What you are describing happened to me a couple years ago not long after I bought the boat from it's original owner.
Check air pressure and look for fuel on the air side. If you see fuel on the air side, one of two diaphragms could be leaking fuel (90psi) to the air side (80psi) causing a flooding condition which clears up after a couple minutes. This only happened to me a few times... for the most part my engine started normally. When I took the boat in for service and explained this issue, my mercury mechanic knew right away. There are three spring loaded rubber diaphragm regulators in the Optimax, two that have fuel on one side and air on the other side. One is the fuel regulator that maintains fuel pressure 10psi > air pressure, and the other is the tracker valve which minimizes pressure surges that come from the air and fuel pumps. The third regulator is the air regulator but it does not have fuel on the other side. All three regulators have a rubber diaphragm so if one has worn out then it is good practice to change all three. This fixed my issue and the tracker valve was the one that was leaking by although the other two had already started cracking. The service manual has a great diagram of the fuel/air system with a full step by step description of how it works.
Live with it, or get it checked out and be ready to BOAT
Hopefully I am wrong and it is something else, perhaps something super simple. Good luck and please post once you figure it out.
Tony