Thanks for the reply.
I replaced the carburetor with a brand new one, so I'm thinking that the carburetor vent system is fine.
Also - I took the entire thing apart (again) and checked the vent system - there is a plastic part that covers it - so the model that I have doesn't vent through...
I'm desperate with this - I clean everything - took the carburetor out, clean it - nothing. I even replace the carburator - the mower starts and then it starts leaking trough the air filter and stalls soon after.
I also checked the spark plug - it was full of gas - I clean it, used some sand...