I have a similar problem, throtle was hard to push (pull), and idle kept getting higher. Found that the cable was not my problem. I discovered that the cable conects to a rod. The rod to the
fuel injection. This rod had somehow shifted (into the engine), slid past its indent. I removed the plate that covers the area it enters the engine, gently pulled the rod out to its proper length, so the indent aligns with the plate (which is suposed to keep it in place), problem solved! Or at least I thought. the next time I went sailing, the problem began again. I fixed it again... So I went to the Authorized Yanmar Service Guys to ask for there help, and look at their repair manual. So they looked it up, and told me that there was a circlip (snap ring) missing, so they ordered me the part. NO WAY THAT THIS PART EVER EXISTED IN THAT SPOT! So, I did what I could do , adjusted the plate (filed the holes so that the plate would align better with the notch better) and put it all back togther. So far so good, but I am sure time will tell.
Tell me what you tried?