Try checking the throttle position switch. It is a switch at the base of the throttle that tells the car to idle. It "makes" when your foot is off the accelerator and the linkage returns to the home position. You will find a 2 wire connector coming up from the base of the throttle housing. Disconnect it and check for continuity when the throttle is released, then check for open when the throttle moves. You will find a procedure for setting the proper linkage position in the CD Rom manual.
Best of luck!
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)