My Titan Tornado II powered by a Rotax 912S did not run for 2 - 3 weeks while I replaced a windshield in January 2013. During the replacement very cold weather set into the Northern Utah area (temps don into the teens F.).

After replacement of the windshield, and upon running the engine I noted a mag drop of about 350 RPM when swithcing from "Both" to the "Left Mag" (the condition did not exist prior to the down time and cold temps). The "Right Mag" yields about a 150 to 200 RPM drop. I should mention at this point that aircraft and engine are hangared at a temperature of 45 -50 deg. F., and were not exposed to the really cold temps until pulled out of the hangar for runup and flight.

Could the cold temps be a factor? Nothing else that I am aware of has changed.

I installed new spark plugs to see if that was the problem, and experienced the same mag drops on runup. The new plugs are gapped to .028 inches. Is that too wide of a gap for the colder temperatures? What should they be gapped to for the conditions?

I also ordered a new Spark Plug Cap to compare cap engagement with the "old," as during installation of the new plugs I noted via feel that cap engagement was not equal on all spark plugs. As I checked the new cap with one sample plug--one where plug cap engagement felt looser than the rest, I found the new cap to engage more positively than the old.

If re-gapping dosen't solve the mag drop problem, I thought that my next step would be to replace the spark plug caps (not cheap).

Is there a more logical course of action to resolve the excessive mag check RPM drop? Or a better way to diagnose the problem?

  • Re: Excessive Mag drop during Runup

    by » 11 years ago

    Hi Ron,

    .028 is way to wide for really cold weather and it will increase the mag drop.
    The Rotax 912 plug gap should be .023 to .027.
    Cold weather gets smaller gaps and hot weather gets larger gaps. If you are down in the teens for cold then you may want to be down at the lower gap setting of .023. During the cool early spring air around .025 and the hotter summer months .027.
    Re-gap them and apply the thermal paste and try again.
    If this doesn't fix it all then make sure the carbs are synced.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Ron Parr

  • Re: Excessive Mag drop during Runup

    by » 11 years ago

    Thank you for the suggestions Roger. I'll try re-gapping, and check carb sync and see what happens.


  • Re: Excessive Mag drop during Runup

    by » 11 years ago

    Hi Ron

    Just a thought, and it might sound strange, but try warming you CDI ignition modules before you start next time with a hair dryer or heat gun.

    They are prone to problems that seem to disappear when warmed up.

    Before you do anything else, try this and see if it makes a difference.

    Cheers Mark

  • Re: Excessive Mag drop during Runup

    by » 11 years ago

    I had a loose Spark Plug Cap on my 912UL and replaced it with a new cap (expensive). A friend of mine had a loose cap on his 912S a few days later, took a pair of pliers and squeezed the cap slightly and put it back on the Spark Plug. Seems to have worked fine on his 912S.

  • Re: Excessive Mag drop during Runup

    by » 11 years ago

    Hi All,

    I re-gapped my plugs to .025 inches, re-synced the carbs, and the problem with excessive mag drop disappered.

    ...but a new problem has surfaced. My 912S is now a bugger to start. The RPM is reading 800 RPM during the start cycle (way too low). You know how if you advance the throttle while the "choke" is on while cranking it won't start? It's almost like one carb side is starting and the other isn't.

    One thought I had was to make sure the choke is opened on both carbs when applied. Any other thoughts?

    The carbs were synced at 3500 RPM, but I did notice what I thought was a pretty big range in pressure on the sync gages between the left and right sides when throttled back to 2500 RPM (about 2 1/2 to 3 inches. I wondered if that is too much for the start cycle.

    Once it catches the engine runs great.


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