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Thread: Carb Heat for better fuel atomization?

  1. #81

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    Quote Originally Posted by skywagon8a View Post
    What did you do at the time that the temperature decreased? Pull the throttle back a bit?
    No, the climb continued at full throttle.

  2. #82

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    Quote Originally Posted by skywagon8a View Post
    What did you do at the time that the temperature decreased? Pull the throttle back a bit?
    I did increase my carb heat in the climb as my CAT dropped to 20 with full power. That would of course also richen the mixture (and reduce MP). An interesting note though when I went back and looked at the data. Unlike the use of carb heat in cruise the use of carb heat (40 degrees CAT) at full power does not appear to equalize the cylinders. I get an egt temperature difference in cruise of about 250 with no heat and about the same at full power. With carb heat in cruise my egt temp difference is like 45. It does not appear I get the same effect at full power, stays over 200.
    Last edited by bubb2; 03-19-2017 at 03:15 PM.

  3. #83
    Tom3holer's Avatar
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    Six seconds is a long time between samplings. A lot can happen between samplings so saying you know within a couple of degrees what peak is, is an incorrect statement. You can change the sampling rate, at least you can on the JPI 600 I have to 1 or 2 sec and suspect you can on yours as well.
    If indeed you are running LOP at to with the mixture pulled out slightly you have to realize you have a problem that fuel injection will not fix.
    At TO power the mixture is purposely set excessively rich to insure against detonation. It should be, as mentioned above 150-200 ROP. As I mentioned before because you are indicating 24gph is NO indication you have adequate fuel flow/mixture to all cylinders.
    Again I suggest you do the induction leak test I mentioned several posts back.
    Most of these setups run just fine so doubt it is a carburetor problem. It is most likely an induction issue.
    I have asked more questions here that I care to admit, Have received a wealth of helpful information all of which has helped me get up to speed on these engines/aircraft we love to fly. We are all here to help and learn and most importantly to enjoy this most wonderful ability our machines offer us.

    Tom
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  4. #84

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    Flew the airplane with no air filter. Not much different than with one. The pressure restriction theory did not seam to hold up. I cycled the carb heat on and off several times. Each time the fuel flow increased with carb heat. About a gallon increase in cruise accompanied by about an inch drop in manifold pressure. OAT was 11 and the unheated carb temperature today was +1. Full carb heat could only get it up to 35. That 5 degree difference from my usual 40 degrees seamed to make a difference as my egt differential improved with the heat but not as well as when I can get the carb to 40. Never went below 100. I immediately did a compression check when I landed and confirmed all cylinders at or above 78.
    I have been advised to ensure the intake manifold tube assembly's are aligned. I plan to do this, pressure check the system, then collect more data.

  5. #85

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    Are the screws holding the carb halves together loose? The mounting flange nuts tight? Does the carburettor move when the carburettor heat is operated? I am now thinking you are looking for something really simple but easily overlooked. Throttle position changing when carb heat is operated because carburettor is loose and moving. Air leak at carb gasket because the screws have loosened.

  6. #86

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    Quote Originally Posted by N86250 View Post
    Are the screws holding the carb halves together loose? The mounting flange nuts tight? Does the carburettor move when the carburettor heat is operated? I am now thinking you are looking for something really simple but easily overlooked. Throttle position changing when carb heat is operated because carburettor is loose and moving. Air leak at carb gasket because the screws have loosened.
    The carb appears tight, no movement. I did do a pressure check on the intake system last year when it was becoming clear there was something odd going on. My balance tube has a treaded plug I am able to hook up to compressed air. I blocked the carb inlet with paper towels and pressurized the system checking for leaks with soapy water. None found. There was obvious air leakage past those paper towels. Once I adjust the intake tubes and replace all the gaskets I'll re-check the system. This time I think I'll use a plastic garbage bag or something similar to get a better seal when I block the carb inlet.

  7. #87
    Dave Calkins's Avatar
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    What is the issue we are chasing here? (Thread title: carb heat for better fuel atomization)

    uneven EGT's?
    wide EGT spread?
    FF change with carb heat application?


    one very high CHT? If its one high cylinder, maybe its a cylinder problem that has gone undiagnosed. Valve guide or seat? Rings? Wrong spark plug heat range? Weak or high resistance spark plug?

    XXXXX one lean cylinder? you stated in first post that peak on a certain cylinder is 1490egt and full rich at 1000msl its 1450egt. Not rich enough! Right? How is the CHT FOR THAT CYLINDER?



    where you live Bubb? Hope you find the prob and share the data! Where you live?
    Last edited by Dave Calkins; 03-19-2017 at 02:29 PM.
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  8. #88

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    Quote Originally Posted by Dave Calkins View Post
    What is the issue we are chasing here?

    uneven EGT's?
    wide EGT spread?
    FF change with carb heat application?

    one very high CHT? If its one high cylinder, maybe its a cylinder problem that has gone undiagnosed. Valve guide or seat? Rings? Wrong spark plug heat range? Weak or high resistance spark plug?

    where you live Bubb? Hope you find the prob and share the data! Where you live?
    I'm out in Chugiak.
    I do not think it is a cylinder problem, All cylinders check OK. The main problem is that in cruise, full rich, cylinder #1 is very close to peak egt. It runs about 40 degrees ROP (don't think this is an induction leak as this is one of the richest cylinders on take off). This results in a very excessive fuel flow. Very wide egt temperature differentials indicating inconsistent fuel/air mixture being distributed to the cylinders. This wide variation is most apparent at full power when cylinder 4, for whatever reason, indicates 300 degrees higher than the others (in cruise it's one of the richer cylinders). I suspect it may be too lean at full power. The egt differential reduces with power reduction but is still above what is considered normal for a carburetor. The main puzzlement is this same engine becomes one of the best fuel/air distributed carbureted engines you could find with the application of carb heat. Does anyone here have a carbureted 520 engine that runs with an EGT temp differential lower than 45 degrees (this engine does but only with carb heat, about 250 degrees with carb heat cold)? I've talked with guys who have GAMI injected 520's who see numbers that low (even lower). I would love to see numbers from other engines.
    The CHT's on this engine are very low. I think the max I have ever recorded on any cylinder was 320. Normal temps for #1 in cruise is 305. #4 has the hottest EGT at full power, about 1550 (over 300 degrees hotter than the coolest cylinder) yet CHT on that cylinder might get to 310 in climb and cools back down to about 300 in cruise (this is a 1575 pound 1954 180 so climbs to my normal cruising altitude of 1000 feet are of pretty short duration). Oil temp is also very low, I have almost the entire oil cooler taped over and only get oil temps in the 140's in winter.
    Last edited by bubb2; 03-19-2017 at 11:43 PM.

  9. #89
    Tom3holer's Avatar
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    Did you ever find out what was going on?

  10. #90

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    Absolute EGT readings are meaningless. Individual EGTs can vary dramatically for a host of reasons, including variation between probes, curve of the exhaust pipes, distance of the probes from the exhaust valve, distance of the probes from exhaust pipe curves (both up- and down-stream), etc. Even 300ºF differences are absolutely nothing to worry about. That's why Alcor (the company that originated EGT measurement systems for airplane engines) refused to put any temperature readings on their gauges - they feared pilots would obsess over them, rather than viewing the system as a "trend" management tool as was intended.

    It's the CHTs you should be concerned with for long-term engine health. There can be slight differences in CHT probe readingss, but swapping probes between cylinders should produce very similar CHT readings for that cylinder. The notable exception to this is that the "ring-type" probes that mount under the spark plugs typically read 50-80ºF (or more) higher than the "well-type" probes. The well-type probes provide more realistic readings that are aligned with the engine manufacturer's operating recommendations. (Yet most single-cylinder EGT systems provided by the airframe manufacturer use the "ring-type" probes... Go figure...)

    At higher power settings, significant differences in CHT readings – especially when combined with higher-than-desired CHT readings on one or two cylinders – may well indicate that there is a problem with the cooling airflow over those hotter cylinders. A well-designed air cooling system can be rendered ineffective by leaking baffles, torn/cut baffle seals, mud dauber nests in the cylinder fins, etc. Those CHT differences (especially when resulting in elevated CHTs) are DEFINITELY important, and should be investigated/resolved ASAP.

    High CHTs damage exhaust valves, seats, and guides. A bent or warped exhaust valve creates "hot spots" on the piston and cylinder head that can lead to massive problems. Keeping your engine's CHTs well below the redline in high-power situations, and below the yellow-zone in cruise-power situations will increase the engine's longevity tremendously.

    PS - The CHTs you're reporting above are excellent, and would appear to indicate a very healthy cooling system. Unless you see cruise CHTs above about 380ºF, I would be very happy with your engine's cooling system!
    Jim Parker
    '65 Champion 7ECA - Flying
    ?? Bearhawk Patrol - Building
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