How Important is OMC Ignition Timing?

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    legendre


    Replies: 389
    Topics: 8
    #14607
    quote FrankR:

    The factory service manuals say to have it at WOT, but the fact is it really doesn’t matter. The only reason I can see for saying to have it at WOT is that would hold the armature plate steady while you are messing with it.

    Ah of course! You’re right, it doesn’t matter – as we’re only concerned with the angular relationship between the armature plate and the points cam on the crankshaft.. how far one leads or lags the other.

    What matters in this case is the angular displacement of the armature plate from the crank position. If the plate moves, the crankshaft would be moved to reestablish alignment of the marks, or vice-versa.

    Actual position of the armature plate is only of concern if we’re measuring timing angle with respect to TDC, which is a fixed point of reference.

    Thanks for catching my blunder! Very much appreciated.

    Avatar
    grover

    US Member - 1 Year
    Replies: 12
    Topics: 2
    #14615

    Once I used a timing fixture, I never used a feeler gauge again. The spark is hot and the motors start and run noticeably better.

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    elgin2

    US Member - 2 Years
    Replies: 111
    Topics: 24
    #14630

    Franks tools takes all the guess-work out of timing for sure.

    Tom
    Tom

    US Member - 2 Years
    Replies: 455
    Topics: 45
    #14631

    OK, this covers how to determine when the points open. If you don’t have a timing tool, you can still get this done… I use some kind of stationary pointer that can be screwed on to the block, and get close to the flywheel.

    First, make sure the mag plate is locked down and won’t move. Adjust the first set of points using a feeler gauge, then get the flywheel to where that set of points is just opening. Put a mark on the flywheel where the pointer is. Then use a tape measure to determine the circumference of the flywheel. At one half the circumference from the first mark, put a second mark. (This is 180 degrees from the first mark.) Line the second mark up with the pointer, then adjust the second set of points to where they just open.

    I was quite surprised how far off the initial settings (just using gap) were on a Scott 10 hp Pennant I had a while back. It was probably 15 or 20 degrees off. The motor ran better, and idled a LOT better with the timing adjusted correctly.

    Tom

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    aquasonic

    US Member - 2 Years
    Replies: 451
    Topics: 36
    #14632

    Many thanks to all who weighed in on this subject. This thread has been very educational for me, and it has convinced me to purchase one of Frank’s timing fixtures. It will be interesting to see the results first hand.

    RICHARD A. WHITE
    RICHARD A. WHITE

    US Member - 2 Years
    Replies: 1763
    Topics: 136
    #15408

    So this process should be able to work on nearly all 2 cylinder opposed motors? With that being said, one need to find where one set of points opens, using a Ohm meter, then ensure the other set opens exactly 180 degrees from that.

    Am I correct? I have some Mercs, that need this done as I have always used feelers gages to accomplish this, but it is close is not always good enough.

    Thanks

    Richard

    http://www.richardsoutboardtools.com
    classicomctools@gmail.com

    Avatar
    legendre


    Replies: 389
    Topics: 8
    #15422
    quote Tom Manley:

    OK, this covers how to determine when the points open

    Pardon this, but I wasn’t able to send you a PM.

    Are you the same Manley that I know from the Minneapolis bike scene, in the 1990s?

    Avatar
    legendre


    Replies: 389
    Topics: 8
    #15424
    quote Richard A. White:

    So this process should be able to work on nearly all 2 cylinder opposed motors? With that being said, one need to find where one set of points opens, using a Ohm meter, then ensure the other set opens exactly 180 degrees from that.

    Am I correct? I have some Mercs, that need this done as I have always used feelers gages to accomplish this, but it is close is not always good enough.

    Absolutely. Establishing that correct fire-point for the ‘first’ cylinder is important.. but setting the two fire points exactly 180′ off is even more important, if the motor is to run smoothly.

    In other words, the relative (cylinder vs. cylinder) timing relationship is probably more critical to running quality than precise absolute (cylinder vs. crank position) timing.

    Avatar
    legendre


    Replies: 389
    Topics: 8
    #15425

    And since we’re on the topic.. I’m still waiting to hear from one of the experts, where an OMC motor should +precisely+ time, with respect to crank position.

    In this case, it’s specific to the Johnson TD20 – where (in terms of degrees, mm or inches) should the spark occur?

    RICHARD A. WHITE
    RICHARD A. WHITE

    US Member - 2 Years
    Replies: 1763
    Topics: 136
    #15427

    I am recalling the motor I am working on now states the proper "timing" is .265 BTDC. This is on a 1954 Mercury Mark 20H . That being said, if I set the points to open at that point and set the second cylinder 180 out from that this beast should be dare I say perfect?

    http://www.richardsoutboardtools.com
    classicomctools@gmail.com

    RICHARD A. WHITE
    RICHARD A. WHITE

    US Member - 2 Years
    Replies: 1763
    Topics: 136
    #15428

    And now since I do not have a degree wheel, or the proper timing template called out for in the manuals, I will find the diameter of the mag, plot that out in CAD, with 2 marks 180 degrees apart, fabricate a pointer and do this properly, then make one for OMC motors too. Gosh this is fun, LOL

    http://www.richardsoutboardtools.com
    classicomctools@gmail.com

    The Boat House
    The Boat House


    Replies: 3399
    Topics: 91
    #15435

    You're only as smart as the person you're talking to.

    • This reply was modified 2 years, 3 months ago by AvatarThe Boat House.
    • This reply was modified 1 week, 3 days ago by The Boat HouseThe Boat House.
    Avatar
    legendre


    Replies: 389
    Topics: 8
    #15465
    quote Tubs:

    When a timing specifications is given it is made at
    a fix point.

    Correct, and it’s normally quoted in degrees before (or after) TDC.

    quote :

    The components of the ignition will now
    make adjustments so the timing is correct as to load
    and RPM.

    That’s an oversimplification. What happens, is that timing changes as the speed control is moved up & down the range. The spark advance increases as the control is moved up through the range.

    quote :

    Now in the motor your discussing the
    timing is controlled by you.

    Ok, sure – we understand one another..

    quote :

    There is no timing
    specifications because the mag plate isn’t fixed and is
    constantly moving.

    That’s a mis-statement. There most certainly +is+ a timing spec – and again, it’s normally quoted in degrees BTDC. On any motor, there will be timing specs for idle and/or full speed operation. Even in the case of manually variable timing as on this old Johnson, there are still lower and upper limits for ignition timing.

    quote :

    The timing would need to be only
    matched to the carburetor.

    Ideally, timing is linked to RPM and load, which is why some engines have features like centrifugal and vacuum auto-advance mechanisms – but you’re correct that on this Johnson, the timing is directly tied to the position of the main carb butterfly. That’s a reasonable compromise, as the engineers figure that at a given throttle position, the engine speed will be X and as such, the timing should be Y. This is very reasonable on an OBM as the load is very predictable – all water acts about the same, driven by the same prop at a particular RPM.

    The advance will at first be too high when the throttle is first moved from idle to full, but everything falls into place once the engine RPM catches up, which is (or ought to be!) shortly after the adjustment.

    Having said all of this, there are still two hard datum to be known – the correct advance at low / zero throttle, and the same at full throttle. If the factory doesn’t quote it, that’s too bad – but it still can be deduced and measured using the correct timing tools.

    The Boat House
    The Boat House


    Replies: 3399
    Topics: 91
    #15479

    .

    You're only as smart as the person you're talking to.

    • This reply was modified 2 years, 3 months ago by AvatarThe Boat House.
    • This reply was modified 1 day, 2 hours ago by The Boat HouseThe Boat House.
    RICHARD A. WHITE
    RICHARD A. WHITE

    US Member - 2 Years
    Replies: 1763
    Topics: 136
    #15490

    So, I made me a tool today, Used ohm meter set to audible to stop make the effing racket just as points break.
    Reading the manual. it states that you set the gap approximately 1/4 inch past the initial opening. This is for the Phelon ignition. Does that sound right?

    Thanks

    http://www.richardsoutboardtools.com
    classicomctools@gmail.com

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