Most mopeds don't have batteries so the current isn't constant and the horns tone varies with engine rpm. At idle they're pretty much useless, if your horn has an adjustment screw on the back side of it you can try adjusting it, but your engine has to be turning at a pretty good clip, so have someone hold the front end down while it's on the center stand as you adjust it. If you can't find a screw on the back to adjust the tone and it still sounds like crap, you'll just have to get another horn.
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