Yanmar Low RPM's under load.

[Paddym]

I have a 2015 FP which has Yanmar 3JH5E 40hp diesel engines with 1300 hours. The port side engine when running in neutral will rev up to 3500 rpm's. Once placed in forward, it will only go up to 2100 rpm's. At all times the exhaust is relatively clear. I have changed the engine oil, fuel filter, and water separator filter. This engine does not have an air cleaner filter. I have adjusted the throttle cable and pushing it forward does not increase the rpms. The exhaust elbow was removed, inspected, and was clean. The water pump">raw water pump is relatively new and has good flow. The diesel looks clean and I use an additive. The SD60 sail drive has new oil and the seals were replaced a couple of months ago. I have Kiwi folding prop's on both engines. The hull and prop's were recently professionally cleaned.
The starboard engine runs fine in forward and reverse.

What am I missing here? Any help would be appreciated.

[PippaB]

Fuel flow. With a very clean portable container of fuel, disconnect the fuel line as close to the engine as possible and insert into the container. Run the engine. If it will get up to full speed then it is time to start chasing through the fuel system. If it won't, then the problem is on the engine.

PS - Welcome to CF.