914 Woes...

"A blocked exhaust doesn't happen overnight - and I would feel it at low RPMs also"

Its unlikely but it can - internal baffling material has been known to collapse blocking in part/whole the passage of exhaust gas.

Easy check -  get someone to hold their hand over the exhaust exit pipe, crank the engine - should feel multiple strong air pulses. If not or weak, you have a blocked exhaust.

Today I discovered, advancing the throttle makes the enging go ape - shakes violently.

That must be a clue. Now I'll change the small air lines - they are quite fragile

and as old as the rest

Have a look on carbs and make a good synchronise and it will be OK

To be sure, we measured resistance from plug cap to the other plug cap - all is well.

The vent line between the carbs is ok. Also the thickerone between the inlets.

We have discovered, that the 2/4 carb doesn`t do much.

We took off both fuel bowl and top - nothing wrong.

I am at a loss here.

Please explain what you mean when you say "We have discovered, that the 2/4 carb doesn`t do much."