Just diagnosed the same symptoms on my 80s mercury mariner 60hp..
Since mech says everything's okay.. Look at everything before motor.
Is tank venting? Fuel line sucking air? U might have to check prime bulb, some after market brands don't work well.. Position is with arrow pointing upwards just before engine..
That's assuming mech is correct.. If not.. Might be carbs, faulty coil, plug, plug lead, switch box, the stator might be packing up..
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