Today I found out, that the engine is drawing air at the rear of the intake - especially at that spot at the end where there is only a pin instead of a bolt.
Tightening the bolts helped a bit, but not enough - looks like I need a new gasket.
In could be that the manifolds are not installed properly (exhaust and intakes are not parallel to each other) . This in usually caused by assembling and tightening the two manifolds first than installing on the engine. To properly assemble, all three gaskets need to be tightened at the same time so each can fully seat.
