Smoking under acceleration

My 2.5 N/A 90 is producing heavy black/grey smoke under acceleration. the harder i accelerate the worse it gets. i know these engines are sluggish but mine really is SLOW it will not pull above 45 on flat ground and show it any gradient and it doesnt want to know anything over 20mph! (while spitting out a s**t load of smoke) it runs clean on tickover after the initial little puff however it does sound like its misfiring but only very slightly. the truck sounds like a V8 and it clearly isn't (no holes in exhaust or blown out manifold gaskets either) any suggestions?? i have scoured the forums and just cant find anything that matches my problem
P.S i have already tried adjusting the injector pump and i can make it smoke worse but no better

P.P.S tried also adjusting fuelling but to no avail so have reset it to factory standard.

HELP!!

Thanks in advance guys

Hi you may need to have your injectors rebuilt !! They are prone to going weak over years of use and do not hold pressure as they should !! A garage that repairs diesel type vehicles should be able to perform a pop test!! This tests each injector for leaks and spray patern etc , it's not too expensive compared to a 200 tdi they are much dearer to replace !! After that check valve clearances and pump timing !! Good luck ..

forgot to mention its drinking fuel aswell

ok so super simple fix. my valve clearances were out by about 1cm each! once sorted it ran like a dream until the battery just died. now i have a new battery its like having a new truck. yayyyyyy!