Thanks again brutus. The returnspring is now removed and the throttle moves completely freely all the way from fully closed to fully open. I must admit that the previous problem to fully close the throttle was NOT the returnspring in itself but rather that upon closing the butterfly/axle had to move slightly sideways in order to sit correcly in the bore and come to fully closed position. And in that process it had to compress the returnspring. What I did now was to add a compression spring on the opposite side behind the throttle arm in order to hinder the valve/axle from vibrating sideways and cause excessive wear. Hope you can follow me here..??!!??
The arm extension is made of Makrolon(Lexan to the americans) and it is very tough stuff (just ask the riot policemen..) so alkylate fuel will not touch it.