The adjuster on my after market RR horn doesn't seem to do anything. The adjuster worked fine on the stock RR horn. I remember having to make a tool to adjust it, I think the screw head had 7 points.

1970 Road Runner 505 cid MCH CNC ported Stealth heads MP 528 camshaft 4 speed GV overdrive 11.98 @ 117 on street treads