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Hi folks, yet another "large RPM drop" post.

The 912ULS in my RV-12 worked great until I replaced all the spark plugs at a recent oil change. Since then, it's been failing mag checks when I turn off ignition B. Most recently I replaced all the spark plugs again with new DCPR8E. I do my mag checks at 4000 rpm, and currently get a 1000 rpm drop when turning off B. Despite the 1000 rpm drop, the engine remains quite smooth at 3000 rpm. Things I've tried:

- Swapping the 6-pin connectors (new-style ignition modules). Swapping either connector makes it fail when turning off "A" instead. I think that means the problem is "downstream" of the CDI modules, pointing to coils/wire/spark plugs.

- Checked resistance of primary side of all 4 ignition coils. All read 0.1-0.2 ohms on my crappy meter (manual says it should be 0.25-0.35, but I suspect my meter just isn't accurate that far down). I wiggled the wires a bit to look for an intermittent open but it seems solid.

- Checked resistance from plug cap to plug cap of opposite spark plugs (e.g. 1T-3T, 2T-4T). All are 15k-17k, as expected.

Anything else I should be looking at?

  • Re: Large RPM drop during runup

    by » 2 weeks ago


    Drop on B.  Move 2 and 4 plugs from A to B lower to upper, upper to lower.   Does the failure remain on B or move to A?  no change?  Move 1 and 3 plugs.   No change ?  Drop still on B.   It's not a plug...

     


  • Re: Large RPM drop during runup

    by » 2 weeks ago


    Are you using the thermal paste on the spark plug threads and if so, ensuring the paste does not come in contact with the plug electrodes?


  • Re: Large RPM drop during runup

    by » 2 weeks ago


    Yep, thermal paste on the thread with care to make sure it doesn't get on the electrodes. Good call Stephen, I'll try swapping upper-lower one side at a time next time I'm out there.


  • Re: Large RPM drop during runup

    by » 2 weeks ago


    I’ve had new out of the box plugs that were bad, something to consider.


  • Re: Large RPM drop during runup

    by » one week ago


    The plot thickens...

    We started today by taking all the spark plugs out. The top plugs from cylinders 1 and 2 were quite black (after less than an hour of ground runs), 1-2 bottom were grey, and 3-4 top and bottom looked very clean. I saw somewhere that it's normal for 1 and 2 to run richer, so I wasn't too worried about that, but we did see some signs of thermal grease contamination which we carefully cleaned off. After reassembly, there wasn't really any improvement. But we used an IR thermometer and found that cylinder #2 was substantially cooler than the rest. Additionally, a timing light on that ignition lead wouldn't fire reliably. After once again replacing the top plug (and boot, because I had a spare), we ran the engine for a few minutes at 4000 rpm; it was pretty rough at first but gradually improved. After 5 minutes at 4000 rpm the mag drop was 100 rpm on either ignition, and after 5 more minutes I tested again and it was down to 60 rpm, same on both sides. Beautiful!

    So I shut down, put the top cowling back on, and taxied out. Short of the runway, the drop was nearly 300 rpm when turning off ignition "A"... almost out of spec, but "almost out" means within, so I went flying (staying close to the airport to be safe). The engine performed great throughout climb and cruise. I tried turning off one or the other ignition in flight, and there was minimal drop (50-100rpm) and no roughness. However, the engine started to sound a little rougher in the pattern coming in for landing. After landing I tried another mag check at 4000 rpm just to be sure, and the drop was 600+ rpm on ignition "A" only, <100 on ignition "B" only.

    Maybe the plugs are fouling? I know these engines are happiest close to full power, but the plugs shouldn't foul to the point of failing a mag check in the time it takes me to taxi to the runway... Any other ideas or things to check?


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