Continental engine stumble

Discussion in 'Maintenance Bay' started by Lawson Laslo, Jan 9, 2021.

  1. iflyvfr

    iflyvfr Pattern Altitude

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    This needs to be a sticky somewhere, like maybe the C150/152 forum. Dan, are you aware if this issue also manifests itself during initial climb after liftoff? I used to own a 150 and it stumbled after takeoff a couple of times and my online searches led me to this info on some other forum. I think I smoked the tires once to avoid running off the end after getting about 25-50' in the air.
     
  2. RyanB

    RyanB Super Administrator Management Council Member PoA Supporter

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    Nothing. 100F is plenty warm for a small block four-cylinder. The green arc on the oil temp begins at 75F on most of these.
     
  3. Lawson Laslo

    Lawson Laslo Pre-takeoff checklist

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    Ok so went for test flight today
    Didn’t stumble too much on takeoff, went to do some stalls first one with full mixture no carb heat and it stumbled upon recovery, next mixture leaned on recovery it stumbled for about 7 seconds and lost rpm
    Lastly tried with carb heat on the whole time and there was no stumble what so ever on increasing power

    going to increase the idle mixture tomorrow
     
  4. PaulS

    PaulS Touchdown! Greaser!

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    Nope, I would not do this. Why fly with an engine that is acting up? Get it fixed, then fly it. It could save your life.
     
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  5. Lawson Laslo

    Lawson Laslo Pre-takeoff checklist

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    Yes definitely not flying till it’s fixed
    Hopefully will figure it out tomorrow
     
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  6. Tom-D

    Tom-D Taxi to Parking

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    some times trouble shooting is flying ?
     
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  7. Lawson Laslo

    Lawson Laslo Pre-takeoff checklist

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    I will check for intake leaks tomorrow also, maybe thats the issue?
     
  8. JAWS

    JAWS Cleared for Takeoff

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    A previous post mentioned an idle rpm rise at shutdown of 75 rpm, indicating that the idle mixture is too rich. Should be 25 rpm ideal, 50 max.

    If it is running rich at idle and hesitating, adding more fuel via the accelerator pump will only make it worse. And carb ice would make the engine operate badly throughout the rpm range, not just in one spot. Turning on carb heat will change the airflow and make the hesitation stop or get better, while richening an already rich mixture.

    What part number is on the installed carb? Read the SB I linked to previously.
     
  9. Lawson Laslo

    Lawson Laslo Pre-takeoff checklist

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    Thanks for replying
    Carb mention in the SB is a10-4894
    My carbs a 10-4894-1
    I assume the -1 means it has new parts?
     
    Last edited: Jan 11, 2021
  10. Tom-D

    Tom-D Taxi to Parking

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    The idle mixture is a separate circuit from the accelerate pump they do not effect each other when the engine is idling. the accelerator pump isn't not pumping.

    The hesitation occurs when the idle circuit stops providing fuel before the butterfly valve is open enough to create a vacuum strong enough to provide fuel thru the main jet.

    When this condition occurs the engine usually quits.
     
    Last edited: Jan 11, 2021
  11. Tom-D

    Tom-D Taxi to Parking

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    There is a condition that feeds fuel from both circuits at the same time, the accelerator is still feeding fuel and the main jet is operating normally so there is too much fuel.
    The engine will quit while both circuits are feeding fuel, then the accelerator stops feeding fuel, the engine starts again.
     
  12. Jim Carpenter

    Jim Carpenter Pre-takeoff checklist

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    Does your plane have a Brackett air filter? I've run across several instances in the past, where a change back to the OEM filter type helped with a power-on stumble issue. I don't know if this was because somehow the Brackett foam element was clogged, or somehow impeding induction air flow, or what. Totally non-scientific, but it might be a place to look. And, this is not by any means meant to be a slam on Brackett, they make a good product, I've flown many aircraft with their filters with no problem.
     
  13. Dan Thomas

    Dan Thomas Final Approach

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    There are multiple reasons it might stumble after takeoff. Carb ice is only one, though it's certainly a common one.

    Ice tends to form faster with the throttle closed or only open a little. That's why the carb ice risk charts show icing worse at low power or on approach. The throttle plate chokes off the airflow, causing the engine to suck hard getting the air around that plate, and the velocity of the air goes way up. Increasing the velocity of an airstream lowers its pressure, and decreasing pressure lowers its temperature, so moisture (water vapor) will respond to that by freezing to the plate and the carb's walls.

    So when you're taxiing around on the ground in an environment conducive to icing, ice can form. The engine's RPM will drop as the ice starts blocking the airflow around the throttle plate, so most pilots just open the throttle without realizing what's happening. This is the big mistake, and it's entirely due to poor instruction or simply intellectual laziness on the part of the student. Doesn't want to learn a bit of physics. There are many people who blame "the antiquated designs of our engines" but the fact remains that the affordable airplane will usually have a carburetor and if you want to reach old age you'd better understand it and learn to manage it. Flying is like that: reality doesn't care about a pilot's uninformed opinions.

    So anyway, if you accumulate some ice while taxiing and then take off, the massive airflow going past the wide-open throttle plate isn't experiencing the same temperature drop it was at or near idle, and if the temps are right it starts melting that ice. The water and chunks of ice going into the cylinders don't burn very well at all and the engine barfs a bit. That same roughness convinces far too many pilots to turn the carb heat off after pulling it when experiencing power loss, and they end up with a dead engine. Again, poor training and understanding. It's sad. It wasn't like that when I was young; pilots easily learned to manage the engine because many of them still drove cars that had chokes and could ice up sometimes and those engines needed understanding. Now we have cars that largely operate themselves and newer pilots want airplanes that take that load off their minds. Ain't gonna see that anytime soon, so learning stuff the right way the first time is important.

    Does this count as a rant?
     
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  14. Tom-D

    Tom-D Taxi to Parking

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    When the fuel supply is depended upon the idle circuit, (throttle plate closed) any ice will cause the fuel supply to stop. When this happens the Venturi action is not enough to keep the engine running.
    This is why the accelerator pump is needed to continue run.
    But
    The accelerator pump must not provided too much fuel or it simply floods, and it takes a while to clear.
     
  15. Tom-D

    Tom-D Taxi to Parking

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    As soon as that happens, fuel flow will stop.
    Simply fuel flow thru the idle circuit is dependent upon an equal pressure being applied, flow is created by "sucking" no sucking, no flow, engine stops.:)
     
  16. JAWS

    JAWS Cleared for Takeoff

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    Great post.
     
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  17. JAWS

    JAWS Cleared for Takeoff

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    Yes it does. It should have the atomizer nozzle installed, which is good. Did this issue just crop up in the last flights or has it been there since the carb replacement? Why was the carb changed?

    Get your idle mixture set, check the airbox and intake tubes, check the intake filter for condition (or just replace it - cheap maintenance). If still no improvement, then deeper troubleshooting will be needed.

    I have heard of installing a baffle in front of the air filter to help the small Continentals from icing up, IIRC. Might be another rabbit hole to look down.
     
  18. Lawson Laslo

    Lawson Laslo Pre-takeoff checklist

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    Just happen all of the sudden
     
  19. Dan Thomas

    Dan Thomas Final Approach

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    I was a flight instructor, too, and watched students open the throttle as the engine began to die as carb ice started forming at idle. It didn't quit after they opened the throttle. It takes more ice than that to stop the flow. When in the shop with the door open I often heard other students doing the same thing out on the ramp: they'd start it up, first flight on a nice but humid summer morning, and after it idled for a minute the RPM would start to fall off and it would threaten to quit. They'd just open the throttle and they'd get the RPM back. I'd go out and tell them to set the RPM at 900 and pull the carb heat. They'd watch the RPM rise to 1000 or 1100 or so. Carb ice.
     
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  20. Tom-D

    Tom-D Taxi to Parking

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    It is the accelerator pump that makes it able to do that, the added fuel from the accelerator pump is why it kept running, if the Venturi action is below the hole in the carb's Venturi it will quit again, the trick was you adjusted the throttle to get the carb is on the primary jet, where the metering is done by the oriface of the metering jet or the mixture control.
     
    Last edited: Jan 12, 2021
  21. Lawson Laslo

    Lawson Laslo Pre-takeoff checklist

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    Mechanic and I enriched idle mixture and solved the issue, no more stumble.
    Thanks everyone for you help troubleshooting!
    Tailwinds and Blue Skies!
     
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  22. GeorgeC

    GeorgeC Administrator Management Council Member PoA Supporter

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    So, mine was stumbling like this last year. They did things at annual and the hesitation at acceleration went away, but it seemed to idle lower and rougher (enough that I was occasionally worried that it would die on the landing rollout), and I wasn't getting the rpm rise at ICO. So a few months ago, I had the idle mixture enrichened a touch, now I get ~75rpm rise at shutdown... but it seems that the stumble is back.

    Thanks, I'll have to try that next time.
     
    Last edited: Jan 14, 2021
  23. wanttaja

    wanttaja En-Route

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    Stromberg carb on my C85. The mixture doesn't have enough range to kill the engine, so can't check for a rise at shutdown.

    Was landing on a 15 degree day in another Fly Baby, and the engine did die on rollout. Managed to coast to a taxiway. This Fly Baby didn't have a starter, so I had to climb out (encumbered by cold-weather flying gear), hand-prop it, then climb back in.

    Ron Wanttaja
     
  24. Tom-D

    Tom-D Taxi to Parking

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    Stromberg carb.....
    Is another animal.