Do a compression test first but if ign and compression are good and your sure your fuel system is good it could also be a broken reed valve or reed valves not sealing properly. They will cause more of a issue at idle speed than full throttle. And they can make a odd noise if not sealing right because you will have some crank case pressure coming back though the carb/carbs effected. Just something else to check if all else looks ok.
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1983 wellcraft v20 steplift Mercury 250efi on the bracket!!!!
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