Leaning mixture on ground near idle power

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woxof

Pre-takeoff checklist
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woxof
I was reading about mixture leaning on the ground recently for prevention of plug fouling and someone wrote....

"most carburetors have an idle mixture circuit that is not affected by mixture control position. At very low RPM the idle mixture circuit controls the mixture, as the RPM increases out of idle range the mixture control becomes controlling. I lean for taxi but realize when I am all the way back to idle this does nothing. While powering up during taxi leaning is effective. Also if I am stationary waiting for a clearance I run the engine at 1200 rpm. This scavenges exhaust gases better as well as allowing the mixture control to be effective."

Someone else responded....

"Wow, not my experience in any of the planes i've flown, all with Lycoming engines. While there is a idle system leaning has been effective even at the lowest RPM, and my experience is that it helps reduce plug fouling. Except on cold days i would not idle above 1000 rpm."

So what do you think. Will the leaned mixture help prevent fouling even when the idle circuit is in use. Do you recommend a higher power setting(when appropriate) to be off the idle circuit. Does it vary with certain carbs? Any other suggestions.
 
Only the old Bendix/Stromberg carbs have a mixture control that won't affect idle mixture. Most anything else that has the idle cutoff (ICO) function controls the flow to both the idle and main nozzles. Leaning on the ground involves pulling the mixture close to ICO.
 
woxof said:
I was reading about mixture leaning on the ground recently for prevention of plug fouling and someone wrote....

"most carburetors have an idle mixture circuit that is not affected by mixture control position. At very low RPM the idle mixture circuit controls the mixture, as the RPM increases out of idle range the mixture control becomes controlling. I lean for taxi but realize when I am all the way back to idle this does nothing. While powering up during taxi leaning is effective. Also if I am stationary waiting for a clearance I run the engine at 1200 rpm. This scavenges exhaust gases better as well as allowing the mixture control to be effective."

Someone else responded....

"Wow, not my experience in any of the planes i've flown, all with Lycoming engines. While there is a idle system leaning has been effective even at the lowest RPM, and my experience is that it helps reduce plug fouling. Except on cold days i would not idle above 1000 rpm."

So what do you think. Will the leaned mixture help prevent fouling even when the idle circuit is in use. Do you recommend a higher power setting(when appropriate) to be off the idle circuit. Does it vary with certain carbs? Any other suggestions.
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You cannot hurt anything by leaning aggressively on the ground/while taxiing, and your plugs will love it. Do not overthink this.

If you over-lean the engine will quit. Big deal. Restart and don't be THAT aggressive. Do a web search for "Key Reprints from The Lycoming Flyer" for leaning tips.

Bob Gardner
 
bobmrg said:
You cannot hurt anything by leaning aggressively on the ground/while taxiing, and your plugs will love it. Do not overthink this.

If you over-lean the engine will quit. Big deal. Restart and don't be THAT aggressive. Do a web search for "Key Reprints from The Lycoming Flyer" for leaning tips.

Bob Gardner
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Yep. You learn how to lean really fast once you start flying the big bore engines.
 
Regardless of leaning concerns go to fine wire plugs which not only reduce fouling but cost less in the long run. In fact they even be free as at least one test shows a few percent increase in power for the same fuel flow over massives.
 
bobmrg said:
You cannot hurt anything by leaning aggressively on the ground/while taxiing, and your plugs will love it. Do not overthink this.

If you over-lean the engine will quit. Big deal. Restart and don't be THAT aggressive. Do a web search for "Key Reprints from The Lycoming Flyer" for leaning tips.

Bob Gardner
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remember --- most engines have several cylinders still on the rich side, when they have been leaned by the suggested method.
 
before we get into a PC one must remember, we are only talking about a MA-3 SPA type carb.
those carbs have two ways to get fuel out of a carb.. the first is the mixture control valve, the second is the accelerator pump. all other fuel must pass the thru the mixture valve.
 
Try slowly pulling the mixture control out. Does RPM increase? There's your answer. This isn't rocket science.
 
Snowmass said:
Regardless of leaning concerns go to fine wire plugs which not only reduce fouling but cost less in the long run. In fact they even be free as at least one test shows a few percent increase in power for the same fuel flow over massives.
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The type of plugs has nothing with mixture.
 
DoubleD said:
Try slowly pulling the mixture control out. Does RPM increase? There's your answer. This isn't rocket science.
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When you set your mixture this way, you will leave 2 or more cylinders on the rich side.
 
I didn't suggest stopping there. Doing this answers the OP's question.
 
I have a fadec system, mixture is set by computer ,have only had plugs foul once in all the many hours.
 
bobmrg said:
You cannot hurt anything by leaning aggressively on the ground/while taxiing, and your plugs will love it. Do not overthink this.

If you over-lean the engine will quit. Big deal. Restart and don't be THAT aggressive. Do a web search for "Key Reprints from The Lycoming Flyer" for leaning tips.

Bob Gardner
Click to expand...

What Bob said. 20 years (has it really been that long?) experience flying 172s and 182s tells me that if you lean on the ground and forget, the engine will remind you when you try to apply take-off power by quitting on the spot. Re-start and don't lean so much.
 
I used to fly a Cherokee 6 with an O-540. The only way I could keep it from fouling a plug was to lean to near cutoff on the ground and then only push the mixture forward on the takeoff roll.
 
In fact, Continental says that as long as the engine power is at max continuous or less you can do whatever you please with the red knob from full rich to so lean that the engine is misfiring without worrying about damange.

The comments on the idle loop are fine, this is one of the two reasons you have to lean to the point of the engine nearly stumbling: one, if you don't, you're not much changing the mixture at idle and two, it serves as a good reminder if you fail to enrichen before putting the power in.

Lycoming's warnings on mixture settings are all on the RICH side of peak as well.
 
Ghery said:
What Bob said. 20 years (has it really been that long?) experience flying 172s and 182s tells me that if you lean on the ground and forget, the engine will remind you when you try to apply take-off power by quitting on the spot. Re-start and don't lean so much.
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Exactly. I had an instructor on a flight review express concern when I leaned on a long taxi to the active runway. His rationale was that I might forget it was leaned. That struck me as odd, as (provided you lean aggressively) it will run rough and then quit as the throttle was advanced during the run up. If you lean it aggressively again after the run up, it'll do the same thing as you try to advance the throttle on the runway on takeoff. In any case, I thought it was ironic when I was directed to throttle up to 1800 rpm and lean for 1 minute before shut down to clear the plugs.

On the other hand, to be fair, if a student leans just a little bit on the ground, rather than leaning aggressively, and then forgets to go full rich prior to take-off it can result in an overly lean mixture on take off with rough operation, partial power or excessive CHT.

Then again, it's a checklist item and the student should be using the checklist. Letting the student who displays sloppy, pro forma checklist habits attempt a take off with an aggressively leaned aircraft isn't a bad way to underscore the need to actually follow the checklist and confirm each action rather than going through the motions.

The critical item is to lean aggressively, so that it'll quit when you throttle up more than a few hundred RPM, or leave it full rich. It's the non committal leaning in the middle that can be a problem if a pilot forgets to enrich before adding power.
 
That instructor was probably not taught how to correctly lean and like so many others, was taught to treat the red knob like an on/off switch for the engine.

To quote, Dr Bruce, Sigh...


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Larry Vrooman said:
The critical item is to lean aggressively, so that it'll quit when you throttle up more than a few hundred RPM, or leave it full rich. It's the non committal leaning in the middle that can be a problem if a pilot forgets to enrich before adding power.
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Agreed. Lean to almost quitting.

I learned this long after my private rating with a 172RG that would foul plugs if you looked at it cross eyed.

Aggressive ground leaning ever since. Engines are always happier. Haven’t seen a fouled plug in over a decade. Etc.
 
If you don't want to spring for fine-wire plugs, consider installing the UREM37BY plugs if they fit your engine. Check the Tempest website for an application chart. Or the Champion REM37BY if you can get the new production versions. I'd prefer the Tempest.

The UREM37BY plug has extended electrodes and a very shallow well, making fouling very unlikely. I had considerable trouble with the O-235K2C in a Citabria fouling its plugs in the flight school. Lots of circuit work will do that. Sometimes had to clean the plugs at 25 hours. Installed the 37BYs and the problem vanished.

proxy-image
 
In a Cirrus, SOP for a lot of folks is to lean a warmed-up engine at idle until rpm peaks. Note the letter in the M-I-X-T-U-R-E placard where the lever is, then just lean to there in the future.

As a side note, use a before takeoff checklist and you'll never have to worry about "forgetting" the mixture.
 
I was always told to lean until you hear a slight RPM rise.. I usually go just a tiny bit further than that

Take off is easy, (A) use a checklist (B) get in the habit of just pushing everything forward

Not leaning on the ground because you might forget to go full rich before take off is dumb, expecting your own failure is not a good strategy.
 
Tantalum said:
Not leaning on the ground because you might forget to go full rich before take off is dumb, expecting your own failure is not a good strategy.
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Agreed. Kinda like not raising the gear in a retract because I might forget to lower it again before landing.


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If you're properly leaned for taxi, the engine will stumble or quit when you go full throttle at takeoff.

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ateamer said:
properly
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Being the operative word. You'll see a lot of people just pull the thing out half an inch.. then read it on the T/O checklist but not actually do anything
 
You cannot possibly, in your worst-case scenario, wildest imagination, even begin to hurt the engine by leaning aggressively on the ground during taxi. If you do it right, you will greatly reduce lead and oil fouling, and you will not be able to advance the throttle for takeoff power without the engine quitting (thus reminding you to go full rich for takeoff if you miss this checklist item). Lean like crazy for taxi, until the engine stumbles then just enough enrichened so that it will take taxi power.

Now, if your usual taxi to the end of the runway is a one-minute affair, maybe it won't matter. But if you fly to "big-city" airport where you seemingly taxi to Timbuktu and back to get to the active, you may find your plugs fouled by the time you get around to the runup or takeoff power application. Then lean in cruise always to best power or best economy, and in climb as suggested by your POH (for Grummans anytime above 5000 feet, which is fairly conservative). You will be rewarded with excellent fuel consumption numbers, and squeaky-clean plugs.

The whole leaning topic seems to be a zombie thread on POH, which probably means that too many pilots are not getting proper instruction about how to manage the little red knob. (And to be honest, in my training, leaning was a pretty ad hoc topic.)
 
chemgeek said:
The whole leaning topic seems to be a zombie thread on POH, which probably means that too many pilots are not getting proper instruction about how to manage the little red knob. (And to be honest, in my training, leaning was a pretty ad hoc topic.)
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Yes. For some reason engine management of these bone simple aviation engines is some kind of dark art

Honestly, if someone spends an afternoon geeking out on the Engineering Explained channel on YouTube you can get a good pool of knowledge
 
Tom-D said:
You should get a slight decrease ?
Your way, the mixture is a bit rich.
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The way I had it taught was to start full rich, then lean until you get a slight increase in RPM. I think the logic is that at full rich idle the engine is too rich, so by leaning out you get a better combustion and a bump in RPM.. go past "peak" and you'll see a decrease and eventually kill the engine. Except for the OP's thing, which apparently has an "idle circuit"

The Pipers I've rented and flown always had something like this in the checklist:
https://s3.amazonaws.com/atp-program-docs/checklists/checklist-pa-28.pdf

Same for the Cirrus, at least SR22T/TN
https://takeflightsandiego.com/assets/documents/SR22T G5 FOM (1).pdf (top of 3-25)

Granted, it's a nearly imperceptible rise, almost like you can hear it better than see it on the RPM gauge
 
Larry Vrooman said:
Exactly. I had an instructor on a flight review express concern when I leaned on a long taxi to the active runway. His rationale was that I might forget it was leaned. That struck me as odd, as (provided you lean aggressively) it will run rough and then quit as the throttle was advanced during the run up. If you lean it aggressively again after the run up, it'll do the same thing as you try to advance the throttle on the runway on takeoff. In any case, I thought it was ironic when I was directed to throttle up to 1800 rpm and lean for 1 minute before shut down to clear the plugs.

On the other hand, to be fair, if a student leans just a little bit on the ground, rather than leaning aggressively, and then forgets to go full rich prior to take-off it can result in an overly lean mixture on take off with rough operation, partial power or excessive CHT.

Then again, it's a checklist item and the student should be using the checklist. Letting the student who displays sloppy, pro forma checklist habits attempt a take off with an aggressively leaned aircraft isn't a bad way to underscore the need to actually follow the checklist and confirm each action rather than going through the motions.

The critical item is to lean aggressively, so that it'll quit when you throttle up more than a few hundred RPM, or leave it full rich. It's the non committal leaning in the middle that can be a problem if a pilot forgets to enrich before adding power.
Click to expand...

Forgets? Doesn't everyone make a last minute check of controls and instruments before adding takeoff power? Landings are mandatory, takeoffs are optional.

Bob
 
This certainly is an issue with fuel injected planes too. My Bonanza much prefers to be leaned quite a lot on the ground, but I actually had to abort a takeoff due to not having pushed the mix far enough back in. The engine didn't quit, but it certainly wasn't making power. It's a fairly fine line too. With the fact that I already need to put TCP in my planes due to fuel issues around here, ground leaning is definitely a big deal.
 
Sounds like it makes sense.

And.....
1) My first CFI in Louisville had the mix at full rich from start to engine shut down. Never did lean anything at anytime.
2) Second CFI in ATL would on a hot day pull the mix out once we were in the air "about this much" if it was a hot day .... and that was it.
3) As my 2nd CFI moved to airlines right before my check ride, my 3rd "finish up" CFI showed me how to lean by pulling back until engine slowed down, and then pushing it back a smudge until engine went back up and then a few twists rich to make sure we were rich of lean peak. Once we were in the air, not for taxi.
 
WDD said:
Sounds like it makes sense.

And.....
1) My first CFI in Louisville had the mix at full rich from start to engine shut down. Never did lean anything at anytime.
2) Second CFI in ATL would on a hot day pull the mix out once we were in the air "about this much" if it was a hot day .... and that was it.
3) As my 2nd CFI moved to airlines right before my check ride, my 3rd "finish up" CFI showed me how to lean by pulling back until engine slowed down, and then pushing it back a smudge until engine went back up and then a few twists rich to make sure we were rich of lean peak. Once we were in the air, not for taxi.
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And those are the kinds of CFIs that ruin engines.
 
bobmrg said:
Forgets? Doesn't everyone make a last minute check of controls and instruments before adding takeoff power? Landings are mandatory, takeoffs are optional.

Bob
Click to expand...

Given people sometimes also forget to lower gear when landing, I guess people do forget things taking off, landing, and anything in between. Not everyone makes a perfect every time last minute check of controls and instruments. No excuses not to use the check list, and you will be responsible for consequences if something bad happens because you forgot something. But yes, people will indeed forget things.

Plan to be as good as possible, do what you can to not do wrong in first place, know you won't be perfect, and be good enough to be able to overcome/correct when you do something wrong.
 
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N1120A said:
And those are the kinds of CFIs that ruin engines.
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Serious question - running rich would not ruin an engine, just foul the plugs. Running lean would ruin an engine. Thus, maybe the flight school leaned to (pun intended) running rich at the cost of more plugs vs running properly, but at a higher risk of someone running too lean and costing money to fix a over heated engine?

Just trying to figure it out. Or maybe the simple answer is the right answer - easier just to skip that mixture leaning thing.
 
WDD said:
Serious question - running rich would not ruin an engine, just foul the plugs. Running lean would ruin an engine. Thus, maybe the flight school leaned to (pun intended) running rich at the cost of more plugs vs running properly, but at a higher risk of someone running too lean and costing money to fix a over heated engine?

Just trying to figure it out. Or maybe the simple answer is the right answer - easier just to skip that mixture leaning thing.
Click to expand...

Fouling plugs and leaving lead deposits in cylinders and other parts of the engine. Causing you to run hot. Ruining the engine.
 
When I taught at Burnside-Ott back in the late ‘70’s, we had the students lean from hour 1, even back and forth to the practice area. Law of Primacy and all that. My impression at the time was it was largely to save fuel. And Item #1 before every maneuver was “Mixture—FULL RICH”.
 
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