Black smoke on start up

Sounds like gas washing oil off cylinder walls when engine is shut off?  Not sure if that would be float valve bad or float level set too high.  Assume dual carbs?  Make sure your oil doesn't smell like gas or is reading too high (other issues there).

The engine is a dual carbs.
how can I check if the oil smells like gas?

Find the oil dipstick, just to the right of the bottom fan belt pulley, remove it and check the small marks on the bottom to see it looks overfilled (level is above the top mark).  

Then, smell it.  if it smells of gasoline, rather than just oil, you have gas in the oil.

Thanks for the info. Will do the test. The car has only it first 300 miles on it. So I hope it will be fine.

Could it be that the black smoke came from the fact that I pumped the gas petal a bit to much before starting?

Yes - in warm weather it shouldn't take but a single pump. At 300 miles the rings aren't even seated yet.  I suspect newness is more the cause.  I would not worry about it.  Do check oil though and change it before 1k miles - even adjust valves then on a new engine.

Thank you all! Indeed, when I don’t pump there is no black smoke! Next week is the oil change and valve adjustment.