2.5 NA conversion, running problem

I have a 1979 88inch series 3. when i bought it it was fitted with a 2.5 NA engine from a sherpa van which although pulled the vehicle along nicely to begin with, was clearly in a bad state of health after a year or so.

The opportunity arose to have a 2.5 NA from my friends 110 when he rebuilt the car, so as it was cheap i snapped it up. after a lot of fafing finding out it wasnt a straight bolt in replacment, it is now fitted however does not run properly.

the engine starts fine when glow plugs are warmed first and will run on all 4 cylinders if held on or close to WOT however if backed off at all the regs start to die away and it cutts out.

fuel starvation is the suspected problem but any guesses what could be causing it, all pipes are tight and dont leak as far as i can see. though i will go and replace them all soon i think if i cant get it sorted.

is it common for the pumps to be faulty? i did hook the hoses up wrong to start with, before quickly relising and correcting them. could i have damaged it?? any other ideas, what it could be. the lift pump seems to work fine and there is fuel getting to the tops of the injectors when cracked off and cranked over.

HELP!!!

Theres a few things that could be, how many miles has the motor done?
Low cylinder compression is possible, do a compression test, another is injector timing, have you checked this.
Bad injectors might be another, though this would cause white smoke, does the engine smoke at all on start up or while driving?
Also have a quick check of the air filter make sure its clean.