Idle question

[Jess from VA]

So my bike ('99 IS) was gone through by a real pro this past winter and running perfectly.  61K.
Warm idle has always been 900-1000rpm.

Last Saturday I rode all day (lazy cruising and fast freeway), and on the way home, my idle crept up to 1500 or so.  I backed the throttle to off with force, could detect no cable hangup, blipped it a number of times, and it just hung there at 1500 at every stop (fan on).  Today I backed it out, fired it up with choke for a minute, and she warmed up and ran at about 950 smooth.  I didn't let it get hot enough for the fan, but no sign of high idle (and no sign of sticky throttle cable, though I took nothing apart).

Never happened like this before (have needed to barely tweak the throttle screw up or down on rare occasions over the years, like anyone else)

Why? 

[Jims99]

Mine will jump up about 200-250rpm when hot. Idles great around 950 most of the time, but after a long day she’ll be around 1100-1150. Fan only kicks on when sitting in traffic or parked running.

[da prez]

  Jess , start with basics. If the throttle free play is to tight , it may happen. A little hard to get to , but the cable may need adjustment. Start with the minor adjustment at the twisty thing. You know , the right hand one. There  is an adjustment at the throttle grip. Loosen and turn the cable end in a full turn. There should be a little throttle free play.  If you need to go in deeper , be sure the cables are lubed.

                                         da prez

[Chrisj CMA]

Jess, if it doesn’t happen again chalk it up to a glitch. Machines like people aren’t perfect. If it happens again, start trouble shooting. I’d start with all possible sources of vacuum leaks. Maybe someone smarter than me can explain why it makes the idle go up but my wife’s Ford Escort idled a bit high when we got it. Soon the check engine light came on. Took it to my mechanic and all they did was change all the vacuum lines. No more check engine light and a significantly lower (correct) idle.

[Jess from VA]

That bike is fully de-smogged with the best parts.  Nothing plugged, all removed, except the OE petcock vacuum.

I'll have to ride it again before I decide to look further.

Thanks guys.

[98valk]

lead condition. open up the idle mixture screws 1/2 turn.

NOTE: A lean problem gets better as the engine heats up.

    If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!
http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html