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Thread: Carb ice type symptoms on lake take-off

  1. #1
    Member Float Pilot's Avatar
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    Default Carb ice type symptoms on lake take-off

    I was working on basic take-offs and taxiing with a client ( a new I.A. in town ) on the lake. It was light rain (45F) and a little mist off in the distance.
    Almost every time we got the C-172 off the water the engine would start to loose RPM when we were over the causeway road at
    the end of the lake. Right during a pull up.
    Carb heat application cleared it up every time. Using carb heat during half the take-off run reduced the
    problem by about 80%. With just a little drop in rpm for a few seconds.


    I suspect that water is being thrown up into the intake during high power plow or even step taxi and it happens to turn into ice by the time we were over the other end of the lake. After flying around for a minute or two with some carb heat usage, the problem goes away all together. Although if we flew around in a high power climb we eventually managed to pick up carb ice in flight today due to the high moisture situation.

    The air flow in the cowling might be goofed up by the super large hole around the new smaller exhaust pipe. ( PHOTO) at certain angles of attack it might be let cold damp air directly blast the carb itself. Or maybe it just goofs up the proper air-flow and allows water to inter the engine compartment.

    One thought from another pilot as that I have a cab float out of adjustment. He thinks the aplication of carb heat is reducing teh rpm enough that the carb can then keep up with demand. The seaplane prop does indeed let me crank up the rpm...

    Talk about annoying.... and dangerous.
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    Sounds to me as though it might be the carb float. Do you adjust the mixture for max power and best performance on takeoff? Ya might try that . . . I was taught (too many years ago) to leave the mixture control alone until above 6,000-feet. Best performance, and longer engine life, has been best for me if adjusting the mixture on takeoff. That may or may not be covered in your POH. Your EGT will be your best guide if you choose to try that.

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    After the float install, I fueled up , but forgot to use my filter funnel.
    I then flew straight home with only one landing.

    The next day I was doing some flight testing to see how she did on floats. Stalls, steep turns , glides..
    Suddenly on one take-off she lost 400-500 rpm after pitch up. 10 seconds of Carb heat and level flight made it go away.
    It was rainy and misty so I thought I was just making carb ice.

    After that one event it seems to have carb ice symptoms on every take-off. Regardless of the weather.

    And after the fin install I really looked at the rpm during the take-off run. It is only up around 2400-2450 rpm.

    Leaning it makes it drop of without ever going up.
    At high power cruise, like 2500 rpm, you can hear power surges or fluctuations. They are not much, maybe 75 rpm but they are there.
    And she still does not want to whip up high rpm even in cruise.
    Once she is backed off to 2300 rpm or less she runs smooth.

    The ground (water) mag checks work fine.... At cruise flight ( 2400 rpm) she drops 125 for each side and does not strutter.
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    [QUOTE=Float Pilot;1398771]After the float install, I fueled up , but forgot to use my filter funnel.
    I then flew straight home with only one landing.

    The next day I was doing some flight testing to see how she did on floats. Stalls, steep turns , glides..
    Suddenly on one take-off she lost 400-500 rpm after pitch up. 10 seconds of Carb heat and level flight made it go away.
    It was rainy and misty so I thought I was just making carb ice.

    After that one event it seems to have carb ice symptoms on every take-off. Regardless of the weather.

    And after the fin install I really looked at the rpm during the take-off run. It is only up around 2400-2450 rpm.

    Leaning it makes it drop of without ever going up.
    At high power cruise, like 2500 rpm, you can hear power surges or fluctuations. They are not much, maybe 75 rpm but they are there.
    And she still does not want to whip up high rpm even in cruise.
    Once she is backed off to 2300 rpm or less she runs smooth.

    The ground (water) mag checks work fine.... At cruise flight ( 2400 rpm) she drops 125 for each side and does not strutter.[/QUO

    As for me, I wouldn't mess with it at this point. I'd just remove and replace the carburetor. If leaning promotes a drop in RPMs, it sounds as though insufficient fuel is being delivered to the cylinders. I'm certainly no A&P, but what else could it be? Full power on takeoff is supposed to result in additional fuel being delivered as an engine cooling device. Leaning should increase EGT temps, but certainly should not result in the drop you are finding. Just my $0.02 .............

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    After having a Super Cub rebuilt, including a new engine, I test flew the little bird. After about twenty minutes the engine failed. I switched tanks and the engine came fully back to life. After switching tanks back and forth a few more times, and getting the same engine outfter results, I flew her back to my mechanic. It turned out that the mechanic, rather than applying a small amount of Fuel Lube to the fuel line connections at the wing root, it seems he had simply dipped the fitting into the can of Fuel Lube, blocking fuel flow. After a careful cleaning, and re-applying the lube more carefully, everything settled down and the problem never again arose.

  6. #6
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    I second the thought that your carb float is out of adjustment. Applying carb heat enriches the mixture so you could be running on the lean side.

    I don't think your intake is allowing water into the system, unless the filter element is waterlogged. Have you checked it? Make sure it still has the oily coating still as that should keep the solid water out of it.

    The larger opening for the exhaust could be a factor, but I think it is by allowing the hot air from the cylinders to escape too fast and that allows the carb area temperature to run cooler, resulting in more carb icing. As you know, cool air comes in over the top of the cylinders and flows down thru the fins and carries heat away from them. It then is supposed to flow out the aft lower end of the cowling and into the atmosphere. Positive pressure on top and negative pressure below. If it is escaping around the exhaust, the result could be lower temps around the carb and more susceptibility to icing. Just an idea?

    Keep us posted on what you find!
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    I think maybe there are two problems....

    1. The water spray / carb ice problem which may be partially related to the huge hole around the exhaust pipe.. It is letting some water spray into the lower cowling and some of that is hitting the air box. So you know it is balsting the air box and carb with cool air and goofing up the cooling air flow.
    I did a couple landings and jumped out to look as soon as I could kill the engine... The whole lower cowling had water all over and some was inside the cowling dripping off the breather box and scat tube for the carb heat.
    I need to find somebody to film the water spraying off the front of the floats.

    2. And the second problem is the lack of power and a power fluctuations at high power settings. It does this in cruise flight even 5 minutes after having left the water. And it is becoming worse...

    >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    A new local wrench and breaker of plastic wing tips and I opened up the carb drain plug and let the fuel run into a glass jar. It had some junk inside there....
    Some black grit and a small bug.
    The finger screen on the carb was clean and the gas-strainer screen was clean as well.
    But there was some grit in the bottom of the gas strainer sump.
    All that had been cleaned at annual a few months ago. So there is some new junk in there..

    The air box was a touch loose, as was the carburetor itself. Both were tightened. The idle cut mixture control ( economizer) was turned almost all the way out...It was kinda loose. I went back in a turn. It idles a bit slower now and shows some rpm rise with leaning to cut-off now.
    The air filter is new and clean. I even changed oil.

    I ran it with and without the filter. There was no difference.

    The throttle cable has full travel, as does the mix cable and carb heat cable.

    It starts and idles ok, but I was only obtaining 2450 rpm while 3/4 the way down the lake. I was getting more than 2550 on wheels with this prop.
    The ground mag checks were OK, with about 75 rpm drop on both mags.

    At 2300 rpm or less it runs smooth.
    BUT when at full throttle, even in level cruse, it does the uneven power pulsing thing again. The more rpm the worse the throbbing...

    Before I pull the carb apart I need to order a gasket kit for a MA-4. Maybe a float kit... I do not know if anyone ever installed a new type float in there..

    A local IA Joe to be hot stuff with carburetors. He also has a solvent tank...

    On the plus side.... The float are rigged perfectly..... It trims out straight as an arrow.

    The down side is that I have now lost 3 clients...

    Alex
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    Member algonquin's Avatar
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    Alex, does it just lose RPM or does it get ruff and lose RPM? You have a hard one to find there. You address the usual stuff like lose screws etc., have you checked the mag timing? I realize that's stretching but if off enough to run smooth you might not get full power. But then again that won't clear up. Joe is really good at fining stuff ,my money would be with him. If you want I'll run over and brainstorm with you, just give me a call in Sov, 2020. Tom

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    Fuel flow test went OK. 35 seconds for a gallon to flow through. The tank vents are OK as well.
    We removed the carb and the floats were the old brass colored hollow metal type which should have been changed to the solid blue epoxy floats.
    When moving the floats up and down the valve would get stuck in the seat and cut fuel flow.
    A parts kit is on order from Spruce. Hopefully it works...
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  10. #10

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    Alex, I have a friend coming in from Chicago, he just started his flight instruction in a 172 with a Garmin 1000, I want him to get some flying in while he is up here. Do you do hourly instruction or just packages? and do you think your plane will be ready in the next week or so?
    Wes
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    Yes I will prostitute myself for various forms of payment. By the hour or minute works for me as well.
    I hope it will be up and running. I have already lost 3 scheduled SES ratings clients.
    I am becoming very frustrated.
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    How is the needle getting stuck in the seat? Is the tip stepped? If the metal floats are sound (no leaks) there is nothing wrong with them. The composite floats are always suspect though. You can solder the metal floats... The composite ones just dissolve or melt away...there have been 3 or 4 different versions of the composite floats....anyway, make sure your floats are clearing the sides of the float chambers. If the floats are hanging up, you will experience a loss of power....buy your parts in Anchorage and get them the next day....why ship from Spruce? Just adds down time....

  13. #13
    Member Float Pilot's Avatar
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    Instead of using the new solid blue epoxy floats as per SB-2.. I ordered the Kelly Aerospace metal floats which are specifically excluded from the bad float list per a service bulletin from Kelly Aerospace. I figured since they made the valve and seat I ordered,,, all the parts should singing together like the Carpenters. The metal floats that were in there were listed by part number on an old AD.
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  14. #14
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    Exhaust fairing on order from Power Flow and has been mailed.

    The carb has new parts inside and has been cleaned. Somebody had two crush washers stacked up in the needle valve seat. So it could not make full movement. YIKES!!!!

    Now I just need it to stop raining and maybe let the mosquitoes back off their lake-side assault a little..
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  15. #15
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    Carb installed and test flown today... Works great !!!!
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  16. #16
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    ta daaaa...nice trouble shooting and corrective action...happy fathers day to all

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