[SoCal LS-L]

Originally Posted by oab_au

Well I still think it is a lean mixture problem, caused by, an unknown air leak, or fuel delivery.

The symptoms of a bad idle, getting better as the engine speed increases, and backfiring through the inlet, are all a sigh of a lean mixture.
Chris, a lean mixture burns slower, still burning when the inlet valve opens, to cause a backfire into the inlet manifold. A very retarded spark will do the same, but it won't get better as the revs go up.

I would go all over the engine, at idle, with a stethoscope listening for a leak.

Harvey.

I believe he mentioned the backfire thru the intake had been corrected?

In all fairness, it may just seem that it runs better at 4000RPM, as at that speed anything less than catastrophic misfire is heavily shrouded by engine speed, and also the IRIS is open to help the air along the lengthened track.

Some careful spraying of starting fluid along the intake system should help identify any intake or vacuum leaks..... however I think that given the symptoms and the amount of new equipment he has already installed and eliminated as a cause, engine management should be scrutinized and a stock ECU at least swapped in to see if anything changes.