man I feel for you, for 3 years I had a Perkins 4108 that would start and run great at anything below 1600 RPM any higher it would over heat. I made around 3 to 4 knots so it did not stop me from sailing but for the life of me i could never figure it out. In the 3rd year it overheated and blew the head
so i replaced that never changed that 1600 RPM thing. The replaced head did not last long at all before it blew.
I desided to repower
the boatmy choice was another Perkins 4108 that I found for $5K it arrived with a head problem (injector would not seat) bit it is my belief that the motor
will out live me now. The one thing I would have don different is where I got the motor
. There are reasons people buy from TranAtlantic aka. TAD. even at a higher price
The point I am tryin to make is before you spend a dime on fixing it, think about repower
. any Perkins around has to be over 20 years old.