why does my transit smoke when i start up?

why is my transit van blowing out blue smoke when i start it up in morning? it seems not to be firing on all cylinders for a couple of minutes but after a short time the blue smoke subsides and full power is achieved, we've changed head gasket and checked valves e.t.c, any ideas please as the old bill are on my case regarding emmissions.thanks

john cheeseman
November 2009