I recently picked up a Volvo 140dl which was repowered with a cummins 4bt. Every hydraulic function on the machine works but its very slow mainly tracking the machine and trying to do a dual function as in dig and scoop or swing and dump bucket. The machine isnt reading any rpm which makes me think the hydraulics are operating as if the machines idling even though the engine is running full throttle. I can get into the service screen and monitor the pressures but have nothing to compare it to. The serial for the machine is VCEC140DA00210139
Sounds like an engine speed sensor issue. The EECU is not getting a signal hence not stroking the pumps as engine rpms increase.
This most likely the problem. A Cummins 4BT has no way of communication with the ECU's in the 140D. Never seen that done but I wouldn't expect any help from your dealer. Main relief pressure setting is not going to affect slow operation unless the relief is open an dumping oil all the time. Without EECU input the VECU is probably not going to control the pump correctly so the only way you run it would be in manual mode, which is possibly what they have done.
Ive tried running it in manual mode and it doesn't make much of a difference. The tracks move a little better in manual mode but all other functions stay the same.
As stated the issue is most likely the "repower" that cobbled into the machine. Without EECU input you are probably only running in what would be manual mode. Pump output is regulated by the VECU so chances are that the machine will never perform as it should. Also don't expect a dealer to work on this machine.