The white plastic butterfly is the governor vane. Air pressure from the cooling fan on the vane, limits the engine's speed: the vane pushes in the opposite direction from the way the governor spring pulls.
The obstructed vane explains why you suddenly had a temporary improvement when you fiddled with the linkage at the carburetor end: you must have reshuffled the stack of grass, but the fan would push it back into its jammed position quite soon afterward.
If you have the vane moving smoothly, and the right springs, the only other thing you should do is set the minimum and maximum speeds precisely by bending the brackets where the two springs terminate, but unless you have a tachometer this is impractical.
Thanks for coming back with the final outcome - I'll now close this thread. If anyone needs it re-opened, just send a PM to a moderator.
