How Important is OMC Ignition Timing?

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This topic contains 87 replies, has 22 voices, and was last updated by RICHARD A. WHITE RICHARD A. WHITE 4 years, 3 months ago.

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    legendre

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    #17003
    quote Phil B:

    The timing tool, or “gapping the points” exist to adjust timing of the spark (the points opening) relative to the position of the coil & magnet.

    And in all other instances I’ve ever encountered, the timing is set relative to crankshaft position – in terms of degrees BTDC.

    quote :

    Timing of the spark relative to TDC (advance) is controlled by the position of the magneto plate (which also controls the throttle via the magneto plate cam)

    Again, this is a red herring. Ignore the fact that the timing follows the throttle lever position.. and answer this:

    What determines the final timing advance figure, when the engine is running at full speed? Is it simply a matter of where the spark is strongest, or where the spark is most appropriate to the crankshaft position in terms of degrees BTDC (as with +every+ other design)?

    And no one has answered the big question – Why does the OMC timing tool exist, when the "timing" is simply a matter of aligning the magnetic TDC with the coil’s pole shoe? Why would you need such a tool, when the relationship you desire can be set with a ruler, or with a set of factory-applied marks? Does the OMC timing tool index on the magnet position, or on the crankshaft position?

    (Hint: It’s because the magnetic TDC isn’t what we’re after… it’s a particular crank position, as with every other timing spec.)

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    phil-b

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    #17004
    quote legendre:

    What determines the final timing advance figure, when the engine is running at full speed?

    The position of the magneto plate (the "stator")

    quote :

    And no one has answered the big question – Why does the OMC timing tool exist, when the “timing” is simply a matter of aligning the magnetic TDC with the coil’s pole shoe?

    Think of the tool as a "gap setting jig" it doesn’t control what you think of when you hear the word "timing"

    quote :

    Why would you need such a tool, when the relationship you desire can be set with a ruler, or with a set of factory-applied marks?

    Uh, the tool depends on factory applied marks. It’s just a pair of "pointer arms" to connect the crank position with the marks on the magneto plate.

    If your ruler has a round hole in the center, with a slot for the crankshaft key, and ends which narrow down to a point and droop, you have your own timing tool.

    quote :

    Does the OMC timing tool index on the magnet position, or on the crankshaft position?

    YES (both). The magnet is in the flywheel, which is keyed onto the crankshaft.

    To use the tool, you remove the flywheel, and put on the tool, while has a slot for the crankshaft key, just like the flywheel does.

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    legendre

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    #17006
    quote Phil B:

    YES (both). The magnet is in the flywheel, which is keyed onto the crankshaft.

    Are you sure that the magnet is always in the flywheel? On the OMC magneto I know, it’s in the rotor, which is a separate part from the flywheel. While the flywheel must be removed to access the points, the rotor may remain in-place. In fact, it must, as one of the tuning parameters is rotor / coil shoe gap..

    Right?

    And once again, why not just put a mark on the rotor, to align with a coil shoe, if our only concern is that the magnet(s) properly time the ignition event to the point where the magnet(s) – on the rotor – are properly aligned with the coil pole(s)?

    Do you not see how this would obviate the OMC timing tool? Did OMC omit those marks, to prevent the average owner or handyman from accurately setting the ignition timing, unless they were willing to purchase a tool that was never widely distributed? Huh?

    None of this makes any sense.. since when is an engine timed, based solely on peak spark power? Have you ever encountered any other engine, or family of engines, that didn’t time the ignition relative to crankshaft position (in degrees, BTDC)?

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    chris-p
    Canada Member - 1 Year
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    #17008

    Timing is fixed. That is all there is to say about that really.

    The timing fixture was not meant to be a common household item I believe. It was for the techs in the shop. Frank just happens to make them so we can all reap the benefits of using them, compared to simply using feeler gauge.

    Tubs
    Tubs
    US Member - 1 Year
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    #17013

    The magneto rotor could be marked with a line at the factory for magnetic dead-center, and a corresponding mark could be placed dead-center on the coil’s pole shoe. Then it would be simply a matter of adjusting the points to open when the two marks came into alignment – as that would be the magnetic dead-center point, hence the AC zero crossing which is the point of maximum primary current and strongest spark.


    legendre
    Setting the gap is much easier than having to lift the flywheel on and off to
    aligned a mark on a flywheel.
    What you have posted here is the purpose of the timing tool. It indicates the
    " magnetic dead-center" of the magnets in the flywheel so they can be aligned
    with the corresponding mark found on the mag plate that shows the
    "dead-center on the coil’s pole shoe".

    I’m puzzled as to why your struggling with this.

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    phil-b

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    #17025
    quote legendre:

    Are you sure that the magnet is always in the flywheel? On the OMC magneto I know, it’s in the rotor, which is a separate part from the flywheel. While the flywheel must be removed to access the points, the rotor may remain in-place. In fact, it must, as one of the tuning parameters is rotor / coil shoe gap..

    The tools Frank makes (superior) reproductions of are (to the best of MY knowledge) only for OMC "universal magneto" engines, with magnets in the flywheel. On those engines there is no shoe gap spec, just a machined reference boss to line up the coils with. Frank also makes a coil alignment jig for that task.

    quote :

    And once again, why not just put a mark on the rotor, to align with a coil shoe, if our only concern is that the magnet(s) properly time the ignition event to the point where the magnet(s) – on the rotor – are properly aligned with the coil pole(s)?

    Do you not see how this would obviate the OMC timing tool?

    OK, I do now! I don’t know why they didn’t find/make a "built-in" way to do it, since OMC engines were made to be more friendly to shade-tree mechanics than Elmer’s were….

    quote :

    None of this makes any sense.. since when is an engine timed, based solely on peak spark power? Have you ever encountered any other engine, or family of engines, that didn’t time the ignition relative to crankshaft position (in degrees, BTDC)?

    The tool is for setting the points (the fixed timing between magnet, points and coils), not the adjustment of "advance"

    The change in spark timing relative to piston position (advance) is done by rotation of the magneto "plate" which also controls the carb, so when the engine is "wide open", the spark is "earlier" (relative to the piston position) to give the flame front time to propogate (since the pistons speed up, but the speed of combustion doesn’t).

    Adjustment of the "advance" is done by tweaking the cam on the bottom of the magneto plate, which is always backwards from what I expect– for a given number of degrees of advance before TDC the adjustment determines how much fuel the engine gets.

    The mark on the cam on the bottom of the magneto plate is flat at the low end– at the low end, speed is changed only by advancing the spark (wasting less fuel?) while keeping the butterfly closed. The mark on the cam indicates the point (which no doubt could be expressed as degrees BTDC) when the throttle begins to add fuel as the timing is advanced further.

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    archangel
    Canada Member - 1 Year
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    #17028
    quote Phil B:

    quote legendre:

    Are you sure that the magnet is always in the flywheel? On the OMC magneto I know, it’s in the rotor, which is a separate part from the flywheel. While the flywheel must be removed to access the points, the rotor may remain in-place. In fact, it must, as one of the tuning parameters is rotor / coil shoe gap..

    I believe that this comment illuminates the heart of the disagreement here. Legendre has taken his knowledge of other motors — specifically motorcycles, if I remember correctly — and attempted to translate the theory directly to outboards via his understanding of his TD-20. The trouble is, the TD-20 does not have the OMC universal magneto that everyone else is talking about. He isn’t aware that the universal magneto is a different configuration than the TD-20 and has wrongly assumed that the TD-20 uses the universal magneto. Another instance of model specifics being as — or more — important than general theory.

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    david-bartlett

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    #17069
    quote Archangel:

    quote Phil B:

    quote legendre:

    Are you sure that the magnet is always in the flywheel? On the OMC magneto I know, it’s in the rotor, which is a separate part from the flywheel. While the flywheel must be removed to access the points, the rotor may remain in-place. In fact, it must, as one of the tuning parameters is rotor / coil shoe gap..

    I believe that this comment illuminates the heart of the disagreement here. Legendre has taken his knowledge of other motors — specifically motorcycles, if I remember correctly — and attempted to translate the theory directly to outboards via his understanding of his TD-20. The trouble is, the TD-20 does not have the OMC universal magneto that everyone else is talking about. He isn’t aware that the universal magneto is a different configuration than the TD-20 and has wrongly assumed that the TD-20 uses the universal magneto. Another instance of model specifics being as — or more — important than general theory.

    "Bingo!"

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    legendre

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    #17125
    quote Archangel:

    (…) The trouble is, the TD-20 does not have the OMC universal magneto that everyone else is talking about. He isn’t aware that the universal magneto is a different configuration than the TD-20 and has wrongly assumed that the TD-20 uses the universal magneto. Another instance of model specifics being as — or more — important than general theory.

    Thanks for elucidating this point.. I recognized the issue a couple of days ago, and was intending to get around to addressing it. But frankly, the whole debate was getting a little long in the tooth, and I just didn’t have much enthusiasm remaining.

    You’re certainly correct that I was using the older, TD-type magneto as my reference.. but sadly, the main contested point(s) still apply to the ‘universal’ magneto. This, despite the fact that the universal is ‘inside-out’ compared to the TD. Here’s what the disagreement boils down to:

    – Position #1 (mine) – Ignition timing on any engine is strictly a function of crankshaft angle, usually expressed as a figure of degrees before top dead-center (degrees BTDC). Spark energy remains important, but thanks to careful design & manufacture, the range of sufficient energy is wide enough that a properly-timed ignition event will always fall within an acceptable spark energy range.

    – Position #2 (others) – Ignition timing on OMC magnetos +only+ is strictly a function of maximum spark energy, determined by the angular relationship between the rotor magnet(s) and the coil pole shoe(s). Actual spark timing remains important, but thanks to careful design & manufacture, the range of allowable timing is wide enough that a max-energy ignition event will always fall within an acceptable angular range.

    Are we at least clear on what is ultimately being contested, here?

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    wbeaton
    Canada Member - 1 Year
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    #17366
    quote legendre:

    It would be helpful if you posted the make & model of the buzz-box you’re using, as anyone trying to duplicate your work with a typical buzzer or continuity light would drive themselves nuts..

    I did that in my very first post in this thread. Perhaps you should read the posts before you comment on them.

    Wayne
    Upper Canada Chapter

    uccaomci.com

    RICHARD A. WHITE
    RICHARD A. WHITE
    US Member - 2 Years
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    Topics: 129
    #17369

    On a side note, I have done some research and found some interesting info on the differences of a good multi-meter vs the "buzz" boxes referred to earlier. Anyone care to take a stab at why to do this right we should NOT use a multi-meter? There is a valid reason….the info comes from an old friend, who is actually younger but has a VERY good knowledge on electronics…not to toot his horn but he was picked to help with some NASA stuff…. :ugeek:

    http://www.richardsoutboardtools.com

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    legendre

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    #17397
    quote Richard A. White:

    (…) I have done some research and found some interesting info on the differences of a good multi-meter vs the “buzz” boxes referred to earlier. Anyone care to take a stab at why to do this right we should NOT use a multi-meter?(…)

    Response lag when using the (digital) multimeter? Buzz-box needing more (or less) current to indicate, thus offering a more positive indication?

    Either instrument is capable of unambiguously indicating an open or closed condition. But one may be ‘better’ at indicating the intermediate, where the points are not quite closed, but not fully open, either..

    RICHARD A. WHITE
    RICHARD A. WHITE
    US Member - 2 Years
    Replies: 1619
    Topics: 129
    #17414

    And here it is:

    The physical point the the points open is NOT the same as the magnetic decoupling of the points which is actually what you are looking for when using your meters. So with that knowledge I now completely get this stuff. I had assumed from the beginning that once the points opened that was it.. I now know that it not it. Whew!!! AND he stated that the buzz box you folks mentioned is FAR superior for finding that exact point. A normal multi-meter, no matter HOW expensive, can’t show you the magnetic decoupling point, only the location at which the points are no longer in contact with each other.
    I think I just got a bit smarter…..

    http://www.richardsoutboardtools.com

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