LOTS of people like the rumbling sound of setting the idle low but, as has been mentioned, it's not good for the bike. If you don't have a tach, the best way I can describe it is quick enough that you can't easily count the rev's anymore. It's not a long-stroke big-twin. 950~1050 RPM's (the ideal rev range at idle) is relatively brisk sounding.
When I bought mine the FI-light on the dash (Fault Indicator not to be confused with Fuel Injection), was on at idle. Would turn off as soon a throttle was turned. Bumped up the idle speed and it went away. As everyone has said, this is due to low oil psi from P.O. wanting that low rumble sound I am sure. Not good for the motor obviously.
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