You need to isolate your problem, there are 3 likely problem areas. I would start by removing the throttle cable from the engine and hand throttling it. The governor arm should be easy to hold at different throttle settings with 1 finger when the engine is running. If this is not the case, you probably need to adjust the springs for it. I am not familiar with the specific engine but the springs are set up as a pretty standard linkage so with a little thinking, most people can figure it out.
The next thing to do would be to disconnect the throttle end as well. Try moving the throttle back and forth and if there is much resistance, something is wrong with it. The morse units can corrode quite easily and I have not been impressed with their build quality. If this is in fact the problem, cleaning and lubing it may help but you also may need a new throttle.
The final thing to do is to pull the cable back and forth by hand. This should be quite easy, like pulling a brake cable through on a bike. If it is not, check for any tight bends or areas where the cable has been flattened. The ends of the cables tend to be problematic and if they get bent at all, they get really stiff. PB blaster makes a special cable lube which you could try here as well.
I replaced a setup on a boat this summer where I was worried that I would physically break the throttle lever trying to push on it. There were two issues there. One was that the governor arm had a lot of resistance and one of the springs needed to be tightened and the other issue was that the morse control was ruined so we replaced it with a much nicer kobelt unit.