Nissan TLDI 50B wont out up to run RPM range

Hi

The past two trips out with my boat, by Nissan TLDI 50 hp will not reach full RPM range like it has been doing. I replaced the water separation filter, spark plugs, and replaced a fuel line. It will run fine up to 4900 RPMs, but thats the max. I was able to get 5500 RPM out of the engine just a week ago. I haven't changed props or anything so I'm kinda puzzled. Also, yesterday I had the drive belt for air compressor break. Got towed in and am in the process of replacing. Could it be possible that the belt was failing, causing the air pump to not give enough air to engine for correct fuel/air ratio???

Doubtful the belt could slip; it's a gear-belt. That belt would seldom break, unless it was subjected to a lot of load, which may be a clue -- possible compressor issue. If you want to work on a TLDI yourself, you will want the Factory service manual, go into diagnostic mode, and see what codes come up, then go forward from there.