Those engines are designed to run at 3600 RPM. Anything below that and they start losing horsepower rapidly. First thing I would check is the RPM.
Next make sure the lift pump is supplying enough fuel to the injector pump. You could have a collapsed fuel line.
Valve lash is 0.20mm or 0.008inch for both inlet and exhaust.
I would also check the injectors, and the compression before I started in on the pump.
Make sure you have the proper weight oil in it for the temperature.
Side note: I just replaced one of those engines because it had spun two rod bearings. If you need any parts, let me know, I have extras. I have everything to put back it together except the rods. Crank has been magna fluxed and turned.