real handy for taxi and runup then.

You are describing why I lean on the ground. Set the carb up at 1,440 MSL but would go to breakfast at Big Bear (6,752), Hesperia (3,390), Apple Valley (3,062) that were all much higher. Would sometime go to Ocean (L52) and that was only 14' MSL.
 
Originally Posted by Walt
I'm with Larry on this one, set the idle mixture correctly, and there is no need to lean on the ground.

Except that you set the mixture at sea level... What happens when your breakfast destination is at 8000 MSL and home base is 4000? That red knob comes in real handy for taxi and runup then.

OMG You're absolutely correct, setting the mixture properly at your home field is a waste of time, it probably wouldn't work well if you were in OUTER SPACE either :eek:
 
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Exactly my point Walt. You don't set your idle mixture for "your" home field; you set it for sea level. Setting it "correctly" for a high elevation, though convenient, would ensure that it was too lean when you DID go down to sea level - and the red knob won't help you then.

Not sure how often you stray from your home airfield, but an 8,000+ altitude deviation is a very real mission for some of us.
 
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If you adjust idle mixture properly, you should have a 50rpm, or so, rise before the engine dies as you “smoothly” pull to ICO. Yes?
If you have a 50rpm rise, one could reason you have room to lean on the ground. I do, and, I do. Whether or not you need to is an argument I will leave to others. I just don’t think it is that big of a deal.

I set my idle mixture for my home field, cause that is where I wrench on it. (250’ MSL) I fly frequently to our Coast home and that Field is 20’ MSL. Don’t notice any difference.

As far as the OP’s question, I move off the stop slightly as both of my machines idle well at quite low rpms, just the way I like it. I don’t have an rpm “number”, I just do that pilot “Shi...” and all seems to be well.
 
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On a hot day, my carbureted Lycoming will die at full rich when I push the throttle forward for takeoff. I did a lot of troubleshooting over several years which eliminated the possibility of vapor lock. The bottom line was that the heat from the oil sump was heat-soaking the carburetor and (probably) causing the 100LL to boil in the float chamber. The fix (for me, again) was to install a 1/16" phenolic gasket between the carb and the oil sump for thermal insulation, add more cooling air to the oil cooler and, while waiting in a conga line for takeoff on a hot day, keep the RPM up to 1300. Even so, on a hot day, I have to lean a little bit when taking off to keep the engine from stumbling. I go full rich as soon as the engine runs smooth. Disclaimer: This applies to my engine only and I'm not suggesting that anyone else try what I am doing.
 
On a hot day, my carbureted Lycoming will die at full rich when I push the throttle forward for takeoff. I did a lot of troubleshooting over several years which eliminated the possibility of vapor lock. The bottom line was that the heat from the oil sump was heat-soaking the carburetor and (probably) causing the 100LL to boil in the float chamber. The fix (for me, again) was to install a 1/16" phenolic gasket between the carb and the oil sump for thermal insulation, add more cooling air to the oil cooler and, while waiting in a conga line for takeoff on a hot day, keep the RPM up to 1300. Even so, on a hot day, I have to lean a little bit when taking off to keep the engine from stumbling. I go full rich as soon as the engine runs smooth. Disclaimer: This applies to my engine only and I'm not suggesting that anyone else try what I am doing.

....and that is doing your pilot ?Shi....?
Every machine I have owned has behaved a bit different. You set them up per the manual, then you deal with the specific issues, fix or fine tune what you can, and operate accordingly.
 
....and that is doing your pilot “Shi....”
Every machine I have owned has behaved a bit different. You set them up per the manual, then you deal with the specific issues, fix or fine tune what you can, and operate accordingly.

I have not had the extent of leaning issue that snopercod discussed but I do religiously lean on the ground.

On that subject I had a "doing stupid pilot sh**" episode a few years ago.
My bride and I were on our first trip to the west and I was getting very comfortable with leaning on TO at high altitude airports. No biggie.
However, on landing at Grand Canyon field, for some reason unknown to me, I pushed the mixture in while turning off the runway. Engine immediately died. So I'm sitting there on the taxiway doing a flooded injected hotstart and the tower asks if I need help. "NOOOO" I say. How embarrasing. :rolleyes:
 
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I think habitually adjusting the red knob to compensate for my often dramatically different airport DA's makes for good awareness at the very least. The opposite situation is like that Stinson crash on youtube a few years ago where the guy took off high, hot and heavy, flew for several minutes into rising terraign and crashed into the trees. ALL WITHOUT MOVING THE MIXTURE FROM FULL RICH!
 
Except that you set the mixture at sea level... What happens when your breakfast destination is at 8000 MSL and home base is 4000? That red knob comes in real handy for taxi and runup then.

This sounds fishy to me. Carbureted cars and motorcycles have no trouble idling at Loveland Pass. Rotaxes and Jabirus use Bing carbs which lean automatically at cruise powers, but do not change idle mixture with altitude. I never heard of idle problems with either of these engines. Why would injected Lycomings and Continentals be inferior? Old habits die hard.
 
OMG You're absolutely correct, setting the mixture properly at your home field is a waste of time, it probably wouldn't work well if you were in OUTER SPACE either :eek:

And then, when the airplane did go down to sea level, it would be too lean. The potential consequences of that outweigh any argument for not manually leaning at the home field.
 
This sounds fishy to me....

Yep, there is no reason at all to take my word for it. But please perform a simple test and report back. Adjust your idle for the appropriate 30 RPM rise at sea level and take note of how far out the mixture comes out to achieve that max RPM. Then fly up to a high altitude airport and do the same thing. My bet is that you will see a lower idle speed, a correspondingly greater RPM rise to peak, and the red knob is going to come out much further.

In other words, the engine is fat at high altitude.

I fly from a 2600MSL strip and routinely lean to max RPM for idle and runup. After doing this hundreds and hundreds of times I have a very good feel for the distance the mixture knob moves. I can tell you that this "normal" is significantly different than when I'm at sea level and also when I'm at 7000 feet. Its also different in the winter at 20 degrees than it is in the summer at 120 degrees. The fat/lean thing is not a "habit".
 
This sounds fishy to me. Carbureted cars and motorcycles have no trouble idling at Loveland Pass. Rotaxes and Jabirus use Bing carbs which lean automatically at cruise powers, but do not change idle mixture with altitude. I never heard of idle problems with either of these engines. Why would injected Lycomings and Continentals be inferior? Old habits die hard.

The thing is cars & motorcycles engines of today are like Rotex & Jabiru (modern design & systems) Lycoming ,Continental ,Franklin & all the old Raidials are 1940's technology ( really not much state of the art past Henry Ford's model "T")

Fule injection is an add on as well as electronic ignition to an antique design that actually works quite well once you understand it's a manual system. Not computer controlled.
 
I still fail to understand how you guys are leaning manually. If I were to manually lean to 14:1 AFR (about optimum for our class of engines) for a no load 750 RPM, my engine would be coughing and pucking well before I got to 1000 RPM. I get manually leaning for an extended idle, but if you are able to taxi at 1000 RPM, I can assure you that your purely on the idle mixture setting at 750, with no restriction from the red knob.

If you don't believe me, on your next flight observe the fuel flow at idle and at your taxi RPM. Knowing that the red knob simply restricts raw input flow into the servo circuits, then you can explain how you are properly leaned at both settings.

unless you are moving the red knob in conjunction with the black knob, you are not at optimum AFR at all settings during taxi/ground ops. This is the simple mechanics of the servo design. And the reality is that you need the proper mixture more at idle (to keep combustion temps as high as possible) than you do at 1000 RPM if your goal is reducing lead fouling. I just don't see how you can manually lean to a proper AFR at idle and still be able to taxi.

I have heard all the testimonials that with manual leaning, users are not experiencing fouling. Well, I don't manually lean and I don't experience lead fouling. So I remain unconvinced that manual leaning is REQUIRED to avoid fouling if your idle is set properly for your altitude.

Larry
 
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Yep, there is no reason at all to take my word for it. But please perform a simple test and report back. Adjust your idle for the appropriate 30 RPM rise at sea level and take note of how far out the mixture comes out to achieve that max RPM. Then fly up to a high altitude airport and do the same thing. My bet is that you will see a lower idle speed, a correspondingly greater RPM rise to peak, and the red knob is going to come out much further.

I'm flying a Jab with the Bing carb. It idles fine from at least negative-1000 to at least 7000 DA in my experience so far. No red knob. Spark plugs look clean after 100 hours, will keep them another 100. Can't believe the fancy fuel metering on the IO-390 won't do at least as good a job mixing fuel as the flood-prone motorcycle carb of yore?

Now, what prompted my rant: a two year old $600k C182/G1000 comes with a (sigh) vacuum pump, which fails to maintain vacuum at idle, resulting in a very loud "CLANG!" alarm every time the black knob is pulled back. No wonder pilot population is declining, we've all gone deaf.

WHAAT?

:p
 
I like the chalky grey/white color inside if my exhaust pipes that i get when i lean on the ground manually. I don?t like the sooty black buildup that i find when i dont lean for taxi.

As far as needle adjustments, i do them once in the fall and once in the spring to get a 50 RPM rise at ICO. It is far too inconvenient to make this adjustment before every flight.

Don
 
...So I remain unconvinced that manual leaning is REQUIRED to avoid fouling if your idle is set properly for your altitude...

For some of us, it's very hard to define "your altitude". My strip (2600MSL) is within 30 minutes flight time from a mountain strip thats almost 7000 MSL AND a strip thats more than 200 feet BELOW sea level. Since I visit both places, my airplane has to be set up for the worst case (sea level), so I'm going to be manipulating the red knob anyway. Certainly you cant be advocating that a local Denver pilot adjust his mixture to "his altitude", right? If he ever goes down to sea level the mixure will be far too lean - so much that it might even quit as soon as he turns off the runway.

The fact is, even if adjusted perfectly, there's a 30 RPM rise as you lean on the red knob. That means there is at least 30 RPM of "fatness" in the mixture. I cant speak for others, but I routinely remove all that fat with the red knob and idle/taxi on the ragged edge to going rough. I do this by ear and it takes a split second to accomplish. Is it required? Probably not if I flew the flatlands, but it sure takes the chugging and black smoke out when I'm at high DA.
 
For some of us, it's very hard to define "your altitude". My strip (2600MSL) is within 30 minutes flight time from a mountain strip thats almost 7000 MSL AND a strip thats more than 200 feet BELOW sea level. Since I visit both places, my airplane has to be set up for the worst case (sea level), so I'm going to be manipulating the red knob anyway. Certainly you cant be advocating that a local Denver pilot adjust his mixture to "his altitude", right? If he ever goes down to sea level the mixure will be far too lean - so much that it might even quit as soon as he turns off the runway.

The fact is, even if adjusted perfectly, there's a 30 RPM rise as you lean on the red knob. That means there is at least 30 RPM of "fatness" in the mixture. I cant speak for others, but I routinely remove all that fat with the red knob and idle/taxi on the ragged edge to going rough. I do this by ear and it takes a split second to accomplish. Is it required? Probably not if I flew the flatlands, but it sure takes the chugging and black smoke out when I'm at high DA.

That makes sense and I suppose I would do the same with that much altitude variation. I usually set my idle when the temps are 70 and I can go as low as -10 without issue. I suspect that is comparable to a couple thousand feet of lower altitude. Still far from the variation you deal with.

Larry
 
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