We have a ten year old Merc 4 stroke 90 HP outboard that is acting up. For its age, it has very low hours, with the only historical problems being gummed up fuel system due to limited use. All fuel lines were replaced about 18 months ago. It was running fine until the last time out when the engine started sputtering and putting out periodic white smoke (at idle and at speed). There is a little fuel or oil coming up from the exhaust output at the prop at idle.

Thoughts? Possible exhaust gasket problem?