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I'm afraid I don't know enough about your outboard to provide you with a solution but the problem interested me and I found something that
I don't know that particular engine, but 3000rpm doesn't sound like a roll-back protection rpm number and I would be more suspect of the fuel delivery
Remove the air filters and see if you can spin the turbines (compressor vanes) by hand. If the boat has been sitting the turbines may be stuck to the
Check your boost. Same symptom as a stuck turbo or leak on inlet side giving no boost. Other thoughts - are the bottom of the hull and the prop clean?
I have found with the merc diagrams here when they only give you one number for the top seal the bottom seals are the same number. The two on the bottom