I see this is a repeat post so I'll try to help but we need a better
description of the problem...
The "clanking" at idle.... is this a mechanical noise like a failed engine
part? Or is it a spark knock like one might get from improperly assigned
plug wires? Is the backfire into the intake or the exhaust?
As with any running problem like this, it is important to have all the
basics right.... Spark plug and plug wire condition.... proper plug gap (in
the ball park, at least), is the firing order correct? Does it have
compression on all cylinders?