71 2.2T Idle question
 

I have a 71 2.2T that has an issue at idle. The car has stock cams, clean carbs and the following:

Main venture 30mm
Main jet 120
Air jet 185
Idle jet 55

Emulsion tube F1

The car will idle at around 900 rpm nicely. When driving though it always returns to ~1500 rpm and then after 30 seconds to a couple of minutes drops back to 900rpm…

Thoughts?

I have a Morelli Distr – could this be a return spring issue?

Thanks
Sam