Hi,,
As the idle is affected when you turn the steering, I assume it's the bit between the throttle grip and the splitter box that needs re-routing. On my bike, I've fitted the cable so that it the splitter box sits on the top frame tube, and the front cable emerges from between tank and frame tube on the left hand side. of the steering head. Then route the cable in a long, easy sweep from beneath the headlamp shell into the throttle grip assembly. The cable should leave the throttle assembly pointing forwards and inwards at about 45 degrees. Set the throttle play adjuster so that you have at least 1mm of free play with the throttle closed at both extremes of steering movement.
The problem usually comes if someone has fitted higher handlebars than standard without changing the throttle cable.
Hope that makes sense.
Graham