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By Pete L
So, lined up ready for take-off after annual. Carry out the power checks, one mag off in turn.

Very occasional miss on the right mag about once every five seconds, although overall RPM in correct band. Think "It's never done that before". A few moments thought, try again, same effect.

Decide eventually not to take it into the air. Taxi back to maintenance hangar.

Redo power check just before shutting down. Instant 300rpm+ drop, so obviously something wrong.

Start up with mechanic listening. Fault now present on left as well to lesser degree.

Quite glad that the "it's not right" antennae are working - I'd never had one of this type of event before.

Conclusion from the mechanics was a plug breakdown exacerbated by cleaning during the annual. Eight new plugs appear to have cured it.
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By Flyin'Dutch'
Good call!