I've always heard people talk about leaning the mixture for ground operations to avoid fouling the plugs-- hell, I've done it myself.

The other day, I was taxiing at a (high) idle while I wound the mixture knob out. I saw no effect at all until the idle cut-off killed the engine.

I've heard some people say that leaning on the ground doesn't really do anything, that the idle circuit (which isn't affected by the mixture knob) is what's in play below about 1200 rpm or so. My experience seems to bear this out.

I guess leaning on the ground doesn't hurt anything, so why not do it? Unless you forget to richen things back up for takeoff, that might not be so good.

You saw no effect? You should see a slight rise (25-50 rpm) before engine shutdown. This is the test to see if your idle mixture is set properly.

I habitually lean on the ground (or water), especially with larger engines. Fouled plugs are no fun.


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My leaning learning came in stages - as did most of my flying.
First stage was NO leaning available in smaller A-65s through C-90s.

Second stage came with rental fleet of older Cessna C100 series.
At that time I was taught the old - do not lean till above 3,000.
There was the lesson on clearing a fouled plug by leaning at high RPM till the engine would shake itself clean. Re-run the mag. test. If improved sufficiently, go for it.

Third stage came with the AGGRESSIVE ground leaning process to avoid fouling plugs. Apparently this fouling came from lower RPM of idle and taxi where valve overlap would allow the last of the exhaust gas, (largely un-burned) to be re-inhaled as the valve OVERLAP would open the intake just soon enough to suck back some of the last of the exhaust gas causing a bit of enrichment. (fouling)

Actually the AGGRESSIVE aspect was added so that if you, (I), forgot to return the mixture to rich, (for the altitude) your engine would stumble, quit, and sit waiting for you to wake up instead of potentially damaging the engine from detonation.

Claim is that the idle and taxi power levels are low enough to allow aggressive leaning without risk of detonation. Seems to work.

Last thing I learned, but hesitant to try at first, was that you could continually lean as long as EGTs did not increase above ground run-up, mag. check, and prop. check values.

Hope it helps
Chris C

I agree with Halestorm and Wannabe. We leaned to maximum Rpm rise on pipeline patrol airplanes before taxi and takeoff. The extra lead, the white dust in the cylinder and white enamel on the plugs, is the culprit. 100 LL has more lead than 80-87 and auto gas, not less. If the oil companies choose to provide one grade fuel to both their rich, big piston engine, customers and their poor, little engine customers, which do you think will get the best fuel for their engine?

I used nothing but auto fuel in Ag planes and had little fouled plugs and valve problems. The patrol companies have fought lead buildup, from 100 LL, ever since it came out. And the little Lycoming in the C-152, that is supposed to love it, is the worst.

Halestorm wrote:You saw no effect? You should see a slight rise (25-50 rpm) before engine shutdown. This is the test to see if your idle mixture is set properly....

yes, this happened at idle cutoff just before the engine quit. No effect before that, which leads me to believe that the mixture knob position doesn't mean much on the ground.

contactflying wrote:I agree with Halestorm and Wannabe. We leaned to maximum Rpm rise on pipeline patrol airplanes before taxi and takeoff. .....

Didn't notice any change in idle rpm when leaning during taxi, until it went all the way to idle cutoff.
Leaning for max rpm for takeoff at higher elevations generally involves running the engine up, to where the high-speed circuit in the carb would be in play. Correct? That's not what I'm talking about.

I lean my 182 to rpm rise, O-470L, it responds as expected. Previous owner recommended it after having fouling issues, I've had none even when I have to make a 1 mile taxi sometimes. My friends with C model Mooney's swear by it, something about their setup makes them foul like crazy if left full rich.

Slight drift: what are the opinions of rpm setting at cutoff? Most seem to pull the mixture at idle, some say to set the tach at 800-1000 and then pull to avoid fouling?

hotrod180 wrote:

Halestorm wrote:You saw no effect? You should see a slight rise (25-50 rpm) before engine shutdown. This is the test to see if your idle mixture is set properly....

yes, this happened at idle cutoff just before the engine quit. No effect before that, which leads me to believe that the mixture knob position doesn't mean much on the ground.

I'm not very familiar with your model engine, but every engine I've flown behind could be leaned effectively on the ground. It's often a delicate procedure, balancing between rpm-rise and engine shut-off, but it should work. What I've read on the subject says there is zero risk to the engine with aggressive leaning for taxi...just lean aggressively enough that you cannot accidentally take off without readjusting the mixture. After start-up I lean to where I can't even taxi forward without adjusting the mixture until the engine is warm.

It's not just fouled plugs...lead build up on your valves can cause them to burn. If the valve doesn't seat properly (because of a bit of lead buildup around the edges) it cannot dissipate heat. Running too rich can be more damaging to your valves than running too lean, though I don't know how much ground idling affects that.

While it's not certified and therefore nobody would use it in a certified aircraft Declan Run-Up fuel additive does the same thing as TCP but costs (much) less and is safe to cary in the cockpit.

Hammer wrote:

hotrod180 wrote:

Halestorm wrote:You saw no effect? You should see a slight rise (25-50 rpm) before engine shutdown. This is the test to see if your idle mixture is set properly....

yes, this happened at idle cutoff just before the engine quit. No effect before that, which leads me to believe that the mixture knob position doesn't mean much on the ground.

I'm not very familiar with your model engine, but every engine I've flown behind could be leaned effectively on the ground. It's often a delicate procedure, balancing between rpm-rise and engine shut-off, but it should work. What I've read on the subject says there is zero risk to the engine with aggressive leaning for taxi...just lean aggressively enough that you cannot accidentally take off without readjusting the mixture. After start-up I lean to where I can't even taxi forward without adjusting the mixture until the engine is warm.

It's not just fouled plugs...lead build up on your valves can cause them to burn. If the valve doesn't seat properly (because of a bit of lead buildup around the edges) it cannot dissipate heat. Running too rich can be more damaging to your valves than running too lean, though I don't know how much ground idling affects that.

Hammer nailed it.

A few years ago there was a discussion of this on another forum. I've never been a big advocate of leaning for taxi, so I contacted a tech rep from the outfit who makes our carburetors....which at the time was Superior, I think. The engineer came back with a response that, yes, leaning during ground ops MAY help to prevent lead buildup, but the mixture has to be taken right to the edge of idle cutoff for it to have any effect. His point was that if set like this, ANY increase in power, even just a bit more rpm to turn during taxi, should cause the engine to shut down on cutoff. His point was that, unless it's that close to idle cutoff, it basically does nothing.

The policy at the school where I worked was to lean for taxi. I can't tell you how many times, however, that I caught students pushing up power for takeoff without first going to full rich mixture. This to me seems to be the only risk to leaning for ground ops. Discipline is key there.

But, as Hammer notes, to achieve any benefit from leaning on the ground, you really need to lean aggressively.

I don't lean for ground ops. I ran an O-360 that fouled plugs regularly, so tried leaning for ground ops, and over the course of a few hundred hours, it made absolutely no difference. And, yes, that carburetor was properly set up.

YMMV

MTV

IO520, I lean till it sputters and then give it a little to keep it running.

Been doing that in everything from 7ACs, to my S108 on up, worked well, just don't try it with a turbine

I used to fly an airplane that had 144 spark plugs and just over 11,000 cubic inches, to say we had to lean that beast for idle would be putting it mildly. That may be a case of apples vs oranges but you get the idea.

With the advent of fine wire spark plugs being used in the R985 on the beaver fouled plugs have become much rarer. When you do encounter a beaver that's still running massives you best be leaning during idle, it can take awhile to locate a fouled plug when you have to look at 18 of them!

Fouled plugs can also be an indication that your ignition system needs attention, those Slick mags really benefit from a 500 hour inspection.

delete

I lean my O-360 Lycoming for taxi, and in some 750+ hours on this engine, it's fouled a plug all of once, and it was my fault. I was in the conga line waiting to take off from OSH several years ago. I had leaned to taxi from the grass to the line, but then I did my run-up while in the line (turned the airplane so as not to blast the guy behind me) with something like half a dozen airplanes in front of me. It didn't occur to me to lean again after doing the run-up. Take off was normal, but when I got to altitude and leveled off with a lower power setting, the engine ran rough--not enough to find a place to land, but rough enough that it was noticeable. On top of that, the humidity was really high, so I think a bit of carb ice formed at times--running with the carb heat on for various times had some effect, but didn't get rid of it.

I tried going to full throttle and rpm while in the air and doing a mag check, but the roughness didn't go away. Soon I landed at KDBQ, and while there did a full power mag check on the ground, including gradually leaning the engine until it faltered. That did it--the engine smoothed out, and there was no more roughness all the way home.

Cary

hotrod180 wrote:The other day, I was taxiing at a (high) idle while I wound the mixture knob out. I saw no effect at all until the idle cut-off killed the engine.
.

Sounds like your mixture adjustment is set up wrong.
There should be a 50 RPM rise. You should see this rise at idle RPM, otherwise it's a red flag. If you aren't seeing that rise, your engine might stumble if you "floor it" on short finals.

I always lean hard on the ground, as far as possible. Good for the engine and good for my bank account.
Also, that way I can't take off with the mixture out, if I outsmart myself and re-lean after I've done my checks. In fact, that's a bad habit which I am training myself out of - my advice would be don't re-lean after doing your checks.

P.S. to keep the plugs clean, you have to lean properly at practically all times, and also running lean of peak a lot will help - anything that ensures all the fuel is getting burnt and burning hot will keep the lead scavenging agents working. My plugs are always completely free of lead build-up. Cruising or taxiing without touching the red knob will still lead to plugs full of lead, just one or the other on it's own isn't enough. You'll see this evidenced a lot on training or rental aircraft when you pull the plugs.

hotrod180 wrote:

Halestorm wrote:You saw no effect? You should see a slight rise (25-50 rpm) before engine shutdown. This is the test to see if your idle mixture is set properly....

yes, this happened at idle cutoff just before the engine quit. No effect before that, which leads me to believe that the mixture knob position doesn't mean much on the ground.

Hotrod 180, thanks for all your questions, since we both have bought a 180 recently, we both have the same questions, you seem to post before me. so I get to learn also.
Bradley

FWIW.....not that I'm the most experienced guy, but I've never had a problem leaning aggressively. I was taught from the get-go to lean for all phases. I think mainly because I learned to fly at high DA....or maybe it was just my instructors style. Lean until you see the highest RPM. In the 180/185's I've owned (3), and 188's I've flown, that's just before idle cut off. There should be a noticeable, repeatable increase in rpm. Probably a little lucky as well, but I've also never had a fouling problem. Knock on wood.

Bill

Battson wrote: Sounds like your mixture adjustment is set up wrong. There should be a 50 RPM rise. You should see this rise at idle RPM, otherwise it's a red flag. ....

As I recall, if you don't get that rpm increase right before idle cut-off, it indicates that the idle mixture is set too lean?

I recently have had an issue with the exhaust "popping" at idle, esp right after a cold start-up.
Could that be related to the idle mixture setting?
I just had the cowl off for an oil change about ten hours ago, I looked everything over but didn't see anything amiss with the intake system or exhaust system.

hotrod180 wrote:

Battson wrote: Sounds like your mixture adjustment is set up wrong. There should be a 50 RPM rise. You should see this rise at idle RPM, otherwise it's a red flag. ....

As I recall, if you don't get that rpm increase right before idle cut-off, it indicates that the idle mixture is set too lean?

I recently have had an issue with the exhaust "popping" at idle, esp right after a cold start-up.
Could that be related to the idle mixture setting?
I just had the cowl off for an oil change about ten hours ago, I looked everything over but didn't see anything amiss with the intake system or exhaust system.

Yes, if you don't get the rise you're mixture is already lean. These big bore continentals are set up with an enriched idle mixture on purpose, to help with cold starts.

Once the engine is running happily we lean, once again our reasoning is to avoid fouled plugs, avoid lead buildup on the exhaust valves and to avoid any dilution effect of the oil film on the cylinder walls.

Is leaning for idle absolutely necessary? Maybe not, you may enjoy many years of trouble free operation without it, but best practice says yes, you should.

If you're worried about forgetting the position of the mixture control your friends "CIGAR" and "GUMP" should help you remember where it is...




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Halestorm wrote:.....If you're worried about forgetting the position of the mixture control your friends "CIGAR" and "GUMP" should help you remember where it is...

All knobs forward except cowl flaps.

hotrod180 wrote:

Halestorm wrote:.....If you're worried about forgetting the position of the mixture control your friends "CIGAR" and "GUMP" should help you remember where it is...

All knobs forward except cowl flaps.

That works for the area in which you live, but not for high elevation take-offs. You need to lean for take off to an approximate best power setting at higher elevation air strips, or your engine won't develop good take off power--it'll be too rich.

Cary