I tend to agree with Chris with the sticking
valve as a possibility. A sticking valve in a marine engine is usually caused by a pinhole water leak somewhere in the jacketed exhaust system .... usually the exhaust manifold. When the engine cools down the vacuum formed inside a cylinder with a valve open can suck water through the pinhole into the combustion chamber or down the valve stem of the partly opened exhaust valve .... thence to form rust. Usually a stuck valve will not allow an engine to start or will start with a very rough idle as one cylinder will not be 'firing'. The loud pfffffffff is the clue that the air is not continually flowing in the proper direction. this could happen if a valve is sticking open sometimes and sometimes not. Its usually an exhaust valve that gets stuck, your description/symptoms sounds more like an intake valve. Has this engine ever had a history of sucking exhaust water back into the engine? Boat ever sink? Is the top the exhaust loop 'well above' the waterline, even when the boat is heeled way over? ,,, some things to think about. If it were my engine, I'd open the valve access plate, turn the engine over with just the starter (compression release valve open) and visualize if the valves are working to their 'full' stroke and not sticking. There are some fairly easy remedies for stuck/sticking valves but the true remedy is finding the cause, not treating the symptoms. As regard that air filter, get a retrofit paper element filter. Even air far out at sea is LOADED with particulate ... to the tune of about 30,000 particles per cubic foot. If your old enough to remember the ill fated rescue attempt of the american hostages in Iran in the late 1970s - The mission had to be called off when several of the helicopters crashed .... they removed the air filters to save weight ... and the engines seized from sucking in atmospheric particulate. Put on the LARGEST pleated paper filter retro package you can find ... after you find the other problem. Good luck.
Terry
