[captainjay]

I am assuming from the port on the stern of your boat you just came out of winter lay up? Checking the filters and exhaust elbow and pipe are both good suggestions. The exhaust elbow will have an area were the water comes in and mixes with the gases, the bend tends to get a build up of soot that severely restricts the exhaust flow. Also look closely at the water area as if it is corroded out it can allow water back into the enigne. Don't remove this until you have a new gasket in hand to reinstall it if it is ok. Also the previous owners warning about not going below 1/2 tank my tell the tale as well. The new low sulfur diesel fuel tends to grow more bacteria than the old stuff. So you may have water and or bacteria or other nasty stuff in the fuel that doesn't burn so well. When you look at your racor is there black stringy stuff in the fuel? If this is the case you may have to have your fuel polished and or the tank cleaned. The process involves pumping all the fuell out of the tank filtering it and then refiltering to clean it up. You can also buy some additives to help kill/fight the bacteria if this turns out to be the case. Start with the simple inexpensive stuff and work up.