It could be any number of things. Did it run even under load or was it rough? Did you drop any cylinders at WOT? Check fuel, carbs, timing as already mentioned. See if you can diagnose it as spark or fuel problem. If you put it in the water you can test spark under load. You might try choking it while trying to plane. If a float is stuck it would enrichen the mixture and make it better. My 250 was laying down similar to what you describe and I had a bad stator. Let us know what you find.
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