Idles fine, revs up fine, revs down SLOW
2008 Ninja 500, stock, clean gas and air filter, throttle cables don't bind and return to idle position fine. Choke cable fine and choke plate opens and closes fully. Idle was set at 1100 rpm (warm) which was fine for most any season in South Alabama. I was riding yesterday and noticed that it seemed to run a bit rough at 40-45 mph so I gave it a little choke and it seemed to run out fine although it was a little peppier (usually the choke makes no difference after warmup and riding), so I knew I had an issue at that point.
I know the carb adjustment screws and the idle thumb screw haven't been messed with since having it in good working order a week ago. I checked all of the above and decided to take a closer look at the carb and vacuum lines as well as the tubes from the engine to carb and carb to air box. All of the above seemed fine as well.
Problem is that it idles fine with no stutter or sputter. The throttle revs up just great. The bike even rides and runs fine, BUT the revs take about 4-5 seconds to come down from 5k to idle (1100k) when in idle and neutral. Most times the revs are correct when going down the gears to a stop, but once in a while yesterday and today I noticed them hanging around 3k at a stop and then 3 seconds later the revs dropped to 1100k. This is with the clutch pulled in.
If I drop the idle down to 900-1000k rpm then the rev down period is normal, but the bike idles rough. If I raise the idle, then it idles fast, but fine, but the revs take even longer to come down.
Bottomline: Idles at 1100 OK, revs up OK, revs down slowly
Idles at 1300 OK, revs up OK, revs down even slower
Vacuum or carb sync or something else?