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Have you tried cleaning or rebuilding it?
Apologies if stating the obvious here.
Harbor Freight sells an inexpensive ultrasonic cleaner. Worked great on some motorcycle carbs and a lawnmower carb using some mild detergent. No solvents to attack old rubber or plastic. Turn the carb over a couple of times while immersed to get at all the little passages inside. Watch close and check it frequently at first - sometimes the carb body really is that cheap pot-metal and it’ll start to dissolve! The t-junction for my 4-carb fuel feed did that.
A rebuild kit can also help if any gaskets are leaking or needles/jets are worn. Sounds like your idle screw might be, if cleaning doesn’t clear it up. Never scrape bonded gaskets off phenolic, though, unless you have a resurfacing plate and a lot of patience to polish the scratches back off. I had to replace the phenolic spacer on my old FJ-40 carb after scraping it up.
Vacuum leaks can also muck up an idle, also ignition advance.
Good luck!
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