Hi John, wildfelr, kodjo and rhusak,
So, the cross over the intake is blocked and a thick gasket between intake and carburator is in place. Also took the best fuel available (98 octan). As far as I can see, it seems to work. At least the engine did not die due to boiling fuel anymore. More tests to come on the weekend, but so far seems to work.
Thank you all for your help!