Engine runs backwards

[Ted928]

My car usually takes a few seconds (~4) before the idle will drop from ~1000 to 700 rpm. If I turn the ignition off before the car has settled to 700 rpm it will sometimes spin backwards (painful to hear). If I give it a few second to let the idle drop to 700 rpm before I switch it off, then it is fine. Is this common? Does it mean I am too rich? The car does not knock upon heavy load so I think the timing is fine. The timing is set at ~15 degrees with the vacuum hooked-up to the dist.

Thanks