OK - 3 common problems on 3 liter 2000 Vulcan engines. Check the vacuum line on the FPR (Fuel Pressure Regulator). If it comes right off the engine can go rich, but a minor air leak can make it go lean AT IDLE. It will have little or no effect at speed.
The connection on the EGR Vacuum line is also a common leak on that engine, both where the hard plastic line connects to the rubber elbow.
They also have intake manifold problems. Disconnect the plug from the ISC (Idle Speed Control) valve to do vacuum checks with propane or ether. Leaving it connected you can not detect the leak because the ISC compensates for any change in idle speed.Check between upper and lower intake manifollds