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Thread: Power to Idle Backfire?

  1. #1

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    Power to Idle Backfire?

    My o-290d2 has the tendency when I pull back the power abeam the numbers to backfire. I call it a backfire because thats what it sounds like and thats what I assume it is. It happens quite a bit. It doesnt happen everytime but it happens often. If I reduce power real slowly it seems to keep it from happening- but I am reducing pretty slowly as it is.

    Am running too rich? Other issue?
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  2. #2
    Grant's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    My o-290d2 has the tendency when I pull back the power abeam the numbers to backfire. I call it a backfire because thats what it sounds like and thats what I assume it is. It happens quite a bit. It doesnt happen everytime but it happens often. If I reduce power real slowly it seems to keep it from happening- but I am reducing pretty slowly as it is.

    Am running too rich? Other issue?
    In the interest of clarity for everyone:

    Backfire is when the intake side of the cylinder ignites the fuel/air mixture in the intake and carb.

    Afterfire is when unburned fuel/air ignites on the exhaust side of the cylinder. (This is what most people call a backfire but it is inaccurate)

    There are some causes listed in the data below. To me it sounds like you have a poor performing cylinder. I would check for intake leaks and the exhaust valves first.

    Backfiring

    When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture. Conversely, a charge that contains more fuel than required is called a rich mixture. An extremely lean mixture either does not burn at all or burns so slowly that combustion is not complete at the end of the exhaust stroke. The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens. This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system.


    Incorrect ignition timing, or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring. A point worth stressing is that backfiring rarely involves the whole engine. Therefore, it is seldom the fault of the carburetor. In practically all cases, backfiring is limited to one or two cylinders. Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions that cause these cylinders to operate leaner than the engine as a whole. There can be no permanent cure until these defects are discovered and corrected. Because these backfiring cylinders fire intermittently and, therefore, run cool, they can be detected by the cold cylinder check.


    In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high power settings. The most likely cause, in this case, is an excessively lean idle mixture. Proper adjustment of the idle fuel/air mixture usually corrects this difficulty.


    Afterfiring

    Afterfiring, sometimes called afterburning, often results when the fuel/air mixture is too rich. Overly rich mixtures are also slow burning, therefore, charges of unburned fuel are present in the exhausted gases. Air from outside the exhaust stacks mixes with this unburned fuel that ignites. This causes an explosion in the exhaust system. Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges. As in the case of backfiring, the correction for afterfiring is the proper adjustment of the fuel/air mixture.


    Afterfiring can also be caused by cylinders that are not firing because of faulty spark plugs, defective fuel-injection nozzles. or incorrect valve clearance. The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns. Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of a rich carburetor. Cylinders that are firing intermittently can cause a similar effect. Again, the malfunction can be remedied only by discovering the real cause and correcting the defect. Dead or intermittent cylinders can be located by the cold cylinder check.
    Last edited by Grant; 09-07-2022 at 08:06 AM. Reason: Grammar
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  3. #3

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    What is your field/density altitude? Could need a carb adjustment and/or require some leaning.
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    BC12D-4-85's Avatar
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    Cardiff how much do you "pull back the power abeam the numbers"? Do you just reduce the throttle opening some, or do you close it to the idle stop on the carburetor? On the ground with a warm engine at just above idle speed if you cutoff the fuel with the mixture control what does the engine rpm do - rise some rpms then quit, or just quit? When was the carb last looked at or overhauled? Is the intake system between the carb and cylinders air tight? It may also be a cylinder issue like noted above.

    Gary
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    sjohnson's Avatar
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    We hear this often from planes in the pattern at mountain strips. Pilots either forget to lean or insist on full rich at high DAs.
    There are three simple rules for making consistently smooth landings. Unfortunately no one knows what they are.
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    stewartb's Avatar
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    Cardiff, is carb heat pulled before you reduce power?
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    Quote Originally Posted by TxAgfisher View Post
    What is your field/density altitude? Could need a carb adjustment and/or require some leaning.
    Typically 3,400 to about 6,000 pattern altitude (where it happens.) Could need more leaning. I am leaned some but decently rich as I saw another post that said don't lean under 5,000 feet for takeoff/landing.

    Quote Originally Posted by BC12D-4-85 View Post
    Cardiff how much do you "pull back the power abeam the numbers"? Do you just reduce the throttle opening some, or do you close it to the idle stop on the carburetor? On the ground with a warm engine at just above idle speed if you cutoff the fuel with the mixture control what does the engine rpm do - rise some rpms then quit, or just quit? When was the carb last looked at or overhauled? Is the intake system between the carb and cylinders air tight? It may also be a cylinder issue like noted above.

    Gary
    I pull back to close to idle. I generally add a bit more power once I get slowed down and turning base to final.

    On ground, with warm engine when I pull mixture I've never noticed an increase in RPM. It sputters and quits- but I generally run a bit rich for takeoff/landing ops. I'm leaned, but not max performance lean based on some other discussion I saw on here.

    Quote Originally Posted by sjohnson View Post
    We hear this often from planes in the pattern at mountain strips. Pilots either forget to lean or insist on full rich at high DAs.
    I'm leaned- but probably a bit rich.

    Quote Originally Posted by stewartb View Post
    Cardiff, is carb heat pulled before you reduce power?
    Depends on my mood I do them at the same time but am not consistent which is first. I saw another post that said always do carb heat first. That the way to go?

  8. #8
    stewartb's Avatar
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    I never use idle in flight so can't say for sure, but if you'd use carb heat early (it works better with some power anyway) and push it cold before you go to idle? I'd expect your problem would go away. Stay in the habit of pulling carb heat and idle power at the same time and you may have your engine quit one of these days. Or you could slow down earlier and maintain some power in the pattern.

    It's like going to the doctor and saying something pops when you do this, and he says then don't do it. Problem solved.

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    skywagon8a's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    On ground, with warm engine when I pull mixture I've never noticed an increase in RPM. It sputters and quits- but I generally run a bit rich for takeoff/landing ops. I'm leaned, but not max performance lean based on some other discussion I saw on here.
    On the ground run the engine to 1000 rpm. Slowly pull the mixture control to idle cut off. Does the rpm rise just before the engine quits or does the rpm drop? It should rise 10 -25 rpm. If it does not rise as you have indicated, turn the idle mixture knob on the carburetor counterclockwise 1/4 turn. Try again. If it still doesn't rise enough, turn the idle mixture another 1/4 turn. Repeat until the desired rise is achieved.

    Then try again in flight. Try the simple things first.
    N1PA
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  10. #10

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    If carb heat is required, need to apply while the engine is making power. I'd imagine you are too rich, my Maule would do the same thing if not leaned correctly.

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    Quote Originally Posted by skywagon8a View Post
    On the ground run the engine to 1000 rpm. Slowly pull the mixture control to idle cut off. Does the rpm rise just before the engine quits or does the rpm drop? It should rise 10 -25 rpm. If it does not rise as you have indicated, turn the idle mixture knob on the carburetor counterclockwise 1/4 turn. Try again. If it still doesn't rise enough, turn the idle mixture another 1/4 turn. Repeat until the desired rise is achieved.

    Then try again in flight. Try the simple things first.
    Start from full rich?
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  12. #12
    skywagon8a's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    Start from full rich?
    Start with the manual mixture control in full rich.
    N1PA

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    Grand Pooh Bah soyAnarchisto's Avatar
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    Following.

  14. #14

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    Quote Originally Posted by skywagon8a View Post
    Start with the manual mixture control in full rich.
    Okay.

    So you know I lean all the time in flight and even during runup when DA is high. Rpms rise. Im not doing it at 1000 RPM though. Maybe 1800 on ground at lowest.

    Is the point to be at 1000 rpm?

  15. #15
    skywagon8a's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    Okay.

    So you know I lean all the time in flight and even during runup when DA is high. Rpms rise. Im not doing it at 1000 RPM though. Maybe 1800 on ground at lowest.

    Is the point to be at 1000 rpm?
    1000 rpm or a little over. Below that rpm the manual mixture control is ineffective, as all the fuel will be controlled by the idle jets within the carb. This procedure is to find the correct setting for the idle mixture control which controls the fuel while the throttle is closed.
    N1PA
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  16. #16
    Charlie Longley's Avatar
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    Leaking intake gasket.

  17. #17
    Gordon Misch's Avatar
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    Or just kinda rich. The fuel, denser than the air, keeps flowing a little longer in the carb than the air due to momentum. Then there is unburned fuel to pop in the exhaust. Mine has always done that with a rapid throttle reduction while full-rich, no biggie, I think.
    Gordon

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    BC12D-4-85's Avatar
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    Assuming no air leaks in the intake and exhaust system, if carb heat makes it decrease suspect lean - if carb heat makes it worse suspect a rich A/F mixture. Close throttle until it gets noisy then apply carb heat.

    Gary

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    skukum12's Avatar
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    My motor was popping in the pattern. Found loose intake tube where they come off the oil sump. Had them re swaged.
    "Always looking up"

  20. #20
    RaisedByWolves's Avatar
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    Sort of related, if you get a really rough running engine with carb heat, check the two halves of the carb. They can loosen, safety clips catch it but enough to suck air. Seems to be worse on the round air filter, give it a wiggle when you walk by. Had it happen on a 172 also


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    Grand Pooh Bah soyAnarchisto's Avatar
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    I"m debugging similar issues. at 1000 rpm, rpms rise right before idle cutoff as they should. I'm always leaned for taxi/takeoff/max power as I'm always at high DA. Field elevation is 5100-5300 at the two fields I fly to most commonly. I recently repaired my airbox which was certainly an intake leak. Popping is much better, but still quite pronounced when pulling throttle to idle stop when in pattern. I notice it happening no other time.

    I rarely run carb heat. I have a carb temp gauge and and it's almost never in the range for carb heat. I'm not following the reasoning behind adding carb heat. If it's too rich and that's the cause of the popping, won't adding carb heat make it even more so? Or are we suspecting a lean condition - due to intake leaks somewhere else allowing extra air into the intake and causing a lean condition? If carb heat improves the popping, what am I to deduce from this? Keep looking for intake or exhaust leaks?

  22. #22
    skywagon8a's Avatar
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    [QUOTE=soyAnarchisto;832336I rarely run carb heat. I have a carb temp gauge and and it's almost never in the range for carb heat. [/QUOTE]
    Temperature is only part of the carb ice situation. Relative humidity/dew point is the other. If you don't want to run it regularly, at least check it before closing the throttle for landings.
    N1PA
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  23. #23
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    I think it's accurate to say that 100% of my observed after-firing in airplanes has occurred when the prop was driving the engine with very low power settings, like a closed throttle. For you after fire guys, try eliminating that situation and see what happens. A closed throttle on the ground usually nets about 500-600 RPM. When you close it in flight what RPM are you seeing? Try adding a little throttle without adding RPM. I'd bet your engine will be happier.
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    Grand Pooh Bah soyAnarchisto's Avatar
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    Prop is for sure driving. RPM is still well over 1000 even more with throttle all the way back. I guess that explains it for me.


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    BC12D-4-85's Avatar
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    Rpm driven by the prop creates high vacuum in the intake system from downstream of the throttle plate to the cylinders. Any air leaks will be made worse and the engine goes lean. Manifold pressure that's around 10-12" near idle on the ground (check it sometime) can go even lower. If not maybe a leak is feeding air.

    Gary
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  26. #26

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    Very interesting discussion. I have been operating a Piaggio P149D with the Lycoming GO-480 engine. This has an auto rich carb and they are notorious for gearbox failure. The wisdom from those who know is to fly the approach with cruise settings on the prop so that the prop never drives the engine but of course one has to close the throttle eventually. It always made the loud pops as the revs dropped on touchdown. Finally it’s dropped a valve. Reading this it sounds like the popping was a warning of a dodgy cylinder…


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  27. #27
    Cub Builder's Avatar
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    As others have mentioned, I had the same situation on my O-320 last year. I finally found it leaking around the intake gasket at the cylinder head on one cyl. New gasket eliminated the after fire popping. I was getting after fire popping primarily during taxi, but essentially any time the manifold pressure is high, the leaky gasket would allow that one cylinder to go lean and it would after fire in the exhaust.

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