How Important is OMC Ignition Timing?

[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..

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.

[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.

[david-bartlett]

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!"

[legendre]

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?

[wbeaton]

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]

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
classicomctools@gmail.com

[legendre]

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]

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
classicomctools@gmail.com

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