Just bought a used Yamaha 400 XS. I bought it a week or so ago and I had it running (roughly) and rode it around a bit *driveway*. I parked it and the next day it would not start. Replaced the battery, still nothing. Dont know what could havve changed. I've toggled the choke and given it a bit of gas. It will kick over but not run.

Any ideas.

heh...it could be so many things who knows where to start! how long ago was it last run regularly? I would first guess that you dislodged some junk and it wedged into some bit of the carbs which is essential to starting.

Any difference between trying to start it with full, half and no choke? Does the engine at least try to catch on full?

It wouldn't hurt to take the carbs off and give them a quick cleaning. Good excuse to begin becoming familiar with them. Also, while you have the seat off, might as well check for blown fuses and loose connections around the fusebox.

So I got her to kick on and run. (New spark plugs and some carb cleaner) She sounds good and I'm having a good response from clutch and throttle. However there is one problem. At half choke she idles right where she should between 1and2 thousand rpms. But when she's warmed up and I set it to full choke the RPM's kick up like I'm revving it and settle at about 4500. They dont decrease until I back off the choke and I've played with the idle screw to no avail.

Thoughts or suggestions?

Air leak somewhere?, when its doing this, spray around manifold boots with WD40...see if it affects revs.

Some bikes do idle up high with the choke on. If the motor is warmed up the choke needs to be off anyway.

With the choke on high, you might expect high idle but it's not normal. I would recommend spending the money to get your carbs ultrasonically cleaned and making sure you don't have a vacuum leak in your boots connecting your engine. If the problem persists, you might have to adjust your idle mix screws but only after you check these other things.

My two cents.

check the plugs for gas by kicking it over a few times and removing them, then if they have gas on them get new plugs because those ones are probably not firing hot enough. When you run it with the choke like you said earlier it fouls the plugs much faster.