I Have A 20 Hp Mercury Motor 2 Cylinder W/ Model #0b142461. It Seems To Run Ok At Low End But Is Slow On Top End Speed! I Have Rebuilt Carb, Adj. Carb To Spec,replaced Both Spark Plugs And Put In Fresh Gas. I Did Notice However That Upon Removal Of The Lower Spark Plug
That It Had A Little More Carbon Deposit On It Than The Upper Cylinder.I Have Checked Plug And Plug Wire For Spark With An O.k. Spark Tester And The Wire Is Fine But The Plug Would Spark Intermittentley! Could There Be Carbon Build Up In The Cylinder Walls Causing It To Misfire Or Do I Have Another Problem? I Also Noticed That The Impeller Is Slightly Bent On 2 Of The 3 Blades.any Suggestions?

Start with a compression test. if not getting a steady spark then test ignition, output voltage test on stator, trigger and switch box output.
You need to replace the impeller.

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