I have a 1999 Volvo Penta Duoprop 5.0 Gi. (The engine has performed great the past 5 years only 750 hours on the engine.) The engine won't go over 2000 rpm under load. ( I can free rev the engine in neutral to 3500 rpm with no problems.) When under load and accelerating the engine backfires and kicks back down to approx 1500 rpm (like it is running in a safe mode.) We have new plugs, distributor, fuel filter, new high and low fuel pumps, and new oil pressure and coolant sensors. No water in the fuel filter. We also ran the boat on a separate smaller tank of gas to rule out bad gas. Any ideas on why this engine is not able to run above 2000 rpms under load?
