Always approach lacing a cable with a C-curve. An S-curve is going to add pound pull to the lever.
Where it might not have been lubed in a long time: Loosen cable and remove the lever to lube the bolt pivot, and cable end ball points with bearing grease/copper anti-seize.
With a sandwich bag, cut the corner of the bag just enough to fit over the cable end. Take a rubber band and close the cut tip around the cable so it's leak-proof. Place the end of the cable so it is held as vertical as possible for flow. Pour just a touch of tranny oil in the bag so it begins to flow down the inner cable. It won't take much so less to clean up out of the bag. Pack a paper towel down the bag to absorb what is left.
I'm just suggesting this; if you do see the the cable is routed C enough, has to be the cable, right? As mentioned, for arg sake, 18 pound pull is every clutch setup on these bikes right? You have the same parts and can pull 50 pounds? You'd have to remove the cable, let the inner cable drop effortlessly, then there is no fray binding things up internally. But say the lever and cable were never lubed all this time... as a variable. It costs nothing to remove the lever bolt and lube both. Once lubed, pull to see it is not the cable or is the cable. And this is the quick and dirty, do one thing at a time; without pulling the whole cable off to watch the inner cable drop.. Because, that's your next move.
I am not convinced it takes 50 pounds of pull with, I don't care who's stock springs used. Racing springs are another story.