(OT) RPM's not returning to idle

[William K Denton]

Hi all, 

Sorry for the OT, but I'm trying to help out a relative. My cousin is
looking at buying a used bike, a 2007 Suzuki GS500F. The bike is pristine
cosmetically and has 987 miles, which mean it sat for 8 of the last 9 years
before getting traded in to a local dealer. My cousin's brother test rode it
yesterday and reported back to me that it wasn't returning to idle speed,
sticking up around 4K RPM. It was also stalling and generally running rough,
in fact he had to push it around 100 yds back to the dealer after it stalled
and wouldn't start. 

Dealer added some fresh gas and the bike ran better, but the RPM's were
still not returning to idle when he cut the throttle. 

I'm looking for experience based opinions to anyone out there who's had a
similar issue and solved it. I'm thinking it might be any of the following: 

vacuum leak 
carb synch issue 
sticky throttle bodies 

... but if you have any other ideas as to what may be causing this issue,
I'd appreciate the words. TIA as always...

Bill in Yardley, PA
wkdenton at verizon.net