2006 340 8.1 Horizon 300hrs port engine Last year it became hard to start. It would idle rough and stop. Required restart 3 or 4 times until it would continue to idle. Any application of throttle would cause it to stop running. If it would continue to idle rough and warm up it would accept throttle inputs and accelerate and run fine. Once at this point leaving the dock and going out in the lake resulted in flawless performance. Shutting down at anchor in the lake with a restart a few hours later the engine starts and idles normally. Checked IAC filter. Tested IAC valve. Filter a little dirty and IAC valve checked OK. Replaced filter and IAC valve anyway. No improvement. Launched boat a few weeks ago. Replaced fuel filters in Cool fuel Module before launch. Both were a little dirty but not excessivley so. None of the "loose paint" problem evident. Same symptoms on first start up of the year. Took it for a run and it ran well. Back to dock and tried to start a few days later. Ran for 10 seconds first attempt. Ran for 5 seconds second attempt. Would not start after that and exhibited what I can only describe as "starter kickback" on next attempt. Decided not to push further until examination. Was afraid of possible hydrolock. Pulled all plugs , which by the way only have about 30 hours on them. None were wet with water. No water in oil. Removed left exhaust manifold to inspect. Gasket was not compromised. No sign of internal water damage but the outer manifold was quite rusty , which is why I removed that one rather than the easier right side. Pressure tested it and it appeared not to leak. Borescoped exhaust ports and all 8 cylinders and no abnormalities noted. Turned engine over with plugs out and manifold off and no abnormalities noted. So ...a ton of work and still no obvious faults. I'm thinking now , possibly bad injectors? Any ideas?