Back firing on power to idle. Lycoming 540 Edition

SixPapaCharlie

SixPapaCharlie

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Only recently noticed this but I started hearing a pop or two a few months back upon pulling the power on short final.
I don't recall it happening in the past. I "think" it is that cool pop pop pop backfiring that I hear all the RV pilots doing when they do their overhead break to power off 180 and I hope it sounds cool to people on the ground.
Maybe they hear it and Go "Oh I am going to look up and see what RV is making that cool sound" Then they look up and it is a Comanche and they are even happier because Comanche!

But historically it didn't do it. And now pretty consistently when I pull power on final I hear the a pop or 2.
 
My O-540 did it a little. My IO-540 does it basically every time I pull the throttle out quickly enough. I guess the physics must be something like, "the same fuel is being pushed in for a moment, but without enough throttle there isn't enough O2 to burn it all, so it passes into the exhaust and pops when it hits a fuel/air mix sufficient to finish burning"

But I don't know why the carb version did it rarely and the injected version does it constantly.
 
rwellner98 said:
My O-540 did it a little. My IO-540 does it basically every time I pull the throttle out quickly enough. I guess the physics must be something like, "the same fuel is being pushed in for a moment, but without enough throttle there isn't enough O2 to burn it all, so it passes into the exhaust and pops when it hits a fuel/air mix sufficient to finish burning"

But I don't know why the carb version did it rarely and the injected version does it constantly.
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Maybe I have gotten more aggressive and am pulling the throttle too fast. I will be intentionally more gradual next time I am up and see if I still hear it.
 
There is a film/puddle of fuel on the walls of the intake manifold. If you pull the power off quickly, the fuel "flash" evaporates and you get too rich of a mixture. And, pop, pop, pop.
 
I'm going to pretend I know what I'm talking about and ask if your plugs are clean, but then @Dan Thomas will come by with the real answer.
 
My head goes to an induction leak that’s leaning the mixture. Are you pulling it back rather quickly when this happens?
 
rwellner98 said:
My O-540 did it a little. My IO-540 does it basically every time I pull the throttle out quickly enough. I guess the physics must be something like, "the same fuel is being pushed in for a moment, but without enough throttle there isn't enough O2 to burn it all, so it passes into the exhaust and pops when it hits a fuel/air mix sufficient to finish burning"

But I don't know why the carb version did it rarely and the injected version does it constantly.
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My io540 is the same. Power off 180s will produce a pretty good cackle. FWIW the flight school arrows make the same noise when they're doing them as well.
 
My 340 and the 320 before that will do the same if I allow the prop to drive the motor. I have since adjusted the high RPM pitch limit on the prop to stop it from happening when I pull the throttle to idle when slowing in the pattern.
 
My -540 begins the popping when I pull power to descend from cruise. The popping can be rather loud. I've looked for induction leaks and the like and haven't found the cause. I'd like it to stop. PS. The only way I've found to stop it is to go full rich on the mixture. If I lean, it gets worse. Extremely annoying.
 
2 annuals ago I had my fuel injector spider overhauled and that completely got rid of similar burping and popping I had while drinking @masloki 's homebrewed beer pulling the throttle back on final.
 
I land power off in my Pitts which has an io-540. When I pull the power to idle it makes all sorts of pops. Everyone says its normal.
 
I’ve heard that getting close to 1500TT is a symptom.

Generally, it gets worse before it gets better. Patience and diligence are virtues.
 
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Let'sgoflying! said:
Lean Afterfire?
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Yup.
kyleb said:
My -540 begins the popping when I pull power to descend from cruise. The popping can be rather loud. I've looked for induction leaks and the like and haven't found the cause. I'd like it to stop. PS. The only way I've found to stop it is to go full rich on the mixture. If I lean, it gets worse. Extremely annoying.
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Lean afterfire, as above. Pulling the throttle closed causes a lean mix in the cylinder that might not fire. It gets pumped into the exhaust where the flame from another cylinder sets it off, or it is ignited by hot carbon in the exhaust system.

Carbureted cars had a lot of trouble with it, and with stalling, until the invention of the dashpot, a small vacuum-operated actuator that had a diaphragm in it that worked a small pushrod linked to the throttle lever. If the throttle was closed rapidly, manifold vacuum moved a lever that rotated a cam that was where the throttle idle adjustment screw landed. It prevented the throttle from completely closing until the RPM fell enough, allowing the manifold pressure to rise a bit (falling vacuum) and the dashpot would back off and let the throttle close.

Now, there are various opinions on this. Some internet experts say that closing the throttle rapidly stops the airflow faster than the fuel flow can stop, so the mix gets rich. But if the popping is worse when the mixture is lean, and no problem when rich, as with kyleb's airplane, this theory doesn't fit.

And, as always, I will place some suspicion on the ignition. A weak spark has a much harder time igniting a lean mixture than a hot spark does. So, when were the magnetos last off for internal inspection, cleaning and adjustment? Are there those awful old-design Champion plugs in the engine? Were the plugs spark tested under pressure when they were out for cleaning and regapping? They HAVE been cleaned and regapped within the last couple hundred hours, right? Have those ignition leads been tested?
 
Dan Thomas said:
Pulling the throttle closed causes a lean mix in the cylinder that might not fire. It gets pumped into the exhaust where the flame from another cylinder sets it off, or it is ignited by hot carbon in the exhaust system.
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New plugs, new mags, new leads (new engine). I'm not pulling the throttle closed to descend. Just pulling it back a couple of inches of MP from cruise causes the popping. Say 22" to 20" for a slow descent.
 
What's the mixture knob doing throughout all of this?
 
kyleb said:
New plugs, new mags, new leads (new engine). I'm not pulling the throttle closed to descend. Just pulling it back a couple of inches of MP from cruise causes the popping. Say 22" to 20" for a slow descent.
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Is this an O-540 or an IO-540? Does the EGT rise when you pull the throttle back that much? If it's carbed, we had an R182 with that HA-6 carb, and the EGT would rise significantly on power reduction, indicating leaning. That carb had other problems, too, and when we replaced the engine with a factory-overhauled Lycoming, it came with an overhauled carb that didn't even atomize the fuel properly, flooding the two cylinders that feed off the bottom of the induction manifold at idle. Lyc sent me a brand-new HA-6 clone carb, a machined body instead of cast, and billed me $8800 for it until they got the first one back. $8800.00, 15 or so years ago. For a carburetor! My whole airplane (Jodel D-11) wasn't worth that much.

If it's injected, the correlation between the airflow and fuel servo levers might be off. It might be wises to contact the overhauler.
 
SixPapaCharlie said:
I never go full rich on landing but I do roll it in a couple turns when I am entering the pattern.
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Is there a reason for that? I have the same plane, same engine and now fly around the same area as you - I always go full rich before landing. Is there a reason not to? High DA? Not really an issue in North Texas unless you land at 100F.
 
E
Dan Thomas said:
Is this an O-540 or an IO-540? Does the EGT rise when you pull the throttle back that much?
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IO-540, new Airflow Performance FM200. The EGT doesn't rise noticeably when I retard the throttle...I've tested/looked. It is annoying as anything, because my expectation is to go from a 11.5 GPH cruise down to a much lower fuel burn as I back off the power on descent, but I have to enrich the mixture to minimize the popping, so the fuel flow doesn't come down the way it should.
 
Chrisgoesflying said:
Is there a reason for that? I have the same plane, same engine and now fly around the same area as you - I always go full rich before landing. Is there a reason not to? High DA? Not really an issue in North Texas unless you land at 100F.
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From back in the days when I was a Cirrus pilot, I have always leaned for cruise and then I just enriching a nudge or two as I descend to keep the temps as close to normalized as possible.
Now I do the same. I rarely land full rich. Back when I trained in a 172, we went full rich and added carb heat in the downwind. I also don't add carb heat on landing in the Comanche.
I couldn't tell you why other than it is just my routine that came from somewhere. Maybe a CFI that told me or something.
 
Sac Arrow said:
Are you enrichening before you pull the throttle?
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Generally, I have rolled the mixture in a few turns before getting to pattern altitude and don't really touch it again until I am taxiing.
So pulling the throttle on short final would be after. What are you thinking might be happening?
 
SixPapaCharlie said:
From back in the days when I was a Cirrus pilot, I have always leaned for cruise and then I just enriching a nudge or two as I descend to keep the temps as close to normalized as possible.
Now I do the same. I rarely land full rich. Back when I trained in a 172, we went full rich and added carb heat in the downwind. I also don't add carb heat on landing in the Comanche.
I couldn't tell you why other than it is just my routine that came from somewhere. Maybe a CFI that told me or something.
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I don't use carb heat either. The POH in the Comanche says to only use carb heat if there is indication of icing. I have a carb temp gauge and if it's in the green, I don't use carb heat during landing. But mixture, I enrich during descent, a tiny bit for every 1,000 ft. I lose in altitude and by the time I'm in the pattern, I'm at full rich (like I want my bank account to be one day). If you have to do a go around, would you apply mixture and throttle then or go full power with leaned mixture?
 
Chrisgoesflying said:
I don't use carb heat either. The POH in the Comanche says to only use carb heat if there is indication of icing. I have a carb temp gauge and if it's in the green, I don't use carb heat during landing. But mixture, I enrich during descent, a tiny bit for every 1,000 ft. I lose in altitude and by the time I'm in the pattern, I'm at full rich (like I want my bank account to be one day). If you have to do a go around, would you apply mixture and throttle then or go full power with leaned mixture?
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I'd Just increase power. I have done it a couple times. I am not leaned all that much just not full rich.
 
Chrisgoesflying said:
I'm at full rich (like I want my bank account to be one day)
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Bwahaha. You own an airplane. It's a matter of degrees of brokeness.

The o-540 dakota i used to fly would cough and sputter on short final if you were full rich. Leaving the mixture out a couple turns made it much happier. I don't remember it ever backfiring though. Never did power off 180s in that plane.

I don't land full rich in the lance either. Go arounds involve an open hand shoving all three levers unless the DA is over 3000ish. Then I do try to set the mixture for a go around before I get to final.
 
SixPapaCharlie said:
From back in the days when I was a Cirrus pilot, I have always leaned for cruise and then I just enriching a nudge or two as I descend to keep the temps as close to normalized as possible.
Now I do the same. I rarely land full rich. Back when I trained in a 172, we went full rich and added carb heat in the downwind. I also don't add carb heat on landing in the Comanche.
I couldn't tell you why other than it is just my routine that came from somewhere. Maybe a CFI that told me or something.
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Just remember, the "full rich" is setup for a go around. If you enrich it a nudge or two, your going to be very lean and not making full power when you firewall the throttle due to the wild pigs running across the runway (that's a texas thing, right?). Best case you'll quickly notice and also hit the mixture knob forward. Worst case, you'll be busy worrying about the pigs, not immediately figure out what's going on, not make enough power to properly go around and end up in the trees at the end of the runway.
 
With an engine monitor, the red blinking numbers will probably get you to push mixture in a few seconds after you’ve started the go/around cuz you’ll be like WTF?

Prop setting will give no such warning and risks detonation like when you try to start a vehicle moving in a higher gear, in addition to poor climb performance. Like forgetting to retract a notch of flaps.
 
In the IO-540 in the -10, when I start a 500fpm descent from cruise, I'll have to pull a little power out to control speed. I usually end up hovering somewhere around 20" MP on the way down until slowing down at or near PA. Each time I pull throttle back (usually 1-2" at a time), I'll add a half turn or so of mixture back in to control temps. If I do it 'right', I'll usually land with the mixture control knob being about an 3/4 - 1" from full rich. If I end up with a slam dunk approach for some reason where the throttle is full idle and the prop is acting like a speed brake, I'll get the popping. ETA: The IO-360 in the -7A was worse about popping than the -10. Maybe b/c the 3-blade prop was more of a speed brake and I liked doing slam dunk approaches in it. Tony said I flew 'Cub patterns' in the -7A.
 
Chrisgoesflying said:
Is there a reason for that? I have the same plane, same engine and now fly around the same area as you - I always go full rich before landing. Is there a reason not to? High DA? Not really an issue in North Texas unless you land at 100F.
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Only reason to go full rich for landing is in the event of a go around.
 
I can get popping in my manual transmission Jeep when I let off the throttle. Engine braking. Road is turning the engine with throttle closed, which means high engine vacuum. The equivalent in aviation is very low MP. I will guess that you can recreate or stop the popping by adjusting MP accordingly. I’ve never had that occur in my IO-470 so I can’t give personal experience except for my silly Jeep correlation.
 
SixPapaCharlie said:
Generally, I have rolled the mixture in a few turns before getting to pattern altitude and don't really touch it again until I am taxiing.
So pulling the throttle on short final would be after. What are you thinking might be happening?
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Backfire on decel is caused by unburned fuel igniting in the exhaust, which would be aggravated by an overly rich mixture. I personally don't enrichen at all unless I'm descending from a high altitude. But I also generally run LOP in cruise as well.

What are you flying? A Bonanza? Presumably something with a constant speed prop. Add power, mixture prop throttle. Reduce power, throttle prop mixture.
 
Sac Arrow said:
Backfire on decel is caused by unburned fuel igniting in the exhaust, which would be aggravated by an overly rich mixture. I personally don't enrichen at all unless I'm descending from a high altitude. But I also generally run LOP in cruise as well.

What are you flying? A Bonanza? Presumably something with a constant speed prop. Add power, mixture prop throttle. Reduce power, throttle prop mixture.
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Comanche 250
 
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