Weak spark on 1973 20 Hp Johnson

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    Evinrude 25


    Replies: 12
    Topics: 4
    #208894

    Hello All,
    I am new to this forum. I am not sure my outboard is old enough to be in this Antique site. I picked up an old and beat up 1973 20 Hp Johnson (the first picture) and want to restore it.

    59

    I have taken a part the power head and rehone the cylinder, both of them got 115/113 psi. The block has been sand blasted and paint back base color. So far I got done step #1.
    Step 2: Spark. It did not have spark so I have replaced two external ignition coils, new points and condensers. I got OK spark on the cylinder #2 and intermittent spark on cylinder #1 (sometime no spark at all). I have set the gap of 0.020″. The only thing I did not replace is the Charging Coil 581130 (Driver Coil). There is only one charge coil on OMC Armature Plate Assembly. Does it have 2 independent coil inside for each cylinder or only one for both? Does the point gap (0.020″) affect the spark quality?
    Do I need to replace the Charging coil too?
    All opinions and helps are welcomed.

    Johnson20-1

    Johnson20-2

    Johnson20-3

    • This topic was modified 2 weeks, 3 days ago by AvatarEvinrude 25.
    • This topic was modified 2 weeks, 1 day ago by MumblesMumbles.
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    fleetwin

    US Member - 2 Years
    Replies: 3278
    Topics: 49
    #208912

    OK, well you have the infamous OMC “low tension ignition system”….Kind of a “hybrid” between the original OMC universal magneto and the CDII system that would come in 1977. This system is often hated, mostly due to a lack of understanding…
    Yes, I would update/change that driver coil unless it seems relatively new, there was many internal changes/improvements made to that piece. The driver coil is a single winding that generates the primary voltage for both coils. But, before you replace it, you need to check for side to side slop on the magneto plate, this is a common problem. If the mag plate is sloppy from side to side, or up and down, point gap will vary and ignition output will be poor. The other issue is the air gap between the driver coil laminations and the flywheel magnets. This must be properly set with an alignment tool. Slop in the mag plate will also result in having the flywheel magnets rub on the driver coil. But, if the driver coil is set in too far from the flywheel magnets, primary output will be weak also.
    Sounds like you already have replaced the the external coils. But, there are a few other things to consider. First, there is a black ground strap the runs from the mag plate to a good ground on the block. This should be checked using an ohm meter on the low scale. Twist and turn the mag plate also, to ensure the lead isn’t chafed/broken/intermittent. The same issues occur with the wiring harness that runs out of the mag plate to the coils. Those leads can get chafed/split/grounded.
    Finally, and I should have lead with this, is the stop switch. Pressing the stop switch shorts both sets of points together, killing primary output. So, it is best to disconnect the two stop switch leads from the ignition harness to make sure the switch is not faulty. It is also possible that the stop leads that are connected to the breaker points are grounded/chafed onto the mag plate. So, you may want to disconnect the stop leads right at each set of points and recheck to rule out that possibility…
    It is also important to realize that both sets of points must be set and working correctly, or you could lose spark to both/opposite cylinder. The primary voltage completes its path back to the driver coil through the opposite set of points that is closed….So, let’s just say one set of points was not closing properly, or perhaps oil soaked…You might lose spark to the opposite cylinder because its primary signal can’t make it back to the driver coil through the points from the other cylinder….

    • This reply was modified 2 weeks, 3 days ago by Avatarfleetwin. Reason: correction
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    Buccaneer
    Buccaneer

    US Member - 1 Year
    Replies: 4004
    Topics: 974
    #208944

    I don’t believe I have any motors with this ignition system, but I have saved your
    excellent description on how it works, just in case! Thanks!

    Prepare to be boarded!

    phil
    phil

    US Member - 2 Years
    Replies: 140
    Topics: 18
    #208968

    Isn’t part of the trick with the low-tension system that the flywheel has two magnets?

    http://www.omc-boats.org
    http://www.aerocraft-boats.org

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    2fast4me

    US Member - 1 Year
    Replies: 219
    Topics: 19
    #208982

    Two sets 180 degrees apart one for each cylinder, two points and condensers, two external coils and one charge coil. Not sure if that was their best idea.

    2Fast4Me

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    fleetwin

    US Member - 2 Years
    Replies: 3278
    Topics: 49
    #209094

    Two sets 180 degrees apart one for each cylinder, two points and condensers, two external coils and one charge coil. Not sure if that was their best idea.

    No, low tension was surely not OMC’s finest hour for sure. More complex and less reliable than regular magneto. This system can be made to work satisfactorily, but requires much more maintenance that regular magneto. The key to success is keeping the mag plate from getting worn resulting in slop. Remember to replace that plastic bushing, 322435, on the engines that use it. I’m pretty sure this engine does not use the plastic bushing though…
    Dan Anderson reminded me that this system came about so a small alternator could be added to these engines. CDII solved all these issues…

    Mumbles
    Mumbles

    Canada Member - 2 Years
    Replies: 4525
    Topics: 432
    #209132

    What you haven’t mentioned is whether or not you installed new spark plug wires and connectors when you changed out the coils. Going by the amount of corrosion in your first photo, I’m willing to bet the spring terminals inside the spark plug boots are badly corroded to or are making a poor connection with the wires core causing no continuity in the wires which in turn is causing the no spark situation.

    DSC00086

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    Evinrude 25


    Replies: 12
    Topics: 4
    #209147

    The new coils have spark plug wire with it

    IMG_0954

    • This reply was modified 2 weeks, 1 day ago by MumblesMumbles.
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    fleetwin

    US Member - 2 Years
    Replies: 3278
    Topics: 49
    #209167

    Yes, I remember you posting about the new coils. But, Mumbles post is a good reminder for the rest of us when checking coil secondary resistance. Those poor/rusted connections often show up as an infinite resistance reading on the high ohms scale…A good coil could be discarded only because of a poor/rusted plug boot/terminal…

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    Evinrude 25


    Replies: 12
    Topics: 4
    #209220

    Hello,
    Found an E-bay of good use Johnson Evinrude 20 Hp Ignition Plate Coil 581131 Driver Coil 581130 Stator of 1975 model and ready for use.
    I am thinking about to swap it since it has two (2) charge coils. Is it the new update or improvement from 1975 model?

    • This reply was modified 2 weeks, 1 day ago by AvatarEvinrude 25.
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    fleetwin

    US Member - 2 Years
    Replies: 3278
    Topics: 49
    #209231

    No, this is not a real set up at all….
    My first thought was that someone mistakenly used driver coils on a conventional mag set up, but that is not the case, there are no spark plug leads coming out of the mag plate. But, on the other hand, I don’t think another driver coil will bolt up to the correct low tension mag plate… The other mounting bosses on the low tension mag plate are for the alternator windings, which has a different bolt set up than the driver coil. So, my only other guess is that someone has attempted to use a conventional universal mag plate so they could bolt up two driver coils. My next question is how this could possibly work, perhaps it never did. The only way I could see this having a chance of working is if the two driver coils are wired in series wired to the points this way. But, I really don’t think the timing would be correct if this stuff was mounted to a conventional universal mag plate… So, perhaps someone machined something up so that two driver coils would mount to the low tension mag plate….
    It seems like you have already bought this item, correct? Hopefully you didn’t pay too much for it. I really don’t see how it could work out, but I will “never say never”. It will be very interesting to see how this is actually wired up once you receive it…It certainly can’t hurt to install it to see if you have spark on both cylinders, only takes a few minutes.
    Again, this is not a factory set up, unless it is some sort of engineering “experiment” you got your hands on. Having an extra mag plate, provided it is a low tension mag plate and not worn is nice…Two extra driver coils and a wiring harness is nice also.
    Very interesting….
    OK, this has really got me thinking….Perhaps one end of each driver coil is grounded to the mag plate, this would provide a return path the primary voltage without having to use the other set of points….I can’t wait to hear if this set up actually sparks on both cylinders…

    • This reply was modified 2 weeks, 1 day ago by Avatarfleetwin. Reason: addition
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    fleetwin

    US Member - 2 Years
    Replies: 3278
    Topics: 49
    #209248

    OK, as you can tell, I am racking my brain trying to figure out what is going on with that mag plate you have purchased….
    Spoke with Dan Anderson who was quick to point out that someone has pressed the driver coil windings onto the conventional universal mag laminations. Dan reminded me that the driver coils are held down with only two screws, these driver coils are held down with three…So, someone has used a conventional universal mag plate, removed the plug wires, installed the low tension wiring harness and pressed the driver coil windings onto conventional universal mag laminations…
    I really don’t see how this could possibly work, but I am smart enough “never to say never”. You will know quickly when you install this set up and see what happens….
    Dan also reminded me of a simple solution here….Now, you have the conventional mag plate, you could simply remove the driver coils, install conventional coils on this mag plate along with plug wires, remove the external coils, and use it as a universal OMC magneto, like found on the 1972 models…I’m pretty sure the flywheels are the same, but will look into that…
    CORRECTION:
    Just looked it up, the 73 models use a driver coil with three screws…Looks like the 72 and 73 mag plates supercede to the same number…So, will say that someone just added a second driver coil to the original mag plate…The flywheels are different between the two systems also…. So, we will wait to see if this set up actually produces spark on both cylinders…

    • This reply was modified 2 weeks ago by Avatarfleetwin. Reason: correction
    Steve D
    Steve D


    Replies: 572
    Topics: 67
    #209251

    All I know of this system is what you, fleetwin, and some others have so nicely and thoroughly explained. So I’m learning as I go. I see what Phil says about the flywheel on these having 2 magnets. I looked at the flywheel from my 1973 Evinrude 25 HP that came with the low tension magneto and noticed that. I’m not trying to confuse the issue or say it should be run like this, but just wondering if that setup with the 2 driver coils would work with the earlier flywheel with just one magnet? Maybe that’s how the seller used it?

    Not saying that this addresses the weak spark issue, just throwing a couple thoughts out there.

    Also, as a side question, why did OMC go to this system in the first place? Was it just an attempt to move the coils out from under the flywheel so they could be replaced easier?

    73E25-flywheel

    • This reply was modified 2 weeks ago by Steve DSteve D.
    • This reply was modified 2 weeks ago by Steve DSteve D.
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    Evinrude 25


    Replies: 12
    Topics: 4
    #209262

    Hello Fleetwin,
    Here is the back of the magplate. As per the description” Good parts removed from johnson 20 hp 1975 model 20BA75B, ready to use ! Fit on many model !” I have tried to look for the diagram of it but I could not find it. Go to http://www.marineengine.com and could not find the 20 hp of 1975 model.

    Mag-1

    • This reply was modified 1 week, 5 days ago by MumblesMumbles.
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    fleetwin

    US Member - 2 Years
    Replies: 3278
    Topics: 49
    #209278

    Yes, that 20hp model the seller described was probably produced and meant for an overseas market, perhaps Canadian. Although, I’m guessing this one was produced in Belgium. You won’t find these models listed on marineengine.com.
    Is your latest picture of your current mag plate, or the one you purchased?

    Install the new strange mag plate, hook it up, check the points/gap, then check it for spark. I will be surprised if it works, but most anything IS possible. I can only imagine this set up could work if wired one of two possible ways. Perhaps the driver coils are wired in series. Or, perhaps one lead on each driver coil is grounded to the mag plate.

    Finally, does your flywheel have one, or two magnets, one on either side of the flywheel?

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