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Ignition malfunction only on top side

in 912 / 914 Technical Questions 6 years ago
I have a ignition issue with my 1992 Rotax 912UL (Old version). When I do my mag checks with my #2 mag (Top) on and my #1 mag (Bottom) off I get a large drop in RPM and lots of roughness. I have tried diagnosing this issue extensively and nothing seems to be making sense. Bellow is the results from the tests that I have done.

In order to diagnose the correct cylinder I turned on the motor with the bad mag for 5 seconds and felt the exhaust pipes for the cool pipe. The bad cylinder is the back right top plug.

I have swapped plugs same cylinder same results.

I know the left side top cylinder fires correctly as the engine runs great with both mags on and pipes are hot all the time.

I pulled the plug connectors on both sides and checedk ~5Kohm resistance on all the plug connectors.

I checked between the two plug connectors that are attached to the bad cylinder (connected through coil) and measured ~ 15 ohms (2 plug connectors and internal wire resistance). This was the same for the other coils. Meaning the two sides are still connected to each-other through the coil (Nothing unplugged)

At this moment I really don't know what else there could be causing this issue.
Can 1 half of a coil go bad?
Where it only fires on side or one of the plugs works but the other doesn't?

I would think the whole coil would go bad not just the top or bottom.
I really would like to avoid having to buy a $300+ coil assembly.

  • Re: Ignition malfunction only on top side

    by » 6 years ago

    There is no Half Coil to go bad.
    Think of the coil as a short-lived 25,000-volt battery.
    One plug is connected to Plus and the other to Minus.
    One plug sparks Tip-to-Arm, the other, Arm-to-Tip.
    They both plugs fire at the same time on every revolution, not every other. Remember this Point.
    When one cylinder is about to complete its compression stroke, the plug lights off the fuel.
    Meanwhile, the other Cylinder is completing the exhaust stroke as its plug fires, doing nothing.
    This is commonly called a wasted spark ignition for obvious reasons.
    The cycle repeats every revolution, alternating wasted and real sparks.
    If one side is firing, the other side is also firing.
    Maybe not where you expect it to fire, but it is firing somewhere.

    The next obvious place to check would be the spark plug cables and connectors.
    If the insulation has become cracked from heat or partially rubbed through, the spark may be finding an easier path to the block than going through the plug.
    You will not find this with an Ohm Meter power by a 9-volt battery. It only happens at the thousands of volts level

    Compressed gas as found in a cylinder just before the plug fires makes a much better insulator than ambient air.
    That is why we use 10's of thousands of volts to jump a gap that would only need 1-2 thousand volts in the open air.
    Consider this the next time you run across an engine that idles fine but misses sparks under power.
    Given the opportunity, the spark will jump out of the Plug Wire if it can.

    How to find it...
    Wiggle wires and see if it disappears.
    Swap wires and see if it moves or disappears.
    Temporarily slide an extra piece of insulation over the suspect wire and see if it disappears. (Firesleve?)
    Run your hand over the wire until it knocks you on your butt. Just kidding!
    Blindly change all the wires and hope you were correct and didn't just waste your lunch money.
    Run the engine behind the barn on a very dark moonless night and notice the pretty blue sparks and corona surrounding the bad wire.
    No Kidding! This really does work! And it is very pretty too! Unfortunately, if you can see your hand, it isn't dark enough.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: RotaxOwner Admin
      
    Report

  • Re: Ignition malfunction only on top side

    by » 6 years ago

    Bill,

    Thank you for the great reply. I had thought it was a wasted spark system which made me even more confused as to why the cylinder was not firing. I am going to try a new plug on that cylinder for both top and bottom just in case I didn't actually remember to switch plugs. Then try to space out the wires as best I can and see if that makes a difference.

    The only thing that makes me hesitant to believe that it's shorting to through the wire is just how consistently off it is. No intermittent spark at all just a dead cylinder even when it's flying and the engine was really shaking. But I'll give it a go now that it sounds like the issue "has" to be coil forward (Wire, Plug Connector, Plug).

    Hopefully it's just a plug. I'll report back when I get a look at it.
      
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  • Re: Ignition malfunction only on top side

    by » 6 years ago

    Well turns out it was a bad plug that I must have missed. So I put all new plugs in and she runs just great now.
    Thank you said by: RotaxOwner Admin, Wayne Fowler
      
    Report

  • Re: Ignition malfunction only on top side

    by » 6 years ago

    Great to hear!
    And certainly way better than blindly replacing a set of coils that would not have made any difference.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
    Report
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