Ignition Check Anomaly

My understanding is, that for a  meaningful ignition check, Rotax 912 should be operated at no less than 4000RPM

Don't understand your comment.  My ignition checks were always done at 4K RPM, as per specs.  Are you suggesting ignition check was done at lower RPM?  My concern is big mag drop on one side before flight; normal check postflight. Jim

Jim Hurd wrote:

Don't understand your comment.  My ignition checks were always done at 4K RPM, as per specs.  Are you suggesting ignition check was done at lower RPM?  My concern is big mag drop on one side before flight; normal check postflight. Jim

That's okay Jim - lots of Rotax drivers make the mistake of doing their ignition checks below the recommend RPM.

How often has this symptom been happening?

Is this a winter problem? Could be front cylinders are still a bit cold when you are doing run ups.

Have you done an ignition check at 4k RPM after a flight? If so what if any symptoms?

Do you have an extended taxi (engine running at low RPM) before shut down? If so suboptimal combustion may be causing some carbon/soot build up on plugs. Could try doing pre-flight ignition check just befor take off -engine nearing full operating temperature & time to burn off carbon.

Loose/poorly fitting plug caps could be increasing resistance (poor spark)  particularly when engine still relativly cool.

If above doesnt help, I suggest a very close look at all ignition related connections, paying particular attention to good tight contacts (including earth/ground). Tug on all connections looking for unusuall/unacceotable movement.

Explain the difference between a mag check at 3500 vs 4000?

If your front plugs are looking rich then you may have poor fuel atomisation casued by blocked drillings in throttle body just downstream of the butteryfly valve. Give them a careful clean out with some carb cleaner and compressed air. I am not sure that this would explain your 200rpm differential though.