Have you removed the grip and greased the handlebar end?
Check the idle screw adjustment inside the fuel fill compartment.
If you loosen the cable adjuster considerably and snap the throttle does it return to idle?

Remember that when you adjust (measure) the cable slack not to include the travel portion from resting position to rolled forward as this is for cruise cancel. Kinda hard to explain clearly, hope you get what i'm trying to say.

 

I would check all the possible causes before you start stripping the carby.
cables, twist grip , return spring etc.

 

.....I've heard of one person that took their carbs off and took the butterfly vales out.....

No need to remove them for a normal cleaning/build. Some carb.s need them pulled to change some of the o-rings between the carb.s but not the 1500s. If you bike has never been apart this should not be your issue.

BTW: I have family in you town and visit there fairly often.

Bill

 

Before doing anything like ripping into your carbs, I'd recommend taking the "pull" cable off of the carbs. with this cable removed you'll be able to see if the problem remains or goes away.

If the trouble remains, then look at sticky trottle linkage or weak return springs on the carbs

If the trouble goes away, then one or more of your 4 cables, or the mixing box is sticky.

This symptom is pretty typical for an over tightened throttle (not enough slack), it'd be a time savings to check the adjustment on your pull cable to assure that you'v got too much slack, and then repeat your testing for return to idle.

 

Before doing anything like ripping into your carbs, I'd recommend taking the "pull" cable off of the carbs. with this cable removed you'll be able to see if the problem remains or goes away.../QUOTE]

+1... Also use that opportunity to Spray-Clean/Lube the (3/4" dia) springs that pull the throttle closed... if they're gunked-up, then the throttle won't close. Test by manually rotating the cable pulley, verifying smooth operation, and repeatable closing of the throttle.

You can do a decent job of that with the carb still installed.