How Important is OMC Ignition Timing?

[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. 😕

[legendre]

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".

[legendre]

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. 😕

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.

[legendre]

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,

[vintin]

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

[The Boat House]

• This reply was modified 6 years ago by The Boat House.
• This reply was modified 3 years, 9 months ago by The Boat House.

[jnjvan]

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

[legendre]

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.

[legendre]

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.

[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?

[Author]

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