Throttle cable dragging

Just to clarify, are you saying that with each throttle cable connected at the carburetor, and left disconnected at the throttle lever, the carbs won't return to full throttle? 

Some thoughts;

New 1mm or 1.5 mm should make little if any difference, both are suitable.

Drag/friction on throttle lever? - Jeffs question (above) addresses this.

Routing problem? All bends must be gradual. No kinks/or sharp bends. The cable should , as far as possible, be aligned with the adjustment ferrule on the carburettor. This may result in using extra cable.

Did you lubricate the cable? If so this could be the problem - modern throttle style cables have an inner sheath that does not like lubricant. 

How did you adjust the cable length to suit application? - damaged outer sheath may be "grabbing" inner cable.

Sign of cable in good condition; After cutting to length, hold cable assembly vertical - inner cable should slide/drop out of its sheath (outer cable) without/minimal assistance.

The throttle spring is supposed to go to wide open if the cable breaks and isn’t meant to while the cable is connected.