Carb heat, fouled plugs, and Lyco powered Cessnas

Jim K

Jim K

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Went to go fly the rent-a-wreck 172M this morning to do some approaches. Runup revealed a fouled plug that no amount of throttle and leaning could clear, so I put her back in the hangar.

I did about an hour of pattern work in this thing yesterday. I suspect the full rich mixture and significant time with carb heat on are what fouled the plugs. I should've done a shut down mag check last night but neglected to.

How prone to ice are the o-320 172's? All my hours are in Pipers, so carb ice is just not something I'm used to worrying about much. The poh for this plane of course says full heat prior to closing the throttle, and I was taught full heat any time you're out of the green arc. I'm wondering if partial heat might be more appropriate. I hate pulling that carb heat and hearing the engine bog down as it tries to digest the excessively rich mixture. Perhaps full heat and a bit of leaning? It was a cool, rainy day yesterday, so perfect conditions for carb ice. I understand Lycos are less prone to ice than the Continentals, but that the exhaust routing in the Cessna still makes it more of an issue than in Pipers?
 
The big issue with pattern work and carb heat on is that you're putting unfiltered air into the engine. That close to ground and you're picking up dust/dirt. That stuff ain't good for engine longevity.

That said, I commend you for playing safe and making the right go/no-go call.
 
Jim K said:
Went to go fly the rent-a-wreck 172M this morning to do some approaches. Runup revealed a fouled plug that no amount of throttle and leaning could clear, so I put her back in the hangar.

I did about an hour of pattern work in this thing yesterday. I suspect the full rich mixture and significant time with carb heat on are what fouled the plugs. I should've done a shut down mag check last night but neglected to.

How prone to ice are the o-320 172's? All my hours are in Pipers, so carb ice is just not something I'm used to worrying about much. The poh for this plane of course says full heat prior to closing the throttle, and I was taught full heat any time you're out of the green arc. I'm wondering if partial heat might be more appropriate. I hate pulling that carb heat and hearing the engine bog down as it tries to digest the excessively rich mixture. Perhaps full heat and a bit of leaning? It was a cool, rainy day yesterday, so perfect conditions for carb ice. I understand Lycos are less prone to ice than the Continentals, but that the exhaust routing in the Cessna still makes it more of an issue than in Pipers?
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Those Cool Rainy days I can almost always demonstrate carb ice during the run up in the 172M I regularly tech in. Don't rush the carb heat check. Note the RPM, pull the carb heat, watch the RPM Drop, (or no drop or Rise if you have carb ice) keep it pulled out until the RPM stabilizes, then turn off the carb heat and note the RPM, if higher than when you started, you had carb ice just from the taxi out.

Might be your 172 is running a bit rich, you might lean it a bit. For Pattern work I usually lean till I see an RPM Drop, enrichen until it reaches the Peak RPM (highest RPM) you saw during the leaning process. Then if the screw in type mixture, screw it mixture in 3 full revolutions. If the push type Mixture push it 1/2 way back in. In most planes this is still a pretty rich mixture. If you have an CHT, monitor it, if the CHT's go above 400 during the pattern climb out then run it richer or full rich.

Brian
CFIIG/ASEL
 
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I was instructed to do a "Low power burnout" during run up in military planes. Ask any old soldier about it.
 
"I should've done a shut down mag check last night but neglected to"

Where in the POH is that ever a thing and how could it ever do anything to detect or clear fouled plugs?


Most likely taxiing full rich.

You can pull that mixture knob out a good inch and still idle properly.

Unfiltered air via carb heat is true, but its not ruining engines.
 
pfarber said:
Where in the POH is that ever a thing
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It's not, but then neither is turning on the intercom or a dozen other things I do.

pfarber said:
how could it ever do anything to detect or clear fouled plugs?
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Good chance the fouled plug would've showed up and saved me a fruitless trip to the airport.

pfarber said:
You can pull that mixture knob out a good inch and still idle properly.
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Do that already. Not convinced it does any good though when you look at how the carburetor works.
 
My one page POH says If I'm on the ground and my engine is running, full carb heat.
They aren't kidding. The engine has died while waiting to warm up because I forgot that rule.
 
You might want to take a look at this Lycoming service bulletin:
https://www.lycoming.com/sites/defa... 100LL for Engines Rated 80_87 Octane (1).pdf

Lycoming addresses increased fouling as a result of using 100LL instead of the 80 octane gas for which these engines were designed. In particular, pay attention to Lycoming's recommended ground operation RPMs and the engine shut-down procedure:

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upload_2022-5-3_20-9-45.png


Almost every pilot I've flown with will let the engine idle at 700 RPM or so when stationary, and will shut down by simply pulling the mixture to cutoff. The old POHs and airplane manuals were written when 80 octane fuel was still available and this type of operation was okay with that fuel. 100LL, though, contains more lead than the old 80 octane stuff, and is more likely to cause fouling without attention to revised ground operation guidance.
 
pfarber said:
"I should've done a shut down mag check last night but neglected to"

Where in the POH is that ever a thing and how could it ever do anything to detect or clear fouled plugs?
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Circuit work fouls plugs a lot faster than taxiing. The low power on approach does it. Checking the mags after the flight does no harm, and can catch more than just fouled plugs. A busted P-lead will give no drop. A failed mag will show up right away. Then the stuff can be fixed so the next flight isn't a disappointment.
 
I've never had issues with fouled plugs in a 172 - certainly nothing i couldn't clear up quickly with a static run. I definitely mega lean the 172. It'll quit by 1500rpm if I don't add back mixture. I suspect ground leaning is a bigger culprit than regular flight in the pattern. I have had a few instances of suspected carb ice. Once on take off ground role, once on take off at 200AGL.
 
Fly a 182 continental 470. It’s an ice box. I’ve had carb Ice on the ground. Longer taxi and idle time. In fact I have learned if I have idled for some time after run up I will pull the heat for about a minute jus to be sure. Ice accumulation during ground idle has been implicated on engine out on take off in the past I believe. I have had during cruise as well. During cruise any drop even minor in MP could indicate possible icing. I’ll write down MP at cruise and then lock the throttle and make it part of my scan. Have pulled heat more then once in cruise to have a bit of a rumble followed by smooth running.
Can’t wait to afford something injected lol.
 
Half Fast said:
You might want to take a look at this Lycoming service bulletin:
https://www.lycoming.com/sites/default/files/SL185B Fuel, Higher Octane Aviation, Use of 100LL for Engines Rated 80_87 Octane (1).pdf

Lycoming addresses increased fouling as a result of using 100LL instead of the 80 octane gas for which these engines were designed. In particular, pay attention to Lycoming's recommended ground operation RPMs and the engine shut-down procedure:

View attachment 106562
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View attachment 106561


Almost every pilot I've flown with will let the engine idle at 700 RPM or so when stationary, and will shut down by simply pulling the mixture to cutoff. The old POHs and airplane manuals were written when 80 octane fuel was still available and this type of operation was okay with that fuel. 100LL, though, contains more lead than the old 80 octane stuff, and is more likely to cause fouling without attention to revised ground operation guidance.
Click to expand...
That's interesting. I do always maintain 1000 rpm at idle, but I've never seen that shutdown procedure before.
 
Jim K said:
That's interesting. I do always maintain 1000 rpm at idle, but I've never seen that shutdown procedure before.
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We used it for awhile on the 150s. It cleared the oil out of the bottom plugs so the next start was easy and clean.

The best thing one can do for the airplane is to put REM37BY (or Tempest's UREM37BY) plugs in the engine if they are approved for it. They will eliminate 90% or more of fouling troubles. https://tempestplus.com/wp-content/uploads/2019/01/Tempest_App_GuideV7.pdf
 
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Dan Thomas said:
The best thing one can do for the airplane is to put REM37BY (or Tempest's UEM37BY) plugs in the engine if they are approved for it. They will eliminate 90% or more of fouling troubles. https://tempestplus.com/wp-content/uploads/2019/01/Tempest_App_GuideV7.pdf
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Wondered if someone would mention these plugs - I'm a believer. I have never had a fouled plug and the lead deposits barely register when I clean and gap them. I use Tempest UREM37BYs and have for years in both my Conti 150 and 172. I've seen what a Lycoming O235's lead-fouled plugs look like after the pilot reported his engine was missing. No thanks.
 
iflyvfr said:
I've seen what a Lycoming O235's lead-fouled plugs look like after the pilot reported his engine was missing. No thanks.
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That's where I first installed them. We had an O-235 in a Citabria, and those engines are famous for running cool, and hard to start, too, unless they have two impulse mags. Those UREM37BYs fixed that engine, and I put them in the 172s and the other Citabria (O-320) as well after that. If we'd still had the 150s at that time they would have gotten them as well, to defeat the oil-fouling habit they have.
 
Half Fast said:
You might want to take a look at this Lycoming service bulletin:
https://www.lycoming.com/sites/default/files/SL185B Fuel, Higher Octane Aviation, Use of 100LL for Engines Rated 80_87 Octane (1).pdf

Lycoming addresses increased fouling as a result of using 100LL instead of the 80 octane gas for which these engines were designed. In particular, pay attention to Lycoming's recommended ground operation RPMs and the engine shut-down procedure:

View attachment 106562
.
.
.
View attachment 106561


Almost every pilot I've flown with will let the engine idle at 700 RPM or so when stationary, and will shut down by simply pulling the mixture to cutoff. The old POHs and airplane manuals were written when 80 octane fuel was still available and this type of operation was okay with that fuel. 100LL, though, contains more lead than the old 80 octane stuff, and is more likely to cause fouling without attention to revised ground operation guidance.
Click to expand...

Is there a similar bulletin or memo for Continental engines, particularly the little Continentals with carbs?
 
If you want to avoid plug fouling, do yourself a favor and lean aggressively during taxi 100% of the time. Lean in flight whenever allowable. It's like magic for plug life. Since I learned to lean aggressively on the ground I have rarely encountered plug fouling (carbon, oil, or lead). If I do get fouling, I usually suspect oil blowby which can be a sign of oil control ring failure or cylinder distress. Flying around full rich will carbon and lead up plugs like no tomorrow.
 
chemgeek said:
If you want to avoid plug fouling, do yourself a favor and lean aggressively during taxi 100% of the time. Lean in flight whenever allowable. It's like magic for plug life. Since I learned to lean aggressively on the ground I have rarely encountered plug fouling (carbon, oil, or lead). If I do get fouling, I usually suspect oil blowby which can be a sign of oil control ring failure or cylinder distress. Flying around full rich will carbon and lead up plugs like no tomorrow.
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I wonder if that will become a lost art after a switch to unleaded avgas?
 
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