Most of them do tend to idle rough, one cylinder being a touch off has a bigger effect on the whole when it is one out of 6 than 1 out of 8. Also the chains stretch and the plastic gear wears a lot and that retards the valve timing causing low vaccume and rough idle. A nice tight chain should also help the MPG by closeing the intake valve earlier(actually where it is intended to close) and helping to build cyl preasure, it will also let the MAP sensor know there is not as much load on the engine as it thinks there is based on a weak vaccume signal, if vaccume is weak it thinks there is more load and dumps more fuel.