Had a similar problem with my car, it could be one of several issues, but most likely it is something pretty straight-forward. I would start as such:
First, what idle speed is your car running at when smoking? If the idle is too low, the scavenge pump won't be driven fast enough to clear the oil out of the drip tank.
Second, I'd check the lines going from the drip tank to the scavenge pump. Disconnect the line where it goes into the pump and then disconnect from the other end. Shoot compressed air (or blow really hard if you don't have a compressor) into the end of the line nearest the turbo and see if any coke particles come out of the other end. If you find nothing there, you can then check the other line (going from the pump to the oil tank) although it would be less likely to find the problem here.
Thirdly, if you still haven't found the root of the problem, try rebuilding the scavenge pump (you can buy a kit from TurboKraft).
If you still are having problems after doing all of the above, conduct an oil scavenge pump test like so (
YouTube - Scavenge Test Idle @ 1,050 while running hot after 20 min 2, 2 5 and 3K RPM) and this can help you better understand what conditions are causing oil to backup into the turbo. If the scavenge line doesn't suck up oil as fast as it comes out of the drip tank, there is a problem (as you can see happening in this video after 3K RPM).