why do i have a major lack of power after changing copper seals on a 1.4td ?

did you clean tips of injectors,-if so you may have damaged/blocked the very small holes in the tips. i assume you have checked for leaks etc and turbo hose is on and secure as is any wiring or sensors that you may have disconnected! [just another point-you didnt let anything drop down onto timing belt that could have got between teeth on belt and caused it to jump a tooth when started.]

k
September 2010