I have an '83 H27 with the Yanmar 1GM engine. It runs fine for about 30 minutes at 26 hundred rpm then drops two to three hundred rpm and stays there no matter how long it is run. Once it is shut off and cools down (several hours) it starts right up and again runs at full rpm for another 30 minutes or so. It doesn't blow smoke or sound funny, just slows down as if the throttle had been pulled back a little. It isn't over heating and the filters have only 35 hours on them. The fuel tank was pulled from the boat and cleaned last season and all rubber fuel lines were changed for new ones. Along with the cleaning and fuel lines I put in a new Racor filter system, the engine ran fine until two months ago and started doing this. Total time on the engine is (guessing) 300 to 400 hours Any idea's? Harry on Dolly Phynne