There are a few things it could be and yes, the idle circuit inside the carburetor is one of them. From first hand experience it could also be drag on the shaft imposed by a too-tight stuffing box that only gets tighter as things warm up and thermal expansion comes into play. Remember, at idle you have only 5HP shaft power (direct drive) so it doesn't take much to overpower it.
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Neil
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others
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