In between the 2 carbs, where the 2 link arms actually link up is a solid steel bushing (in the green circle) that is supposed to roll up and down the "U" shaped channel in the plate that joins the carbs together at the bottom. In my opinion this is a bad design, I found a bearing that can replace this bushing and now my throttle is smother and free-er than ever, possibly even than factory.
Try to pick up a " 688 2rs w6 " bearing. The w6 refers to the width of the bearing being 6mm, the original part is 7mm wide and has 2 x 1mm thick washers either side of it so all I did was change the washers to new ones that were 1.5mm thick to compensate for the bearing being narrower. You can also use this chance to take out some of the play by adding super thin M8 shim washers so that the whole thing isn't as loose. Standard/common type 688 2rs bearings are 4mm & 5mm thick, if you can't find the " W6 " bearing then I'm sure that a 5mm thick one will work with thicker spacer washers. I certainly wouldn't recommend using a 4mm one.
You can check if this bushing is the cause of your stiff throttle by trying to operate both sides of the carbs at once. Try rotating the left carb by its stop lever while you roll the throttle, if it is easy when both carbs are being opened up at the same time, then it is the original solid bushing binding and not rolling freely.
If this is what is wrong with your throttle and you do not want to remove your carbs again and change the bushing for a bearing like I did, then you could possibly just try to spray the bushing with your choice of spray lube/grease (ideally one with a straw so you do not end up caking your carbs with grease).
Please check the easier things first, for example;- that the throttle cable moves freely, that the plastic throttle tube isn't binding on the handlebars and that the cable hasn't become dislodged inside the switchgear - because no one I know really looks forward to removing their twin carbs on these bikes.
I hope this helps some of you out there while you have the carbs out of your bike or even if ya' don't I suppose
