Every now and then (especially when it's hot) my engine won't return to idle and revs to about 3-4 k revs. It seems the spring isn't really strong enough on the throttle pulley and holds the butterflies open only very slightly. Obviously on the bike it would have had a cable from the throttle to shut it aswell but of course i only have one cable to open the butterflies and it relies on the spring to shut them. Has anyone encountered this and is there a fix or shall i just fit a stronger spring.

As long as you are sure that there is nothing else causing this, then go for an extra or stronger spring - it's the cheapest option to start with.

Cheers, Nick

A friend of mine has had this when the butterflies have bent slightly. His is supercharged however. It might be worth whipping them out and checking.