Leaning the Mixture is bad?

J

JC150

Pre-takeoff checklist
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JC150
I trained in a fuel injected C172S SP, and was taught after the engine started, to raise the power to 1200RPM and lean until max RPM. This way, the spark plugs won't foul. I own a Cessna 150 now, and have been leaning the mixture for taxi using the same procedure as I was taught in the Cessna 172S SP.

I was recently told by my mechanic that this was incorrect for an aircraft with a carburetor, and instead should taxi with a full rich mixture because fuel is the only thing cooling the cylinders. My mechanic explained how the engine will overheat if I don't keep full rich mixture on the ground.

I wanted to get some other opinions on this as this is the first I'm hearing about this. I always thought leaning on the ground was important so I wouldn't have a spark plug issue. Thanks!
 
Overheat a 150 engine on the ground by leaning???

Find another mechanic.
 
ditto.
Find another mechanic.
 
Agreed. Taxiing isn't likely to heat the engine no matter where you set the mixture. However, your procedure likely is doing squat on the 150. After you retard the throttle, the mixture isn't likely leaned at all because of the idle loop. To lean that carb, you need to pull out the mixture to the point of almost stalling the engine.

Heating doesn't become an issue until you advance the throttle at runup, which will need to enrichen the mixture for at lower altitudes anyhow.

Of course, at higher altitude airports we have even more fun (I learned to fly at 6000' field elevation).
 
JC150 said:
I trained in a fuel injected C172S SP, and was taught after the engine started, to raise the power to 1200RPM and lean until max RPM. This way, the spark plugs won't foul. I own a Cessna 150 now, and have been leaning the mixture for taxi using the same procedure as I was taught in the Cessna 172S SP.

I was recently told by my mechanic that this was incorrect for an aircraft with a carburetor, and instead should taxi with a full rich mixture because fuel is the only thing cooling the cylinders. My mechanic explained how the engine will overheat if I don't keep full rich mixture on the ground.

I wanted to get some other opinions on this as this is the first I'm hearing about this. I always thought leaning on the ground was important so I wouldn't have a spark plug issue. Thanks!
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Your mechanic is seriously misinformed. Lean it as much as absolutely possible.
 
I'm not sure if I would hang the mechanic out to dry over this issue, because it only shows a lack of engine management knowledge. He might be very good at the mechanical aspects of the airplane.

But yeah, he is flat out wrong. Lean as aggressively as you can.
 
While I wouldn't necessarily fire the mechanic, I wouldn't agree with what he is telling you. Leaning on the ground during taxi won't hurt the engine, and will help prevent fowling out the plugs during ground operations.
 
Say what?!?! ....
...And I can't be allowed to perform more rigorous maintenance on my own aircraft without an AP certificate, but this guy with can?!?!?? That guy? The full rich mixture mechanic real man of genius guy...Holy shhoes.. :D
 
JOhnH said:
ditto.
Find another mechanic.
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Ditto ditto.
If you can't trust what he's saying about something as basic as this (and this is about as basic as it gets), then you can't trust anything he says about anything.

And I need a mechanic I can trust.
 
Wow, is he a licensed AP or just a student??

We run a 235 in our Grumman, ground ops we pull the mixture 1.5" seems to work well for us
 
I don't know why a pilot would seek advice from a mechanic on how to operate an airplane or a mechanic would seek advice from a pilot on how to overhaul an engine. Of course some pilots are mechanics, and if they are experienced in engine management, then I would consider the information. As someone once said, the concert pianist doesn't consult the piano tuner on how to play the concerto.
 
Thank you everyone for your response. This confirms what I had thought.

John, my mechanic and I did a test flight together, and my mechanic noticed I leaned the mixture; that's how the subject came up. As a new private pilot and a new plane owner, I enjoy learning new things all the time, and that's why I joined this forum. My instincts disagreed with my mechanic's views, and the responses here have confirmed what I thought. I didn't seek advice from my mechanic.
 
Retired A&P/IA here: Leaning on the ground prevents excess fuel from fouling the plugs. Leaning in flight is absolutely necessary due to density altitude change as you climb. You are not only saving fuel, you are treating the engine better because a rich mixture leaves unburned fuel which contaminates the oil as well as foul the plugs.

I worked at a flight school where the IP's basically taught students to only pull the mixture back at shutdown. Plugs were constantly fouled, and at thirty hours the oil was black from contamination.

Your A&P is wrong.
 
JC150 said:
Thank you everyone for your response. This confirms what I had thought.

John, my mechanic and I did a test flight together, and my mechanic noticed I leaned the mixture; that's how the subject came up. As a new private pilot and a new plane owner, I enjoy learning new things all the time, and that's why I joined this forum. My instincts disagreed with my mechanic's views, and the responses here have confirmed what I thought. I didn't seek advice from my mechanic.
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Do not let him touch your plane again. :no:
 
Geico266 said:
Do not let him touch your plane again. :no:
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That is a little harsh IMHO. Just don't use the mechanic for advice on how to operate your airplane. It has little to do with his competency as a mechanic. Many pilots are no better then mechanics at spouting old wives tales, like shock cooling, leaning techniques, over square operation is prohibited, pull back the throttle after the initial climb and climb at 24 square, and so on.
 
John Collins said:
That is a little harsh IMHO. Just don't use the mechanic for advice on how to operate your airplane. It has little to do with his competency as a mechanic. Many pilots are no better then mechanics at spouting old wives tales, like shock cooling, leaning techniques, over square operation is prohibited, pull back the throttle after the initial climb and climb at 24 square, and so on.
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How many times does "stupid" have to be demonstrated by someone before you realize they are a pro? ;)
 
The only time leaning is bad is when it is done aggressively at high power (in a Cessna 172, above 75%). Partial leaning may be helpful there.
 
Fearless Tower said:
Overheat a 150 engine on the ground by leaning???

Find another mechanic.
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JOhnH said:
ditto.
Find another mechanic.
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jesse said:
Your mechanic is seriously misinformed. Lean it as much as absolutely possible.
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Rwsavory said:
Your mechanic is mistaken.
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N801BH said:
+1........ And FAST...:yes:
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MikeELP said:
Must be a mechanic that enjoys cleaning and/or replacing fouled sparkplugs. :yesnod:


Mike
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Geico266 said:
+1. :yes:
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PilotAlan said:
Ditto ditto.
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so, is the mechanic right or wrong? :lol:
 
The reason I wouldn't want to keep using the mechanic is that he might not be very good at diagnosing issues with the airplane if he doesn't understant how things work, and how they SHOULD work. Anyone can swap parts out, tighten screws, etc. The only thing easier to work on than a VW bug is a SE airplane....but diagnosing problems is what makes a good mechanic. I'd pass on this guy, unless he's proven himself otherwise (or let him swap tires, but not diagnose fuel/spark/electrical/handling problems)
 
Heck, the POH for the SkyCatcher (not fuel injected, O-200D) specifically talks about leaning for ground ops.
 
Well, if you lean it too far you might have to restart after the engine stalls for lack of fuel.

About the only issue I can think of
 
Excess fuel is the only thing cooling the engine? Must be one of those fancy new water cooled Cessna 150s
 
actually you are both wrong. Leaning your method, you aren't leaning enough. Lean it out so much that if you try to advance the throttle it stumbles and dies.
 
flyingron said:
Agreed. Taxiing isn't likely to heat the engine no matter where you set the mixture. However, your procedure likely is doing squat on the 150. After you retard the throttle, the mixture isn't likely leaned at all because of the idle loop.
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If Idle mixture is not effected by the mixture control, why does the engine quit when you pull the mixture out?

There are 2 ways to get fuel out of the float bowl on the MA3SPA carb, thru the mixture valve and thru the accelerator pump.
 
I wonder if a pilot made a mistake you guys would be as hard on them as you are on the A&P.
 
JC150 said:
I trained in a fuel injected C172S SP, and was taught after the engine started, to raise the power to 1200RPM and lean until max RPM. This way, the spark plugs won't foul. I own a Cessna 150 now, and have been leaning the mixture for taxi using the same procedure as I was taught in the Cessna 172S SP.

I was recently told by my mechanic that this was incorrect for an aircraft with a carburetor, and instead should taxi with a full rich mixture because fuel is the only thing cooling the cylinders. My mechanic explained how the engine will overheat if I don't keep full rich mixture on the ground.

I wanted to get some other opinions on this as this is the first I'm hearing about this. I always thought leaning on the ground was important so I wouldn't have a spark plug issue. Thanks!
Click to expand...

If I taxi with full mixture, I'll be buying another $250 set of sparkplugs really fast. Over and over and over. When my ground is 5500 MSL, nobody does anything at full mixture!

Ask the A&P what he recommends for taxi at high alt.
 
Tom-D said:
I wonder if a pilot made a mistake you guys would be as hard on them as you are on the A&P.
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It depends. If the pilot told me never to pull the yoke because it will make you stall, I'd treat him about the same.

Yes, this error is in that class.
 
Tom-D said:
I wonder if a pilot made a mistake you guys would be as hard on them as you are on the A&P.
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I'm not being hard on the mech. The question is "would I let him work on my plane".

The question fr the pilot is not "would I be hard on him", it's "would I let him fly my plane".

I have friends that will not trust with my money or car keys.
 
Your mechanic is absolutely right...



...if you are in the habit of taxiing at 75% power.

What he's saying is not untrue. An overrich mixture will cool the cylinders. What he doesn't realize is that the temperatures will never get high enough at taxi power for the extra fuel from richening to be necessary. You probably don't have an engine monitor in your C150 to prove him wrong.

I wouldn't discount him as an A&P, but I might question other advice from him, or even his diagnosis of other engine problems without some verification.
 
MAKG1 said:
Like the Cirrus I saw a few weeks ago at PAO?

Not sure it was full throttle, but it sure sounded like it. He must have been taxiing with the parking brake on....
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I think I saw the same guy taking off from KAPC. Jumped in his plane, fired it up, full speed taxi to 18L, took off, no runup.

I'm guessing he did his runup on the taxiway, sans brakes.
 
Sac Arrow said:
I think I saw the same guy taking off from KAPC. Jumped in his plane, fired it up, full speed taxi to 18L, took off, no runup.

I'm guessing he did his runup on the taxiway, sans brakes.
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I once told a girl I was dating that you could identify a Bonanza based on them starting up at about 10,000 RPM and then sitting that way for 15 minutes or so. I totally forgot about that.

One day we were at an airport, a month or so later, and she points to an airplane and was like "is that a bonanza?" .. "Yes, how do you know?" .. "His engine is reved up so high and he's been sitting there forever". It was, indeed, a Bonanza.
 
Sac Arrow said:
I think I saw the same guy taking off from KAPC. Jumped in his plane, fired it up, full speed taxi to 18L, took off, no runup.

I'm guessing he did his runup on the taxiway, sans brakes.
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The guy I saw at PAO wasn't taxiing all that fast. Just really, really loudly. Mike the fuel truck guy and I exchanged "WTF" glances (I was getting fueled at the time).
 
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