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Thread: Mag Problem?

  1. #1

    Mag Problem?

    On run up yesterday mag check drop was normal but something
    just "felt" different. Plane flew OK. (Outside temp 80 to 85 typical
    east coast humidity). Flew 50NM for fuel. After that plane would not
    start easily, battery became completely void of charge.

    A mechanic tried to prop it which did not help and plane did not seem
    like it wanted to start. Recharged the battery and after several
    tries and finally started.

    After landing at night, still difficult to start.

    Is a potential mag problem or just typical Beech hot start problems?





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  2. #2

    Mag Problem?

    Of course, the problem could be bad ignition wires or plugs.
    However, if the plane runs right once started and you're getting
    good mag checks at runup, I would strongly suspect a problem with
    the magneto impulse coupling.

    Most of our planes have one mag (the left, I think) with an impulse
    coupling. The impulse coupling gives the mag an extra "kick" at low
    rpm and also retards the spark timing. This makes the engine much
    easier to start.

    If you happen to have a "shower of sparks" setup instead of an
    impulse coupling, then you likely have a problem there. The shower
    of sparks uses a set of vibrating points to provide a strong spark
    at low RPM by using the aircraft battery as a power source.
    Something like breaker point ignition on pre-electronic autos.

    Best regards,

    Steve Robertson
    N4732J 1967 Super III

    --- In musketeermail@yahoogroups.com, "jm8995" <jm8995@...> wrote:
    >
    > On run up yesterday mag check drop was normal but something
    > just "felt" different. Plane flew OK. (Outside temp 80 to 85
    typical
    > east coast humidity). Flew 50NM for fuel. After that plane would
    not
    > start easily, battery became completely void of charge.
    >
    > A mechanic tried to prop it which did not help and plane did not
    seem
    > like it wanted to start. Recharged the battery and after several
    > tries and finally started.
    >
    > After landing at night, still difficult to start.
    >
    > Is a potential mag problem or just typical Beech hot start
    problems?
    >







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  3. #3

    Mag Problem?

    >From: jm8995 <jm8995@yahoo.com>

    >On run up yesterday mag check drop was normal but something
    >just "felt" different. Plane flew OK. (Outside temp 80 to 85 typical
    >east coast humidity). Flew 50NM for fuel. After that plane would not
    >start easily, battery became completely void of charge.

    Excessing cranking will cause the battery to go dead. <G>

    What causes excessing cranking is the real question.

    In your case, I'd be looking at a weak Left mag. The impulse coupler may be sludged up and failing to actuate (causes no symptoms when running, just hard/fail to start). Could also be E-gap, actual mag timing to engine, leaky coil, pitted points, bad condenser, etc.

    The mag's HARDEST job is firing when the engine is turning at cranking speed. THe amount of energy in the spark goes up with RPM. Its a function of the SPEED that the magnetic lines of force are cut by the coil windings that makes the voltage of the spark vary. Cranking is slow, usually less than 100 RPM. Even idle speed is 5-7 times as fast, and therefore the spark is several times stronger once the the engine is running.

    >A mechanic tried to prop it which did not help and plane did not seem
    >like it wanted to start. Recharged the battery and after several
    >tries and finally started.

    If you happen to have it hit once and fire a cylinder, each successive cylinder gets a stronger spark.

    >After landing at night, still difficult to start.
    >Is a potential mag problem or just typical Beech hot start problems?

    I don't think its "potential", its already upon you, and you will need a skilled mechaninc to inspect and repair to avoid the "shotgun" method of replacing unneeded parts because the mechanic doesn't know hoe to troubleshoot.

    With Slick mags running $600 each exchange, you don't want to throw parts at it hoping that it will magically heal itself.

    --Bob Steward A&P IA
    Lycoming Alumnus, Class of '99
    --On the road in Iowa until Saturday
    >
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