I have twin 2008 Yamahas 115 HP four stroke. One of the motors stalls when cold started. Sometimes it run a rough idle and then when tried to accelerate in neutral it shuts down. Finally the outboard will run kind of rough and starts little by little to normalize apparently after it gets hot. Also there is apparently fuel sheen behind the boat on the water. Once the motor is hot and running for a bit, everything is fine again. What could this be ? THANKS !