Originally Posted by A1an
Whip the pump off and split it, check the diaphragm nearest the engine, if its split it fills the bottom cylinder with fuel and can cause the symptoms you are experiencing.
Thanks for the advice. The engine actually belongs to a friend of mine and was just trying to rule out a few things before sending it to a mechanic. I won't be round to the boat again for 2 weeks so the owner will prob just send it into the mechanic.
It'll be interesting to see what it turns out to be! I was going to whip off the fuel pump on the donor engine but I saw a gasket behind it (although it looked plastic and re-usable) but didn't want to risk creating a problem on the donor engine so left it