Re: Lost a part
It is there to hold the needle from sliding up when the throttle slide moves down. Any washer that fits the slide can easily be used to replace the one you lost. The slot is for the cable to move in. Any washer can be used if you simply cut a small hole or slot for the cable.
Keep in mind the needle clip needs to be held down solidly so be sure the hole in your replacement washer is smaller than the needle clip.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.