[Sans Peur]

Is this every time you start it or just the first time after the boat has been sitting in the slip for awhile?
If it is the first time it may be from just sitting.
I keep a dehumidifier running while I am not on the boat. It helps keep some of the odors down and all of the mold. I also put some citrus cleaner in the mid bilge to keep that from causing odors in the cabin.
If it is every time you start the boat. Do you leave the transom door open when you start it? That may allow some fumes to come into the cockpit and ultimately the cabin. You may also need to check the engine room and make sure there isn't an exhaust leak at start up.