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Thread: Carb Heat, and engine reactions

  1. #1
    Farmboy's Avatar
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    Carb Heat, and engine reactions

    Cruiser and I were talking about this the other day, and so I thought I'd pose some questions here as I couldn't find specific prior discussion.

    He can correct me on his O-320, but I believe he was saying he did not see an RPM drop when he pulled carb heat. His thought was that it was related to lack of good hot air feeding his carb when carb heat was applied.

    On my O-290, I don't see a RPM drop either, but I know I have good heat, as the same muffler and flow will bake the cockpit. But, what I do see is an immediate rise in the EGT's when the carb heat is applied.
    So originally I thought okay it's working, but then I realized that it's backwards. With carb heat applied, initial EGT response should be to go down due to a richer condition. An EGT rise makes me think it's getting leaner.... which in theory would only be possible if the mixture was already lean of peak? Is that theory correct, or am I backwards on this?

    In flight by leaning the engine I can bring CHT's up to 350-370 degrees and have the appearance of a good burn. By richening the mixture up the CHT's will go back down from that. But... without an RPM drop with carb heat applied, and with EGT's that rise instantly with carb heat application.... seems like it's running lean to begin with.

    Thoughts?

    Thanks,
    pb

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    Carb Heat, and engine reactions

    On my O-320 I really have to wait a good 5-10 seconds to see an RPM drop on run up with carb heat because the pipes are cool. In fact, I more notice the rpm rise after pushing carb heat back in than I do the slow drop. It gives good heat though. I can really see a difference in-flight in EGT and CHT when I have it on in cruise (sometimes I do to try and even out the mixture more).

    I havenít watched my EGTs while pulling carb heat on run-up. Iím hoping for good weather to go flying tomorrow - Iíll do that if I can. Iím going to think on why it might do that, too.

    I canít remember the exact process but there was a (Mike Busch?) thing about checking carbs for proper leaning authority. Iíll try to find it and post it here.

    How do your plugs look? Do you have mags or electronic ignition?

    EDIT: cleaned up some language for ease of reading. Feeding a toddler breakfast doesnít make for a full attention span while posting, haha
    Last edited by CamTom12; 05-04-2019 at 08:57 AM.
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    PerryB's Avatar
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    Yes, something's not right. Carb heat (assuming everything's as it should be) will cause a considerable drop in EGT and RPM. At cruise, an unnecessary application of carb heat gives me a drop of (approx) 100 deg. and 100 RPM. Those are off the top of my head, but pretty close. I'll check for solid numbers next time out.
    After Monday and Tuesday, even the calendar says WTF !
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    TurboBeaver's Avatar
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    Quote Originally Posted by Farmboy View Post
    Cruiser and I were talking about this the other day, and so I thought I'd pose some questions here as I couldn't find specific prior discussion.

    He can correct me on his O-320, but I believe he was saying he did not see an RPM drop when he pulled carb heat. His thought was that it was related to lack of good hot air feeding his carb when carb heat was applied.

    On my O-290, I don't see a RPM drop either, but I know I have good heat, as the same muffler and flow will bake the cockpit. But, what I do see is an immediate rise in the EGT's when the carb heat is applied.
    So originally I thought okay it's working, but then I realized that it's backwards. With carb heat applied, initial EGT response should be to go down due to a richer condition. An EGT rise makes me think it's getting leaner.... which in theory would only be possible if the mixture was already lean of peak? Is that theory correct, or am I backwards on this?

    In flight by leaning the engine I can bring CHT's up to 350-370 degrees and have the appearance of a good burn. By richening the mixture up the CHT's will go back down from that. But... without an RPM drop with carb heat applied, and with EGT's that rise instantly with carb heat application.... seems like it's running lean to begin with.

    Thoughts?

    Thanks,
    pb
    Peter,
    I have the exact same problem with my Cruiser. Get cabin heat . Zero carb heat and no "sound clue" either. Mine is weirder than that as after ALOT of looking around for restrictions or deflection problems with the damper clearances in the airbox I cant find it!
    One thing I have found is that even though mine is all hooked up per factory drawings; for some unknown reason the intake scat tube that supplys cold air from vertical baffling behind #4 to top left side of a PA12 muffler. If I apply carb heat all the way in on final, land and jump out and raise cowling imeadiately: that scat tube is almost to "hot" to touch. I show zero rpm drop most of the time.......
    How hot air could be going up that scat tube is a mystery but it seams like it is. I did think after I completely closed off the oil cooler hole and starter hole this winter so there was
    ZERO lower cowling preasures I could hear an audible change after the engine was up to 160 degree oil temps. But still really nothing on the tach.......
    Something is amiss here.

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    CamTom12's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    Peter,
    I have the exact same problem with my Cruiser. Get cabin heat . Zero carb heat and no "sound clue" either. Mine is weirder than that as after ALOT of looking around for restrictions or deflection problems with the damper clearances in the airbox I cant find it!
    One thing I have found is that even though mine is all hooked up per factory drawings; for some unknown reason the intake scat tube that supplys cold air from vertical baffling behind #4 to top left side of a PA12 muffler. If I apply carb heat all the way in on final, land and jump out and raise cowling imeadiately: that scat tube is almost to "hot" to touch. I show zero rpm drop most of the time.......
    How hot air could be going up that scat tube is a mystery but it seams like it is. I did think after I completely closed off the oil cooler hole and starter hole this winter so there was
    ZERO lower cowling preasures I could hear an audible change after the engine was up to 160 degree oil temps. But still really nothing on the tach.......
    Something is amiss here.

    Sent from my LM-X210 using SuperCub.Org mobile app
    Whatís the air box on a cruiser look like? I wonder if youíre somehow pumping air from the air box up into the upper cowl when your carb heat is pulled? To do that Iíd guess youíd have to have a poor pressure differential between the upper and lower cowl at low speeds (on final). How do your baffles look? Do you have a baffle on the front of the engine?

    This oneís weird.

  6. #6
    CamTom12's Avatar
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    Peter, I found this as a test for carb richness. I havenít done it but I bet someone else on here can comment on if it would be helpful or not:

    https://www.powerflowsystems.com/ima...itch_small.pdf
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  7. #7
    WhiskeyMike's Avatar
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    The hot air should be dumped out of the system when the heat is off. You might have changed something (especially in an engine conversion from the old O-235 in a PA-12) so that the hot air is being pushed the wrong way. For sure you should see a drop in revs and hear it. More than once I've found broken rivets in the air box flapper valve or a broken control arm, so that moving the carb heat control did nothing. A clue is that the control moves in and out smoothly - maybe too far, and nothing happens. Better have a very good look inside that air box before the flapper chokes off the carb and the fan stops.
    Last edited by WhiskeyMike; 05-05-2019 at 02:47 AM. Reason: spelling
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    Farmboy's Avatar
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    Thanks guys. Iíll read more about it tomorrow. Although I kept looking I thought no one responded to this, as it never showed up on my new posts search.

    Appreciate the help. Once back on the flight line Iíll start testing it physically.
    One thought as well for you Earle is to add a scoop on the baffle inlet to ensure positive pressure for the carb heat.


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    CamTom12's Avatar
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    I tried it Sunday but my EGTs were still rising from the RPM increase for the mag-check/run up and I had a commercial jet behind me so I didnít get good data on the carb heat check. In flight still showed good though. Thatís where it matters so Iíd check that if youíre concerned about operation.

    When you do your carb heat check, is your EGT already stabilized from the RPM increase?

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    BC12D-4-85's Avatar
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    I'd try leaning to a peak with the carb heat cold then hot at 75% power. See how the four temps respond assuming a multi-probe EGT. Note any differences in peak temp and peak cylinder between heat off and on. Might add to the discussion (or add confusion).

    Gary

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    skywagon8a's Avatar
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    Quote Originally Posted by Farmboy View Post
    One thought as well for you Earle is to add a scoop on the baffle inlet to ensure positive pressure for the carb heat.
    Except consider that the engine is a great big air pump and will suck in air with force from wherever it can get it. A scoop may help with the cabin heat since that is ram air pressure pushing, but is less important with carb heat.
    N1PA
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  12. #12
    Farmboy's Avatar
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    Cam, Gary, I had noticed no drop when I did the first few runups so I started looking at it in flight during the 7 hr flight home.
    Cruise probably 75-80% power and everything stabilized.
    Only thing that changed was EGTís instant rise/drop with heat on/off.

    I donít recall seeing a CHT change for the time period I was checking.


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    BC12D-4-85's Avatar
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    Another thought. I'd remove the air filter and repeat the carb heat-rpm-EGT test. If it's restrictive the engine my be responding to better airflow and a better a/f mixture even with heat applied. If it's a Brackett filter once off throw it where it can't be found and replace with a Challenger or Donaldson if that's possible. The Bracketts slow flow and eat manifold pressure (opinion). If there's a manifold pressure gauge installed all this testing can be confirmed.

    Gary
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  14. #14
    Tim's Avatar
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    Where do I find a Donaldson, or Challenger filter to fit where a bracket was

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    supercrow's Avatar
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    Quote Originally Posted by Tim View Post
    Where do I find a Donaldson, or Challenger filter to fit where a bracket was

    Sent from my Pixel 2 using SuperCub.Org mobile app
    Think I bought my K&N/Challenger filter from Spruce, but it's been a while. East to clean and re-oil yearly and is less restrictive to airflow. I like mine. Sure it can be bought in several places if you want to shop prices.

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    BC12D-4-85's Avatar
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    Brackett filters are good, but from my experience others can flow better. Maybe if dirt capture is more critical than flow they'd be a choice. They can quickly build up captured dirt, insects, and a water emulsion or snow so keep the elements replaced. Any carb heat leaks will let dirt in so I prefer flow over filtration in my environment especially on floats or skis. Part of their flow restriction is the thick wetted foam element but the perforated backing plate has to add some as well.

    Challenger with oiled K&N elements: https://www.challengeraviation.com/index.htm and https://www.challengeraviation.com/ApplicationGuide.pdf
    Donaldson with dry washable synthetic media: http://www.donaldsonaerospace-defens...dfs/007217.pdf and http://donaldsonaerospace-defense.co...dfs/048554.pdf

    Edit: Challenger makes both early square and later round air filters for Cubs.

    Any how test the engines above w/o a filter and see if anything changes.

    Gary
    Last edited by BC12D-4-85; 05-08-2019 at 11:34 PM.

  17. #17
    Gordon Misch's Avatar
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    Quote Originally Posted by Tim View Post
    Where do I find a Donaldson, or Challenger filter to fit where a bracket was

    Sent from my Pixel 2 using SuperCub.Org mobile app
    Aircraft Spruce has 'em.
    Gordon

    N4328M KTDO
    My SPOT: tinyurl.com/N4328M (case sensitive)
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    skywagon8a's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    ...One thing I have found is that even though mine is all hooked up per factory drawings; for some unknown reason the intake scat tube that supplys cold air from vertical baffling behind #4 to top left side of a PA12 muffler. If I apply carb heat all the way in on final, land and jump out and raise cowling immediately: that scat tube is almost to "hot" to touch. I show zero rpm drop most of the time......
    How hot air could be going up that scat tube is a mystery but it seams like it is.
    Sum Ting Wong!
    Either the factory drawings are wrong, someone has misinterpreted them, there is a baffle missing on the muffler or?????. Do you mean full hot on final or full cold? "Apply carb heat" implies HOT. "All the way in" implies COLD. IF your intake tube is HOT after landing, it appears that the cabin heat ram air is being routed through the heat muff and out the "carb heat" intake tube.

    The cabin heat is routed from the ram air pressurized air inlet, through the muff to the cabin heat valve. When this valve is OFF the hot air flows through it and overboard. There is always flow to prevent overheating the muffler.

    The carb heat is routed from it's inlet which could be ram air or just an opening somewhere. Then through the heat muff to the carb air box where it is dumped overboard until you pull the carb heat on (hot). When you pull the carb heat on the ram air flows through the filter past the other side of the heat valve and overboard through the same exit. The carb heat valve just selects whether the carb draws hot air or cool. When the valve is moved to it's other position the flow paths for the two different temperatures swaps direction.

    Rig a vacuum cleaner up to blow through the tubes from their inlets. Feel with your hand where the air is coming out.

    If there is a baffle missing on the muff this air will come out both at the cabin heat valve and the carb heat valve.
    N1PA
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    TurboBeaver's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Sum Ting Wong!
    Either the factory drawings are wrong, someone has misinterpreted them, there is a baffle missing on the muffler or?????. Do you mean full hot on final or full cold? "Apply carb heat" implies HOT. "All the way in" implies COLD. IF your intake tube is HOT after landing, it appears that the cabin heat ram air is being routed through the heat muff and out the "carb heat" intake tube.

    The cabin heat is routed from the ram air pressurized air inlet, through the muff to the cabin heat valve. When this valve is OFF the hot air flows through it and overboard. There is always flow to prevent overheating the muffler.

    The carb heat is routed from it's inlet which could be ram air or just an opening somewhere. Then through the heat muff to the carb air box where it is dumped overboard until you pull the carb heat on (hot). When you pull the carb heat on the ram air flows through the filter past the other side of the heat valve and overboard through the same exit. The carb heat valve just selects whether the carb draws hot air or cool. When the valve is moved to it's other position the flow paths for the two different temperatures swaps direction.

    Rig a vacuum cleaner up to blow through the tubes from their inlets. Feel with your hand where the air is coming out.

    If there is a baffle missing on the muff this air will come out both at the cabin heat valve and the carb heat valve.
    Lol. "All the way in" was meant to imply all the way into the runway on final....... The control was all the way out Pete. ( But good catch!)
    However I agree with everything your suggesting, the exception being the PA 12 muffler has a vertical baffling inside the shroud welded all the way around the muffler, that compleatly seperating the carb heat side from the cabin heat side........
    I hooked a small leaf blower onto the carb heat duct right behind #4 and unhooked it from outlet exit on carb side............ Blew nicely no restrictions. Then hooked that scat back up and it blows into airbox fine.
    The damper is deflecting normally, clunk/ clunk both ways. The damper fit is normal all ways around?????
    I have excellent cabin heat with the exact same set up on the opposite
    side?????? How that ram air inlet hose
    could be hot can only be IF somehow air going backwards up to scat.........
    How that could possibly be is a total mystery.

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  20. #20
    skywagon8a's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    ...the exception being the PA 12 muffler has a vertical baffling inside the shroud welded all the way around the muffler, that completely separating the carb heat side from the cabin heat side........
    Sometimes this baffle is on the shroud and sometimes it is on the muffler. The result being the same.
    N1PA

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    Maybe it's time to add a carb-air temp probe to your arsenal?
    Then you know what you're getting.
    I maintain 40 -50 fahrenheit intake temp with a carburated engine at all times. I fell into that naturally with my 0470-50, then saw that the 1340 Pratt book called for it as well...
    Jose

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    skywagon8a's Avatar
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    Quote Originally Posted by 46 Cub View Post
    Maybe it's time to add a carb-air temp probe to your arsenal?
    Then you know what you're getting.
    I maintain 40 -50 fahrenheit intake temp with a carburated engine at all times. I fell into that naturally with my 0470-50, then saw that the 1340 Pratt book called for it as well...
    Jose
    Yes a temperature probe would be useful. Be careful on how you depend on this as it depends on where the probe is located. You may find that P&W was referring to an optimal temperature for fuel mixture and combustion purposes more than for ice prevention. There is a threaded hole built into the Marvel carburetors for a temperature probe at the butterfly valve which is where the coldest temperature will be measured. This is the location where the carburetor will make and deposit ice.
    Also, sometimes a temperature probe is placed in the air box at the heat valve where it measures the carburetor air inlet temperature.
    The temperature drop due to the venturi effect can be as much as 70*F. IF your probe is measuring the inlet air temperature at 40-50*F and you are operating in humid conditions you could unintentionally be placing the critical section of the carburetor in ice making conditions.

    This is a discussion of Carburetor heat and ice making. https://en.wikipedia.org/wiki/Carburetor_heat
    N1PA
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  23. #23
    Farmboy's Avatar
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    After following a fellow pilot out of the WAD a few years ago and have him get concerned due to engine running issues before landing straight in at Glens Falls, it would take some time to get me to believe Carb temp probes. He said it was well above freezing, and yet his carb was incased with ice when I looked at it. Oddly my carb didn't get any ice on the same flight.

    pb

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    Quote Originally Posted by skywagon8a View Post
    Yes a temperature probe would be useful. Be careful on how you depend on this as it depends on where the probe is located. You may find that P&W was referring to an optimal temperature for fuel mixture and combustion purposes more than for ice prevention. There is a threaded hole built into the Marvel carburetors for a temperature probe at the butterfly valve which is where the coldest temperature will be measured. This is the location where the carburetor will make and deposit ice.
    Also, sometimes a temperature probe is placed in the air box at the heat valve where it measures the carburetor air inlet temperature.
    The temperature drop due to the venturi effect can be as much as 70*F. IF your probe is measuring the inlet air temperature at 40-50*F and you are operating in humid conditions you could unintentionally be placing the critical section of the carburetor in ice making conditions.

    This is a discussion of Carburetor heat and ice making. https://en.wikipedia.org/wiki/Carburetor_heat
    I spent many years without temp probes (except the Otter, and on it I hadn't known the part about temp control, so we flew it like any other plane).
    On all of them I learned where the handle was to make ice, where to prevent it, and where to remove it. This position would vary depending on power setting of course.
    When you have a gauge, it is necessary to learn the same thing.
    With a 4893-1 modified for Pponk on my installation, that temp is 36F indicated. The probe sticks in right by the butterfly.
    With the 1340 in the current Otter when run by the book (yes, the intent is for mixture control, atomization, etc., but it also affects ice) It will not make ice when run between 40 -50 F indicated. That probe is indeed far downstream of the carb, in the case just before the blower. In the old days I didn't know, but I run this Otter exactly by the book and yes, it vastly improves the EGT situation, but also means one does not make ice with it.
    Either way, a temp probe adds valuable information, and if you don't even know if you're getting carb heat, then it would seem to be a good idea to at least see if your number changes when you try that little handle or knob.
    Of course you would want to learn what the indications mean on your plane. Like EGT, the number is just a relative reference with no meaning in itself.
    Lots of talk about a simple probe. They work.
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    There are some other things..
    Gauge or not, I've always tried to learn what minimum setting would melt ice at cruise. - this is so I could use minimum heat and still not need to touch the control (or anything) while over hostile terrain. (like big water for instance).
    To do this, you fly along at steady altitude and power setting until your manifold pressure drops and inch, then very gradually add heat until you see the needle twitch down, then back up to normal. Without a gauge, it was a certain position... with a gauge, it happens to indicate 33F. The handle position will vary with different power or EGT settings.
    If you don't make ice in your installation or where you fly, then good on ya. This applies to ice prone planes or conditions, which seems the norm for me..
    Another thing.. Yes, you can induce ice in a cold carb. If you put heat on for a short period on a cold carb you'll likely cause condensation.. then if you take it off that condensation will freeze. I learned as a young boy to leave it on for at least 1 1/2 minutes if I was going to use it at all. This was in the old days of either "full on or full off".
    The downside to partial heat (given you have a way to properly regulate it- like a gauge) is that the butterfly will tend to wear out faster. I cured this in the Cessna air box with McFarlane bushings. I've now put 3 TBO runs on the same box without needing to repair the box.
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