Yes timing will effect idle. Timing isn't really all that complicated, its just when the spark plugs are fired.
The degree of timing (say 15 degrees BTDC) means that 15 degrees before the piston reaches TDC the spark plug fires igniting the air/fuel mixture.
A more advance timing will give the engine more time to complete burning all the fuel and drive the piston down harder.
A more retarded setting will not give the engine enough time to ignite the air/fuel properly and will still being combusting after the piston starts its way back down causing a negative effect in power.