cause for frequent backfires at cranking?

[VetteMed]

I know the LT5 is prone to backfiring through the intake on cranking, but in the last 2 days, I have had backfires about 25% of the time (3 backfires in about 8 or 10 startups). Of course, the MAP sensor hose blows off. Also, there is some popping from the exhaust upon decel while in gear. None of this was an issue prior to my plenum pull last week (had injectors cleaned by FIC), rechecked vacuum system.

So, what is a likely cause for frequent backfires and pops on decel?

I am running with secondaries eliminated, Haibeck tune, headers.

thanks
Andrew