I think I would check this area for a leak in these air hoses and that muffler. I have read about some of these opti motors having a air filter that would collapse. I can’t seem to find I’d listed for your motor so don’t know if you have it or not. But would sure check.
Mercury Mercury & Mariner Outboard Parts by HP & Liter 75HP OEM Parts Diagram for Air Hoses | Boats.net