Took the catamaran out again today to make sure everything was mechanically sound. However had some issues with one of my engines. Under a load it wanted to back fire and almost die. After checking timing and swapping carbs motor for motor I narrowed it down to the fuel system. I pulled the machnical fuel pump mounted on top the sea pump and opened it up to find the diafram looked good and wear free. However. The nipple for the vaccum line that goes directly to the carb was completely plugged. Is it possible that by clearing the small vaccum port my issues is solved? Or do I need to look into finding a new pump? I have not put a pressure gauge or anything on the lines just simply cracked the engine over and pumped into a jar. I am going to see a out getting the boat back on the water tomorrow was just not sure of the significance of that little vaccum port. I was under the impression it was just for priming on start up and had nothing to do with the engines maintain constant fuel.

That line you are talking about is for in case the diaphragm ruptures the fuel will spill into the carb. It's a required marine safety feature, has nothing to do with running the engine.

the pick up in the tank is most likely culprit.

Sounds like you have a bad coil.

the mechanical fuel pump has valving that controls the pressure,without a fuel pressure gauge their is no way to know if the pressure is correct.i don,t think it is common but i have seen mechanical pumps fail.that being said i think checking the pressure should be the next thing to do.if it is an ignition problem it cam appear to be fuel related because the fuel is not being burnt.as bb said,it could very well be a bad coil or maybe a bad ignition module.also could be bad plug wires or a dist cap with corroded terminals under the cap.i would also replace the fuel filter.

As Mike said, you could possibly rule out some ignition issues by switching coils and/or ignition modules to see if the problem follows the swap. Those are pretty easy items to swap out. Adding a fuel pressure gauge might be the next step, but that can be more difficult with stock hard fuel lines, so I would go for the low hanging fruit like coils and modules first.

low hanging fruit!don,t we wish it was as easy as picking the apple off the tree,lol.

I also recently painted the fuel fill area where the tank vent is also located. Is it possible by painting that I completely sealed off the screen and the tank is not getting any ventaltion? Would that cause similar issues? Going to look into all posted suggestions as well.

Remove gas cap and try it.

Might want to take a look inside the distributor, had a similar problem with a friends boat, cleaned all the corrosion out and it ran fine.