How Important is OMC Ignition Timing?

Home Forum Ask A Member How Important is OMC Ignition Timing?

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

Viewing 15 posts - 61 through 75 (of 88 total)
  • Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 4057
    Topics: 418
    #16769

    With the price of offshore digital ohm meters/multi testers which can split a one ohm reading into tenths so affordable now, maybe it’s time to brush the moths off the wallet and buy some new tools. šŸ˜•

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    legendre

    Replies: 389
    Topics: 8
    #16770

    In case any clarification is required, you can think of event #1 as "magneto timing" and event #2 as "ignition timing".

    Magneto timing ensures that a good, strong spark is ready, on-tap when it’s called for – ignition timing sets that spark off at precisely the right moment (in terms of crankshaft position, usually measured in degrees before top-dead-center "BTDC".

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    legendre

    Replies: 389
    Topics: 8
    #16771
    quote Mumbles:

    With the price of offshore digital ohm meters/multi testers which can split a one ohm reading into tenths so affordable now, maybe it’s time to brush the moths off the wallet and buy some new tools. šŸ˜•

    You’ve got that right. I suggest that particular RS meter as I own two of them, and know them to be available, inexpensive, versatile and trustworthy.

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    legendre

    Replies: 389
    Topics: 8
    #16809

    ETA: Apparently, scotty deleted his post while I was writing my reply..

    quote scotty:

    Thank you, Mumbles. I was under the impression that I had to disconnect the point leads to use the timing tool.

    You only need to disconnect the lead between the points & coil if you don’t have an instrument (meter, buzz-box) which is capable of reacting to the small change in resistance when the points open. Seems like that is pretty clear now, which is good.

    quote :

    I was using an analog meter.

    There’s nothing inherently ‘wrong’ with analog meters, that would prevent use in this application. It’s just that virtually all inexpensive, service-grade analog meters aren’t sufficiently sensitive / precise to detect and display the slight (<1 ohm) shift.

    This said, there are plenty of analog meters that are quite capable of this job, but they tend to end up in laboratory / research applications, rather than on the mechanic’s bench.

    quote :

    It was driving me crazy that every time I reattached the point leads it would change the timing slightly.

    And that’s another issue.. to me, it says that your points assemblies aren’t really secure to the stator plate. Either that, or you’re being too rough with them when you torque the screws that secure the wire contacts. Swapping wires around shouldn’t change timing geometry, but it can, if the points aren’t firmly in place when you play with the screws,

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    vintin
    US Member - 1 Year
    Replies: 555
    Topics: 16
    #16811

    Ignition plates sometime have some slop from wear which can affect timing and the readings we are counting on.

    Tubs
    Tubs
    US Member - 1 Year
    Replies: 2232
    Topics: 127
    #16924
    quote legendre:

    @tubs

    In a flywheel magneto system (as in OBMs) there are two ignition events which must be correctly timed. Event #1, is the peak flux through the ignition coil, which is the point where the spark is the strongest. Event #2 is the correct point where the plugs must fire the cylinder(s). If you’ve ever set timing on a car, truck engine, you are setting event #2 – that’s “the timing” we all know of.

    Think of #1 as an acceptable range and #2 as a very specific instant within that range. Event #1’s range cannot be adjusted, it’s a function of the relationship between the rotor magnet(s) and the ignition coil(s) – and that’s fixed at the factory by the placement of coils, magnets and flywheel keyways.

    Event #2 can be adjusted by varying the points gap – so it’s field-adjustable. We have to trust the factory engineers that the correct point for event #2 (which we will set by varying gap) will fall within the desired range of event #1.. and in any properly executed design, that’s just how it works out!


    Event #1, is the peak flux through the ignition coil,
    which is the point where the spark is the strongest.
    Event #1’s range cannot be adjusted, it’s a function
    of the relationship between the rotor magnet(s) and
    the ignition coil(s) – and that’s fixed at the factory by
    the placement of coils, magnets and flywheel keyways.

    In the magneto we are discussing, with points for each coil,
    event 1 is adjustable and is accomplished by adjusting
    the points and it occurs at the exact point that the polarity
    changes when the magnet passes over the coil as it has been
    explained to me.

    Event #2 is the correct point where the plugs must fire the cylinder(s).

    In the magneto we are discussing this is accomplished
    by moving the plate the ignition components are mounted
    on.

    A automotive distributor (usually) only having 1 set of points, a rotor
    to assign the spark to each cylinder, a source for current rather
    than creating it, and components to adjust the timing only
    clouds the issue as its not relatable.

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    jnjvan
    US Member - 1 Year
    Replies: 37
    Topics: 1
    #16926

    Tubs stated it well. The points gap should be set so that event #1 occurs when the flux in the coil is changing most rapidly. I.e. the point at which the flux polarity changes. The point of rapid change in flux is set by the geometry of the magnets, coil laminations and position of the flywheel (set by the flywheel key). Rapid change in flux results in maximum current in the primary coil, which in turn results in a hot spark when the points open.

    Note that the flywheel key sets the relative position of the flywheel magnets, and the points cam. This is what allows a particular points gap to open the points at the proper time for event #1. A timing tool allows for more precise timing, as it eliminates the tolerances of the cam, eccentricity of the crankshaft, and difficulty is setting the gap precisely.

    Event #2, as stated by Tubs, is the preferred timing for best combustion, and is set by the position of the magneto plate with reference to the crankshaft and piston.

    John Van

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    legendre

    Replies: 389
    Topics: 8
    #16946

    If what you’re saying were correct, then there would be no need for the recently-discussed OMC timing tool.. think about it:

    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.

    Absent the factory-made marks, it would be trivial to make your own set of marks using nothing more than a flexible ruler and scratch-awl. Again, it would obsolete the OMC timing tool.

    Again, event #1 is a range, not a specific angular position per se. Look at the shape of a sine wave – you can see that it is at or near the point of maximum current for about 22′ of the cycle. Event #2 (the firing of the spark plug) can occur at any point across that range, and have nearly full spark energy. Sure, max current is at the peak, but allowing 11′ either side of peak would give what, about 90-95% of max? That’s perfectly acceptable.

    So ask yourself, now, why does that OMC timing tool exist? It exists, because event #1 is taken care of by the design engineers and careful manufacturing, and no normal wear & tear will ever alter the range of event #1. Event #2 however needs to be precisely set – it’s not a range, it’s a fixed angle BTDC at a stated RPM.

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    legendre

    Replies: 389
    Topics: 8
    #16947

    Incidentally, the situation with the magneto is really no different than with a conventional automotive-type battery, point & coil system.

    In the automotive system, event #1 is timed by adjusting the dwell angle, which is set to the best compromise between maximum spark energy / reliability and coil operating temperature.

    Then event #2 – the ‘real’ ignition timing is usually adjusted by turning the distributor housing after loosening the base clamp screw. And just as with our magnetos, changing the point gap in the automotive system affects both events #1 and #2 – that is, it alters both spark timing and energy.

    Same situation, really, just with a couple different considerations.

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    vintin
    US Member - 1 Year
    Replies: 555
    Topics: 16
    #16967

    Would it be a correct assumption that max motor output would be achieved by setting the timing just before the onset of spark knock? I’d like to get the max out of my 57 35hp OMC but of course don’t want to cause engine damage. I understand that spark knock is difficult to hear in a wide open outboard motor. Thoughts?

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    legendre

    Replies: 389
    Topics: 8
    #16971
    quote VinTin:

    Would it be a correct assumption that max motor output would be achieved by setting the timing just before the onset of spark knock? I’d like to get the max out of my 57 35hp OMC but of course don’t want to cause engine damage. I understand that spark knock is difficult to hear in a wide open outboard motor. Thoughts?

    Per my understanding, general theory states that, for a given RPM, more advance is better, so long as it’s not causing spark-knock / detonation. But there are a lot of variables and there’s no way to simply say that more advance will or will not benefit your engine.

    If you want to play, I’d trial & error it.. while watching cylinder head temperatures (if possible) and frequently checking the plugs. A higher octane fuel will also tend to tolerate more advanced timing. +Shrug+

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    legendre

    Replies: 389
    Topics: 8
    #16972
    quote legendre:

    quote VinTin:

    Would it be a correct assumption that max motor output would be achieved by setting the timing just before the onset of spark knock? I’d like to get the max out of my 57 35hp OMC but of course don’t want to cause engine damage. I understand that spark knock is difficult to hear in a wide open outboard motor. Thoughts?

    Per my understanding, general theory states that, for a given RPM, more advance = more torque, so long as it’s not causing spark-knock / detonation. But there are a lot of variables and there’s no way to simply say that more advance will or will not benefit your engine.

    If you want to play, I’d trial & error it.. while watching cylinder head temperatures (if possible) and frequently checking the plugs. A higher octane fuel will also tend to tolerate more advanced timing. +Shrug+

    ETA: Not sure how I managed this, but I’ve tried to delete the post several times. System says I can’t.

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    phil-b

    Replies: 467
    Topics: 32
    #16995

    Timing of the spark is only a range relative to TDC.

    The timing of points opening ("event #1") is a fixed relationship between the coil "shoe" and the position of the magnet (which has a fixed relationship to the high point on the cam, since both the flywheel and the cam are keyed onto the crankshaft. This is why you can adjust the points at any throttle setting.

    Timing of the spark relative to TDC at a given throttle position (& carb butterfly position) controlled by the "link and sync" adjustment cam.

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    legendre

    Replies: 389
    Topics: 8
    #17001
    quote Phil B:

    Timing of the spark is only a range relative to TDC. (…) Timing of the spark relative to TDC at a given throttle position (& carb butterfly position) controlled by the “link and sync” adjustment cam.

    Sure.. but what defines the upper & lower bounds?

    Let’s just ignore the fact that the ignition timing changes with mag lever / throttle position.. and view the motor at full speed, max RPM and hence, max ignition advance.. just as with any motor, running at full speed and full advance.

    What defines that final, high-RPM timing angle figure? Is it a certain, factory-specified number of degrees BTDC (as with every other engine), or just wherever it happens to end up, if ‘timing’ is set for the moment of max spark energy – on an OMC?

    Are you saying that OMC timing theory is literally different from virtually every other engine out there? Every other engine I’ve encountered has ignition timing specified in degrees BTDC – but per you, and those who agree with you, that’s out the window. You’re claiming that the spark should fire at the moment of greatest energy, and the actual ignition event just falls where it may.

    I don’t buy it. And again, ask yourselves: Why does the OMC timing tool exist?

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    phil-b

    Replies: 467
    Topics: 32
    #17002
    quote legendre:

    What defines that final, high-RPM timing angle figure? Is it a certain, factory-specified number of degrees BTDC (as with every other engine), or just wherever it happens to end up, if ‘timing’ is set for the moment of max spark energy – on an OMC?

    I don’t buy it. And again, ask yourselves: Why does the OMC timing tool exist?

    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.

    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)

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