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Thread: Leaning on Taxi

  1. #11
    Richard Warner's Avatar
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    Stromberg Carburetors

    Some Strombergs(aka Tractor Carburetors) have mixture controls and some don't. The Stromberg on my 85 Champ has a mixture control wired to full rich. The 65 that was on it before didn't have one. I had no problem with fouled plugs with either engine.

    Strombergs - My reccollection is that the small engine carbs in Champs and Cubs do not have a mixture control. So you cannot lean when taxiing making the distance to the runway the largest factor in the build up of lead in the plugs when running 100LL fuel. Another reason to find a grass airport where you do not have to wait for takeoff behind a bunch of IFR departures.

    "I haven't had any plug fouling problems yet. Don't know if its the leaning, the Marvel Mystery Oil, or the sacred Paul Poberezy talisman on the altar in my hangar." I vote for the statue of "Pope" Paul.

    Best of luck,

    Wes
    N78PS[/QUOTE]

  2. #12
    Hiperbiper's Avatar
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    Quote Originally Posted by WLIU View Post
    "Now; without touching the mixture knob, try and advance the throttle. The motor will die. What you have just found is the mixture knob DOES control the fuel that is availible to ALL the jets in the carb/fuel servo including the idle circuit."

    You are incorrect in your understanding of how the carburetor idle circuit works. What you example has done is to shut down the mid-range circuit, then supply more air than the idle circuit can provide fuel for. The mixture downstream of the carburetor is now too lean to support combustion so the engine gets quiet. You can kill the engine by providing too much air or too much fuel. The example provided too much air for the amount of fuel that the idle circuit can provide.

    Strombergs - My reccollection is that the small engine carbs in Champs and Cubs do not have a mixture control. So you cannot lean when taxiing making the distance to the runway the largest factor in the build up of lead in the plugs when running 100LL fuel. Another reason to find a grass airport where you do not have to wait for takeoff behind a bunch of IFR departures.

    "I haven't had any plug fouling problems yet. Don't know if its the leaning, the Marvel Mystery Oil, or the sacred Paul Poberezy talisman on the altar in my hangar." I vote for the statue of "Pope" Paul.

    Best of luck,

    Wes
    N78PS
    OK, I should maybe quit putting kits in the MS -3 and -4 carbs 'cause I don't understand how they work.

    What's a mid-range jet? Cars have 'em.

    Airplane carbs have idle, main's and (some) accelerator pumps to help with the idle-to-higher RPM range. The exception is the carbs from the older airplanes and those are not germain to the disscusion here.
    Again; the OP posted this tripe from an FAAST guy that stated you can't lean the motor at idle. All I wish to opine is the carb bowl is a closed chamber; the jets (idle, mid-idle,hi-mid, semi-full or full throttle) draw their fuel from the bowl. The fuel exiting must be replaced by fuel entering. The mixture control controls the mixture of ALL the jets thus...


    http://www.youtube.com/watch?v=9BYm0...eature=related


    This is how it works. Since the 1950's. Like the mags, the airplane carb is quite simple.


    Chris
    You Tube only proves that more airplanes have crashed due to Video Camaras than any other single reason...

  3. #13
    Cary's Avatar
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    I don't pretend to be a mechanic, and what I know about carburetors is pretty limited, but I've been flying out of high elevation airports for 40 years (Laramie at 7377', Fort Collins/Loveland at 5000', Greeley at 4700' are my "most used" airports). If you don't lean aggressively on the ground, and if you have a relatively longish taxi distance, you can almost count on fouling plugs fairly often. You can even foul the plugs in a longish idle/taxi at much lower elevations--I failed to lean sufficiently for the long taxi and idling at OSH 2 years ago, and fouled my plugs (Lycoming O-360), and that's only at 800', probably around 3000' DA that day. So my practice is to start the engine, then as soon as it's running smoothly (like 2 seconds), pull the mixture knob way out. That results in EGTs of 900-1000 degrees at 1000-1100 rpm, and that means no fouling.

    Then when you're ready to start your run-up, push the mixture back in and do any pre-run-up leaning that may be necessary due to higher density altitude--or leave it full rich at low density altitudes.

    You can't hurt the engine by aggressively leaning at low rpms. BTW, working for the FAA doesn't guarantee expertise, especially if the topic is way outside that particular FAA employee's area of knowledge.

    Cary
    "I have slipped the surly bonds of earth...,
    put out my hand and touched the face of God." J.G. Magee

  4. #14

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    I have always.been told.the.carb is set up.for max fuel like limp in on fuel injected cars.
    It is harder to damage.an engine with too much fuel than to little.
    The issue I have had with common practise is starting an engine.
    Crank for 10 seconds then prime. Get the air and oil flowing spark
    plugs snapping then get the fuel in and start.
    Then lean so hard the engine won't run when you advance the throttle.
    My opinion based on everything I have ever read and seen.
    Ray

  5. #15
    FlyingRon's Avatar
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  6. #16
    Cary's Avatar
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    The issue I have had with common practise is starting an engine.
    Crank for 10 seconds then prime. Get the air and oil flowing spark
    plugs snapping then get the fuel in and start.
    Can't agree with that--in fact, most of the time, my engine starts much sooner than in 10 seconds, using the standard POH type of start. The whole idea is to have the appropriate mix of air and fuel, and when that occurs, the engine will start. There is no perfect method which works every single time, but generally speaking priming the appropriate number of strokes, then cranking with the throttle cracked open a little, and she starts. I've found that with my airplane, the "appropriate number" is 3 when cold, most of the time. Yesterday after sitting in a cold wind for 3 1/2 hours after shut down, it took 4. On a warm day, it might take 2. So there's no perfect method. But I wouldn't crank and then prime under most circumstances.

    Cary
    "I have slipped the surly bonds of earth...,
    put out my hand and touched the face of God." J.G. Magee

  7. #17

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    My reason for cranking first is primarily to do with oil flow.
    After an engine sits for a few hours/days/weeks ALL the oil drains down out of the bearings leaving them pretty much metal to metal.
    When the oil pump starts to turn the first thing to get oil is the main bearings, followed by the rods and lastly the cam. These are all pressure bearings and require a constant flow of oil to function.
    So when you prime first and then crank, the engine can fire on the first compression event.
    This bearing has not received any oil yet so the force of explosion hits the rod bearing and the mains when they are metal on metal.
    The force on the bearings is considerably less from compression than from explosion.
    It may take 3 to 5 seconds before the bearings all have oil let alone the poor old cam up there jamming down on the valve springs.
    The lobes of the cam rely on splash from the bearings to get oil, it's not like cams ever wear out eh?

    Of course you can avoid this by adding a preoiler.

    Cat does this, the starter button starts the preoiler and when oil pressure is high enough it engages the starter and away it goes.

    Or you can just crank the engine for a a few seconds, get the oil moving, get some fresh air flowing thru the engine, spark plug are snapping clearing off any moisture, then throw in the fuel. Your engine will love you for it.

    You can hear it when you crank the engine, after about 5 seconds you can hear it cranking faster as the oil begins to flow.

    Starting an engine is when most of the wear occurs, never shut it down and it will run forever.

    The auto industry could learn from this but customers wouldn't like it because that would be a (bad) starting car.

  8. #18

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    Most of the Stromberg carbs as use on Cubs, Champs, have a back suction mixture control. When you move the control in the cockpit (if you have one) a low pressure is introduced into the float bowl through one of a number of increasingly larger holes in a disc attached to the mixture control arm. This is supposed to reduce the amount of fuel flow by way of the vacuum that is created on top of the fuel. Since the engine has to be running to create the this vacuum, it will not shut the engine off like a modern mixture which limits the fuel coming into the carb and can cut it off all together. They never seemed to work very well so most folks just wire them full rich, although I understand they do work better at higher elevation airports.

  9. #19

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    Quote Originally Posted by rwanttaja View Post
    Not with a Stromberg. Mine won't kill the engine at idle, and, in fact, you can't even tell that the mixture is out until the RPM goes past a thousand or so. A lot of Strombergs just have the mixture control wired full rich.

    But I do lean my carb when taxiing...pull it out all the way, in fact. I figure it's gotta be doing *something*, when I'm running 100LL.

    I haven't had any plug fouling problems yet. Don't know if its the leaning, the Marvel Mystery Oil, or the sacred Paul Poberezy talisman on the altar in my hangar.

    Ron Wanttaja
    Marvel Mystery Oil is good stuff!

  10. #20
    Cary's Avatar
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    Ray--interesting. I'll never know so much that I can't learn more. I'll ask my IA what he thinks, though. But anything I can do to preserve that expensive hunk of aluminum out front, I'm willing to do!

    Cary
    "I have slipped the surly bonds of earth...,
    put out my hand and touched the face of God." J.G. Magee

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