Idling without plenum / pipe

[JonesDk]

Hi guys.
when idling without the Turbo pipe on the plenum, it misfires accationally.
But if I put my fingers in front of the inlet in the plenum ( the flange for the pipe) and restrict the intake a little, it runs perfectly without any misfire.

Is there a good explanation on this ?

Never observed this before
Jonas

[JonesDk]

Fixed. Vakuum issue