it doesn't backfire in the karb, but in the exhaust because of all the unburned fuel.
Can that be a caused by the intake-gasket???
It could, but since what you have is not "backfires", but unburnt fuel exploding in the exhaust, this is a misfire problem. A misfire can be caused by too lean mixture, but more likely a too rich one, or even more likely, by ignition problems. Since this also happened with your old carburettor, we can rule out the weber, but the manifold could have a leak near the cylinder head. The gasket is very thin and breaks easily if the manifold is not perfectly fitted against the head, and when it does, it is sucked into the engine and a big gap is quickly opened up. This can cause all kinds of running and starting problems, but they would be more likely to show up in idle and low revs than in higher revs, where the leak would have less significance.
So, I think you should check the ignition: Coil, leads, rotor, cover and the plugs. If all that looks perfect, take the distributor out for a stripdown. The Murena has electronic ignition, which is generally very reliable, but the mechanical components in the distributor can seize up and stop working as they should.
Here's a few photos of the internals of the Bosch distributor:
http://gallery.dinsen.net/v/biler/Murena/technical/dizzy/