Very easy for a engine to red line in a no load condition. Under load is when the problem occours. Start with some basic checks, compression and spark. Spark should jump 7/16" and be bright blue. If those check out ok, have someone run the engine on the water under load and pump the primer bulb, if the engine picks up power, replace/repair the fuel pump. If pumping the bulb doesn't help try squirting some fuel mixture into the carbs, if it picks up it's time to overhaul the carbs.
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