Turbo Lag. I have one too and it takes a little bit of time for the turbo to spin up and catch up to the amount of fuel being delivered. Once the turbo is up to speed, then you will not notice a loss of power. Unless it is really sluggish at the low speeds then there might be another problem, this is my .02$.

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'87 300D Turbo Greasecar
'83 Kawasaki 550 KZ