It is easier to get the boss off with motor off the base,unless you can get some kind of hard steel under it like a dollie,but is easier off. This one should have a metal cable inside a coil like tube. The cable inside most likely has a kink in it. It can be fixed you just have to find the kimk and using a pair of pliers to straighten it as best you can. This may well be the problem you have as if you can't adjust the accelerator then the carby can't give the engine the right amout of fuel it needs to run.
