Hi,

I have a K-Jet in my 1978 240 GT.
It’s not good for K jets to sit but it sounds like you have cleared the injectors of an issue and have seen normal flow.
The pressures seem good too.

I’ll suggest that you check the air flap inside of the rubber bellows below the throttle as it is supposed to respond to an increase in air flows.
The control pressure regulator holds pressure onto the distributor flap and slots open the volume.

It should only take a very slight breathe of air to lift the plate up.
I would get below the flap with an air hose from up under the intake side by the way of air filter housing cover and it’s duct.
The least amount of air that you can introduce up through there the better it will help check or determine its responses off idle.

The flap needs to be centered in the bore and you have to check that with a mirror and a light if there is a problem.
It is delicate mechanism and do not try to make any adjustments until you know it clean and clear as I see they shouldn’t move on their own from what the factory set.

This flap may be reacting sluggishly or is stuck or even the distributor’s pistons may be sticky from old fuel.
It may need a solvent soaking or flush.
Hopefully you will not need to try a disassembly as that is also a very precisely made unit.

Hope for spider webs or a rodent nest in the intake duct.

Phil