Huddy is correct about tappets they often get ignored, Also make sure air cleaner is clean, In my experience the 12j,14j and 15j engines were often a little over advanced for optimum when set with timing pins. I used to retard them slightly by moving pump towards the engine a fraction at a time until it misfires and white smokes slightly at hi revs then move it back a fraction. I would mark the pump position first so you can put it back where it was if no improvement made. If engine is in good nick you may get away with a little more fuel without making too much black smoke, max fuel screw is on the engine side of pump top housing, 13mm lock nut, ten mm adjuster screw in for more fuel but this engine will not stand a lot more than factory fuel level before it fails smoke test.
In the LDV vans (14j series engine) which is virtually identical we often found the fuel pump transfer pressure was a bit too high so a quick fix for sluggish acceleration and black smoke was to slightly reduce the pressure by removing fuel pump inlet pipe and inserting small Allen key into hole where pipe fits back off transfer pressure adjuster 45 degrees or so and retry engine. Be careful here tho you don't need to get dirt into pump and will need to bleed system after refitting feed pipe.