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Hello Mike, my name is***** will assist you.
The float on this engine is electronic. What you see when you look at it is just the outer shell, the float is inside that. What you are having happen to you is this, the harmonics between the boat and the engine are causing the float to "dance" or ride up the shaft of it even when the fuel level is low in the FSM. Basically the engine runs out of gas due to the float showing that it is high. When the engine slows down, the float drops, this turns on the lift pump, you get fuel again and off you go. That is why it only happens at high speeds. the fix for this that Mercury came out with was an updated float with a different diameter shaft to prevent the float from danging back up the shaft. Replacing the float is the only repair. Once this is done, you will not have this issue any more. If you need the part number to the float, please let me know.