4-strokes tend to smoke more than 2-strokes, partly because they have higher combustion temperatures and partly because the combustion is more complete.

I suspect that your fuel tank may be too high for an inverted 4-stroke which is why it runs rough at idle.

You've also got to remember that prop-selection can be mighty important and if the 75 is anything like the 91 you should be aiming for about 10,000 RPMs or maybe a little more on the ground. The TT 4-strokes like to rev a little more than OS or Saito.

But most of all -- it'll need a couple of hours running (at least) before it starts to settle down.

Oh, what am I saying...

It's rubbish -- send it to me and i'll make sure it's disposed of safely and in an environmentally friendly way (so you don't get into trouble)
:-)