too tight will cause less belt deflection, may cause high pitch squeel at idle, and you will loose top end speed (or a belt for that matter).
I don't know if this is a symptom but it seemed when my belt was too tight it got really hot.
To loose a belt will also cause a bog. To tight(secondary) spring will cause high RPM. To steep helix will lower RPM. To loose (secondary) spring will also cause low RPM>