I have a carburated 1996 225 mariner. There is an inline racor filter and an installed glass final fuel filter on the engine block. My problem is when the engine is shut down, oil from the injection oil tank backs up thru my fuel lines and fills the racor filter with 2 cycle oil. This fouls the plugs as soon as you try and start the engine. Someone told me it could be backing up thru the oil pump on the engine but to do that it would need to travel past the check valves in the priming bulb which are good. All three carbs have been rebuilt by my dealer to correct this problem but no good. What to do next?