Mag drop

Category: 912 / 914 Technical Questions 4 months ago
I have a 912 UL engine on a ppc, with only 125 hours on it. Recently completed annual condition inspection, replaced plugs, and all seemed fine. Today started, warmed to 122, and found that running at 3500 rpm the mag drop was only 10 or 20 on one side, but 500 on the other, with engine running mildly rough. Obviously something is wrong with one or both mags. Any suggestions about where to start, or what most likely problem might be would be appreciated.
  • Re: Mag drop

    by » 4 months ago


    Make sure you did not get heat transfer compound on the electrodes of one or more of the plugs, all plug caps are pushed on all the way to the plug, including the wires in in the coil packs. Also try trimming 3/8" off each end of plug wire at coil pack and plug cap reinstall with dielectric grease. Verify ignition ground on the ignition modules and coil packs.

  • Re: Mag drop

    by » 4 months ago


    Checked plugs. They all looked good. Switched bottom to top. Restarted and found everything the same. Same mag dropping 500-600, other still 10-30.

  • Re: Mag drop

    by » 4 months ago


    You can swap the ignition modules and the take a meter and verify the grounds, check for broken wire under insulation near connectors. Verify the resistance from plug cap to plug cap I forget what the value is but if you notice one plug cap goes to the plug wire then to the coil and back out of the coil to another wire and plug cap. The total resistance of all 4 of each of these circuits should match. Or just individually measure the resistance of the parts and look for a discrepancy by comparing.

  • Re: Mag drop

    by » 4 months ago


    "checked plugs" means what? They all appear to be firing? I would put old plugs back in the bad mag side. One of your new plugs is probably bad.

  • Re: Mag drop

    by » 4 months ago


    Reversed mag check p-leads on test switch. 500 rpm circuit drop reversed as well. Checked resistance from cap, across coil and wires to opposite cap. All 4 circuits measured same resistance, so coils, spark plug wires, and caps should be good. Now, maybe a very expensive module to replace, or first replace inexpensive plugs. Will try that tomorrow. If that doesn't work will call Lockwood onMonday. Does anyone have any other ideas?

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