You may remember that last week I broke the throttle open cable while trying to work out what I'd done wrong with the throttle cables and why it was sticking open. Well, I now have a nice, new genuine cable. I've had the old opening cable off, and apart from being broken, it was fine. Not sticky, no corrosion, no damage, no anything wrong. I've had the return cable off, it was a little sticky but nothing that would prevent it working properly. It's been cleaned and lubed The hand grip has been off and thoroughly cleaned and I've applied a little lube on the handlebar. In short, everything is clean, lightly lubed and ready to go.

The cables are routed from the bar, infront of the right fork, the go over the bottom yoke next to the left fork, over the top rad, and directly to the carbs.
I've taken all the wiring connectors apart to get them out the way to ensure nothing is bearing on the cable sheaths.
I've tested the cables in situ without either end connected to anything, and the have as near zero resistence to movement as you could wish for.
The return spring on the carbs snaps the butterflies shut like a guillotine.
If I connect just the open cable, the throttle opens and shuts perfectly.
If I attach the return cable, you have to roll it shut with the grip.
Occasionally it is slightly less reluctant to return if the bars are pointing straight ahead, but it barely returns past about a third from fully open.

Any ideas? I'm hypothesising that maybe the freeplay on the cables isn't quite right and it's causing the whole lot to bind up when it tries to return, but there doesn't seem to be all that much tension anywhere when it's trying.

Right now, I'm off to rip the carbs off as I can't get at the adjuster properly, but any suggestions would be welcome if I return having failed to sort it....

Fixed. A bit of fiddling with the freeplay on the opening cable sorted it. Although that doesn't explain why it did the same thing with the old one when I didn't adjust them...

You probably disturbed the cable routing requiring a change in freeplay to compensate

first of all ditch the close cable its the problem you don't need it,

going from experience/and idea, I did the same as you speedy, and it had me baffled for weeks, to find with the aid of neosophist's help (and too date I could never thank him enough, cheers mart again :D), that my carbs were so badly balanced that the butterflies wernt closing enough and i had to roll the throttle forward to close them enough to get a steady idle at junctions etc,

after that they closed properly and were balanced correctly etc, I did refit the return throttle cable for it to work fine only to remove when I fitted a 3/4 turn throttle which only uses one cable.