They don't idle well and stall more readily with old/worn plugs. Trouble is as a plug ages you don't notice the drop in idle speed and smooth tick-over, as it's gradual. My guess is that any fresh plug would have had the same effect on the tick-over.

As for the musty smell, mine often does that after I've fiddled with things due to the fact I leave greasy fingerprints everywhere, which then burn off when the engine is started.

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