I have a similar problem with my 300tdi when I picked it up it would not accelerate although once out on the road it would cruise at 70-80 I found that the lift pump shaft had lost it's washer off the bottom and thought it was the problem. Since changing, it has improved but is still sluggish. I have adjusted the fip but still not much better, as I can't make it smoke much I assume it must be a fuel problem not air. As it is only a toy and is still running I haven't delved further, so will follow this with interest.