Interesting post. My 1980 XS400 does the same thing. I researched thoroughly in the forum and decided to check the idle mixture screw adjustment. The caps had also been removed on my bike by a P.O. The jets were set to about 2+ turns up. I increased them to 2.5 turns and the problem got even worse. I tried 3 turns with an even higher fully warmed up idle - now over 3K rpm. A couple days ago I set them back to 2 turns and now the warmed up idle is just under 2K rpm. Tomorrow, I'll drop it to 1 3/4 turns out and see what happens. I find these results curious! I don't get lean backfire and the bike runs very well otherwise. The factory setting was supposed to be 1.5 turns out and that may be a little too lean for some engines but maybe not for others. When you don't know the history of what has been done previously to these bikes, it makes corrections a little more challenging.
Now, I know the o-rings on the idle mixture screws can become tired and leak. I have a new set of screws, springs, etc from MikesXS and will replace the originals when I pull the carbs off. At the same time I will install new choke rod rubbers and butterfly shaft gaskets - when they are in stock again.
when you adjust the mixture screws out and the bike revs up, its telling you it wants to be richer. Adjust the speed stop screw down, and keep richening up the idle mixture until you find the spot that making it richer, or leaner, makes the idle drop. The bike is very lean at the point you are now, so when you turn the screws out, it gets happier and revs up.
my xs is set at roughly 5 turns out. im actually going to put bigger pilot jets in it, and try to get the part throttle lean surge out, but still be able to idle nice.
XS400 HIGH IDLE!!
in Classics
Posted
when you adjust the mixture screws out and the bike revs up, its telling you it wants to be richer. Adjust the speed stop screw down, and keep richening up the idle mixture until you find the spot that making it richer, or leaner, makes the idle drop. The bike is very lean at the point you are now, so when you turn the screws out, it gets happier and revs up.
my xs is set at roughly 5 turns out. im actually going to put bigger pilot jets in it, and try to get the part throttle lean surge out, but still be able to idle nice.