Thanks everyone for taking the time to reply. I think I've gotten to the bottom of it, I did the service myself and I thought I had the gaps on the spark plugs correctly but on closer inspection the gaps looked too small so I rectified it to a bigger gap going by the manufacturer's guide. I ran it for a good while then took it to sea where I was getting the right revs and the engine sounded like its old self. So hopefully that has been the problem, I'm about to head back to the boat to run it again, I'll let you know if there's any problems. Thanks again, really appreciate your advice guys.👍