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Before you call the dealer try doing the decarb on the engine.

Lets review: You had a slight problem last year before winter layup. you fogged the engine and then let it sit, all winter. You start it up, run ok, until you let it idle for awhile. Now all that carbon, is nice and gooey but not really going anywhere. Engine boggs down, runs rough. Switch back to old plugs, still runs rough, not a happy engine.
Most engine manufactures make some cleaning product to reduce carbon buildup.
Mercury - QuickClean
Yahama - Ring-Free
Generic - Seafoam

Sal's decarb technique