Maybe I can simplify without getting my butt chewed on. If you want more horsepower out of a diesel you have to be capable of providing it more fuel but not so much that you are just making black smoke and nothing else. I was told by the Kubota dealer that anything over 25 hp had to have emissions systems installed. Possibly your engine has been FACTORY derated to avoid the whole emissions system. This can be done by less fuelling, cam timing and such. Take any engine and if you dyno it there is an RPM where it is the "Sweet Spot". That is where the HP and torque are at their best. Usually when you look at a graph on a dyno run on any engine, the HP and torque CAN drop off as the RPM go further.
I will preface my '07, 5.9 Dodge. It is a smog diesel but didn't have any added emission exhaust treatment because it was built in December of 2006. Anything built after January 1, 2007 had to have exhaust DPF and all that other junk. Dodge went to the 6.7 ltr to help over come some loss from the smog stuff.
Back to my 5.9. It had good power but lacked full potential. It is a common rail engine. My turbo started to leak some oil. I pulled it and sent it to a rebuilder that suggested a modification. The turbo kept the 5.9 compressor wheel but the exhaust side was machined out to accept the 6.7 turbine which is the exhaust side. Without any other mods, this woke up the engine. It may not be EPA legal but it made the difference. I can also add HP because being a common rail engine, My programmer will hold the injector timing a little longer to get more fuel. My stock camshaft restricts my engine because of the EGR mandate. It holds the exhaust valve open with overlap with the intake to somehow give the engine "Internal EGR". To get rid of that I need to go aftermarket and change camshaft profiles.
I know you are frustrated about the lack of power. Your tractor may have potential but has been "CHOKED" by all the EPA bull crap even though it sounds to me that you don't have exhaust treatment. To get around that and pull the full potential out of that engine, you may have to study PRE EPA models of that engine and see if there is a difference somewhere in injectors, injector timing, camshaft, ect. You may be able to interchange internals such as a camshaft from an earlier engine that will wake up your engine. I am not fully familiar with your engine but on my B3200, V1505 engine, fuel timing and injected amount is changed by working with the shims unter the rack pump. Pull shim and the timing advances and will add a little more stroke to the pump plunger.
I started my Diesel work in 1980 when I bought a diesel VW wagon with a NA 1500cc, 48 hp engine. I have rebuilt, tuned and modified the VW, Mercedes 5 cyl turbos and the Cummins 5.9 engines for extra power. Some of it was simple some of it was not
I learned that to get more hp, I had to turn the pump up a bit but not too much to make it smoke like a chimney. I also played with the injection TIMING. I found that advancing the fuel injection timing helped a bit. Believe me, when you are driving a 48 HP car. especially if it is loaded, you want to find any extra hp you can.
