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Mag drop

in 912 / 914 Technical Questions 7 years ago
Hello everyone,

I fly a Vans RV-12 with the 912 uls engine, 2009 vintage with 350 hours. The Mag drops seem excessive at 160 A ignition and 270 B ignition. I changed the oil, spark plugs, had the carbs overhauled and synched. All spark plug caps (ngk connectors) read at 4.6 kohms resistance (normal) so I am not sure what is causing the 'mag' drop. It runs rough still on the B CDI 'mag' circuit. ? Is this a dirty air filter issue
? Is this a stator coil trigger pickup issue
? could it be a focal CYLINDER problem ( I saw trace oiliness on cylinder 2 spark plugs of last change) - is a compression check in order (it was fine 8 months ago by hx)

The ignition harness seems intact but I haven'the tested each plug wire. It is hard to find a rotax specialist in Denver. Please help.

  • Re: Mag drop

    by » 7 years ago

    update: changed out spark plugs again with new NGK caps; re-synched engine at 3000 rpm; still mag drop ignition A of 200 and ignition B of 300 with roughness on later.
    afraid to fly it. reviewed last 9 yrs of annuals: always just 100 mag drop A and B circuits. Just had carbs overhauled/inspected at Lockwood. Told me they keep the needle jet at owners/prior placement; which I never touched. Called builder of my Vans RV-12 and he suggested checking spark plug firing by firing up engine on Ignition A and touching all four exhaust pipes; instead will get inductive timing light to see if each spark plug is firing. Suspect electrical system problem still. Will ask my A & P to check ignition wiring for hidden/occult wire break, especially if get missing spark plug firing on individual testing. Hopefully it won't be a coil (transformer) problem, or module (capacitor) problem; or heaven help me a stator/trigger coil gap problem. I am just the owner and not qualified to take the engine apart.
      
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  • Re: Mag drop

    by » 7 years ago

    Hi James,
    Exchanging the caps made the mag drop worse. That makes me guess that a thorough check of the spark plug wiring makes perfect sense. No. 1, to change the plugs you had to fiddle about with the wires which can make the effect of a broken wire worse
    No. 2, not replacing the wires can deteriorate the connection between new cap and wire which may or may not explain the bigger mag drop
    If I may suggest you take off the new caps and cut back the wires by 1/2 inch and screw the caps back in place. Allow a little drop of silicon on the outer surface of the wires so the caps can be screwed on easier (less friction between rubber of cap and wire; also you can safely tell when they really are in place).
    It's a little effort at no cost. Plus, it can be done by non-scientists like us.
    Good luck.
      
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  • Re: Mag drop

    by » 7 years ago

    Thanks Peter so much for the advice. I went out today and tested the engine running on ignition A and all four exhaust pipes measured about the same temperature; then repeated the same on ignition B (on RPM of only 2200), so I think the bad ignition/plug idea is wrong.

    It is probably something more overt and simple, such as a bad synch job or carburetor issues. I just got the carbs back from Lockwood Aviation in Florida for their overhaul, but maybe something is still amiss. I still get RPM drops at runup at 4,000 of 200 ignition A and 300 ignition B, so a single spark plug or broken wire would not affect both ignition circuits.

    tomorrow I will check the carb floats again (I did have a sunken float several weeks ago), and the carb vent tubes (will take them out of the hanger which crimps them a little bit; wondering if this restricts the carb float chamber pressure equalization at working engine RPM).

    I will keep you posted. My local A&P gets a look at it later this week; but again he rarely works on a rotax engine.

    James
      
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  • Re: Mag drop

    by » 7 years ago

    Today I checked both float bowls; they were pristine and had no debri. all four floats weighed between 3 gm and 3.1 gm each; so were all in spec. I synched again at 2500 RPM. I am still getting A mag drop of 160 and B mag drop of 270. This is within Rotaxes spec drops of 120 diff and 300 max so I will just live with it. I pulled the choke during the mag check and it dropped another 300 rpm and really sputtered so I think it is more a summertime in Denver problem of just the Bing carbs running rich at altitude. I heard they are not really altitude adjusting and likely just are running rich. My airport is 6000 feet AGL and in summer the density altitude is 7,000 to 8,000 feet. I could probably adjust the needle jet setting but will just wait out the summer and fly the airplane. Maybe much ado about nothing.

    James
      
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  • Re: Mag drop

    by » 7 years ago

    just a final update:

    the mag drop dropped back to normal for me; at 150 rpm drop each (actually 130 and 160 a and b) after a better carb synch using a dual manifold helio gauge by an experienced a/p for the rotax; as well as setting the needle jet to the second setting; since i fly out of 5,000 feet msl with DA's of 7,000 to 8,000 feet in the summer. back to normal. so great.
    Thank you said by: RotaxOwner Admin
      
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