Just bought ultra clean unmolested 84 240 (133k), ran perfect with pretty good acceleration when floored. After changing air filter, car runs smooth but acceleration is now very sluggish above 3000 rpm.

 

Recheck the filter and make sure nothing is stuck in the housing blocking flow.
Also there is a Valve in the Air box you might want to screw it open or remove it totally.

 

Thanks Tech,
I checked the filter, ok and valve is open, is it possible the air supply hose after the mass air sensor is torn? The car accelerates smoothly but weak under partial throttle, just really struggles under full throttle.

 

Yes it is possible should be easy to spot.