Welcome to the forum and XS400 ownership! Read through past threads, especially those on carbs. You will find a lot of helpful info. As others have said, these CV carbs don't like being gummy and dirty. Heck, they don't even like to stand idle through the Winter not having gas running through them regularly. Besides cleaning, one of the biggest problems is bad "O" rings at the bottom of the idle mix screw on the top of each carb. Yours may be covered by a tiny plug with a hole in it. Drill out the plug to get to the screw. If the plug has already been removed, suspect that someone has messed with the screw and maybe broken off the long, slender tapered tip by cranking the screw down too hard. Remove each screw and carefully inspect. Also, be certain to get the "O" ring out. Often, it will stick down inside the hole. Hardened, leaky "O" rings can affect idle and low rpm performance. Start with 3-3.5 turns out and fine tune from there if necessary.
Too bad you young guys don't know how real engines used to work, ones you could fix on the side of the road if they quit. LOL My '77 Mercedes 230 gasser once had a coat hanger holding the carb (side draft) tight against the rubber isolator block on the intake manifold when the block developed a split and air leak while on a trip. Got home w/o incident and replaced the block at my leisure!
...
