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.

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  • Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 4057
    Topics: 418
    #16352
    quote Richard A. White:

    I obviously fail because every time with wires connected, I get continuity. So please explain with pictures preferably exactly WHERE you place the black and red probes.

    Richard

    I use a set of leads with alligator clips on them to set up the timing. One lead is clipped to ground while the other one is clipped to the small screw holding the wires onto the points. The reading in the first pic is with the points closed and the test circuit going thru the points to ground. The reading in the second pic is with the points just opening and the circuit being thru the primary winding of the coil to ground hence the higher reading. Doing it this way is very simple and there is no chance of altering the timing while putting pressure on the screw while tightening it up.

    frankr
    frankr
    US Member - 1 Year
    Replies: 4200
    Topics: 43
    #16358

    The instructions with the timing fixture plainly state that some ohm meters can tell the difference between about an ohm and zero ohms, while other meters cannot tell the difference. At least I thought that was made clear. I’ll have to review what they say.

    Avatar
    legendre

    Replies: 389
    Topics: 8
    #16361
    quote wbeaton:

    quote legendre:

    This is the issue, and it’s why you can’t use a light or buzz-box – neither can really tell the difference between 0 and 1 ohms….

    If you’re using a continuity light or buzz-box, there’s no discernible difference between 0 and 1 (zero and one) ohms – either state will illuminate the lamp or buzz the box.

    Nonsense. Buzz boxes are used to set airplane magnetos so you can be assured it’ll work just fine on an outboard. The buzz box won’t make any noise until the moment the points open.

    Totally different situation. Compare the wiring diagram for one of these OMC magnetos to that of your aircraft magneto, and you’ll quickly realize the difference. Also, look at the pics in Mumbles’ post, just above mine – the difference between open & closed points is only 0.7 ohms!

    Your typical continuity tester will regard anything below a few ohms as a closed-circuit. One example, the continuity ‘beep’ on one of my meters has a 100 ohm threshold, IIRC. That would never work for one of these OMC mags.. it would just beep continuously. See the issue?

    Avatar
    wbeaton
    Canada Member - 1 Year
    Replies: 607
    Topics: 66
    #16367

    I’m not interested in arguing with you, legendre. My video clearly shows a buzz box setting the points without issue, which you state is impossible. That’s all I’ve got to say on this subject.

    Wayne
    Upper Canada Chapter

    uccaomci.com

    Avatar
    vintin
    US Member - 1 Year
    Replies: 555
    Topics: 16
    #16377

    My experience is similar to what Mumbles is seeing.

    Measured with all of the ignition wiring intact I’m getting .6 ohm with the points closed and 1.4 ohm with them open. The .6 is likely wiring resistance in the meter leads.

    There is a very clear change in readings when the points open.

    I’m using a fairly inexpensive Radio Shack digital VOM. (volt, ohm, milliamp)

    Avatar
    chris-p
    Canada Member - 1 Year
    Replies: 2461
    Topics: 157
    #16379
    quote legendre:

    quote wbeaton:

    quote legendre:

    This is the issue, and it’s why you can’t use a light or buzz-box – neither can really tell the difference between 0 and 1 ohms….

    If you’re using a continuity light or buzz-box, there’s no discernible difference between 0 and 1 (zero and one) ohms – either state will illuminate the lamp or buzz the box.

    Nonsense. Buzz boxes are used to set airplane magnetos so you can be assured it’ll work just fine on an outboard. The buzz box won’t make any noise until the moment the points open.

    Totally different situation. Compare the wiring diagram for one of these OMC magnetos to that of your aircraft magneto, and you’ll quickly realize the difference. Also, look at the pics in Mumbles’ post, just above mine – the difference between open & closed points is only 0.7 ohms!

    Your typical continuity tester will regard anything below a few ohms as a closed-circuit. One example, the continuity ‘beep’ on one of my meters has a 100 ohm threshold, IIRC. That would never work for one of these OMC mags.. it would just beep continuously. See the issue?

    Legendre,

    Just a thought, and don’t mean to be rude, but some of the guys on here have a lot of experience with this vintage of motor, and trouble shooting them. They have been doing this for a very long time, and it is what they know. I have learned, even when Im totally certain that Im right, Im usually wrong, when an experienced veteran of this hobby like Frank states differently to my belief at that time. 999 times out of a thousand times anyways! I think Frank was wrong once? Maybe not. šŸ˜‰

    Wayne has taught me a lot about this hobby as well, and sold me my first vintage motor.

    You can most definitely set the ignition with Franks timing fixture, and a buzz box or light. I have done it hundreds of times as well, all with great success.

    These guys are here to help us non mechanics learn!

    Tubs
    Tubs
    US Member - 1 Year
    Replies: 2232
    Topics: 127
    #16439

    Always learning here.
    Had a light bulb moment following this thread.
    Started following this board in the late 90’s.
    I was in my 50’s and had always thought that
    the plugs fired when the points closed. One of
    the first things I learned was that the spark
    actually occurs when they open. From the RBM
    guys I learned that there is a ideal time for the
    points to open to get the best spark.
    Frank recently posted. " The purpose of using the
    timing fixture is it is more accurate, that is the points
    will open at exactly the right moment" and" once
    you set the points with the timing fixture, if you
    then check them (the points) with a feeler gauge, they
    may or may not be exactly .020". I think got it.
    Gaping the points is a timing adjustment. The
    factory gives you a setting that will cause the points
    to open "at exactly the right moment". If the gap is to
    small the points will open too soon. Too large and they
    open late. You still have spark but not the best spark.
    As there can be some variation from one set of points
    to another even though they are set to the correct gap
    they can be off slightly from that "exactly the right
    moment". The purpose of Franks timing tool is not
    to get the plugs firing 180 degrees from each other
    although that is the result. Its to get them to fire
    "at exactly the right moment". From following an
    RBM discussion years ago I learned that the best
    spark occurs at the exact point that the polarity changes
    as the magnet passes over the coil. So – after giving this
    some thought what I have finely gotten out of this is
    whether using a feeler gauge or a timing tool what your
    attempting to do is to have the points open "at exactly
    the right moment" where the polarity changes as the
    magnet passes over the coil to get the strongest spark.
    The result is they idle and run better. Franks timing tool
    insures you have accomplished this. It may not always
    be the case when gaping the points.

    Avatar
    legendre

    Replies: 389
    Topics: 8
    #16747
    quote wbeaton:

    I’m not interested in arguing with you, legendre. My video clearly shows a buzz box setting the points without issue, which you state is impossible. That’s all I’ve got to say on this subject.

    Ok now – there are buzz boxes, and there are buzz boxes. Obviously, the one used in your video must be unlike 90% of all other basic, continuity-type buzzers or lamps. May I ask – does it have an adjustable sensitivity / threshold control? This wasn’t mentioned in the video.

    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..

    Again, your average work-a-day continuity tester isn’t capable of sensing the small (< 1 ohm) shift in resistance when the points break open – and this is what we are trying to get across to our fellow members – yes?

    Avatar
    chris-p
    Canada Member - 1 Year
    Replies: 2461
    Topics: 157
    #16750

    DO a search for buzz box on ebay for pictures. They are not a continuity light, they are designed to sense the resistance shift when the points break.

    I have one like this.

    http://www.ebay.com/itm/300694897331?_t … EBIDX%3AIT

    I like Waynes though, as it has an audible alarm along with the light.

    If you have a cheapo continuity tester, multi meter, etc…, then yes, the points leads need to be taken off. If you have the proper tool, they don’t.

    I did it for a while with a simple analog meter, but I always needed to remove the leads. I found at times, when re connecting them, the force of the screwdriver would throw them off a touch again. That is why, the timing fixture, along with buzz box, is ideal.

    Avatar
    chris-p
    Canada Member - 1 Year
    Replies: 2461
    Topics: 157
    #16751

    Here are some others

    http://www.ebay.com/itm/New-Vertex-magn … b2&vxp=mtr

    http://www.ebay.com/itm/Timing-Buzz-Box … e1&vxp=mtr

    http://www.ebay.com/itm/Magneto-Timer-B … 02&vxp=mtr

    Avatar
    legendre

    Replies: 389
    Topics: 8
    #16757
    quote Chris_P:

    If you have a cheapo continuity tester, multi meter, etc…, then yes, the points leads need to be taken off. If you have the proper tool, they don’t.

    Then we’re all in violent agreement.

    This was my point from the get-go.. if you are using a +common+ continuity light or continuity ‘beeper’ function on a meter, or even many common bzz-boxes, you won’t get the indication that’s required.

    If you use the correct instrument – like a DVM / VOM with sufficient sensitivity, or one of these highly sensitive buzz-boxes, then you are fine.

    It seemed to me that this particular distinction was what was tripping up several members, and as such, it’s what I tried to correct.

    Avatar
    legendre

    Replies: 389
    Topics: 8
    #16759
    quote Chris_P:

    Ok yes, these are all purpose-built buzz-boxes for reading magneto points.

    I never once stated that such a bespoke instrument could not be used for the intended purpose.. only that the most commonly-available, inexpensive devices were for the most part, totally unsuitable.

    Avatar
    aquasonic
    US Member - 1 Year
    Replies: 326
    Topics: 31
    #16760

    When you purchase Frank’s Timing Fixture, it comes with instructions on using a mutimeter. The instructions clearly state that if the multimeter/ohmmeter does not read less than 1 OHM that the leads must be disconnected.

    Whether it’s an analog, digital, or buzzbox, aren’t we talking about the capability of the instrument to read less than 1 OHM?

    My inexpensive analog meter was somehow able to get the timing very close on a Johnson CD-12. In fact, the timing was much closer than just setting the points to 0.020" with a feeler gauge. I was able to observe the before and after with a timing light.

    Yes, I had to disconnect the leads, and I would prefer to just hook up a higher quality meter to the screw at the connection post without having to disconnect the leads, but the lesser quality instrument did get the job done.

    Now, I have a birthday coming in a couple of weeks so…

    Avatar
    legendre

    Replies: 389
    Topics: 8
    #16762
    quote aquasonic:

    Whether it’s an analog, digital, or buzzbox, aren’t we talking about the capability of the instrument to read less than 1 OHM?

    The capability to read differentials of less than one ohm.. that is, it’s primarily a matter of precision, rather than accuracy.

    But yes, that’s the core of the issue.

    Edit: If you want a cheap but very handy and capable meter, as an example check eBay for a Radio Shack mdl. 2200811 or 22-811. Should be less than $20 used, in very nice shape. It will read those low-ohm differentials plus all kinds of other useful functions like capacitance and frequency. Heck, check item #131500421636 for one, $26.00 shipped, brand new in the box.

    Avatar
    legendre

    Replies: 389
    Topics: 8
    #16768

    @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!

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