tuning help needed

[yellrhd]

OK guys (and gals) I'm at a loss.

I've got a brand new (1200 mi.) bone stock 1600 with dual Solex 35 PDSIT's and an SVDA (got both from aircooled .net)

The car runs great while driving aceleration etc..... except
My problem is I can't get it to stop popping/backfiring? when I down shift and take my foot off the gas or when just sitting at a light with my foot off the gas.

I did a search here and came up with a list of possible causes and have addressed each one as follows.

Carbs= synchronized and set per instructions from aircooled.net
timing=7.5BTDC as per instructions from John@aircooled checked with light.
Intake leaks= None (checked three times)
exhaust leaks= None
Valves=checked and set per Bently manual

The only thing I can think of now is should I change out the Bugpack glasspack for a quietpack? thinking the lowbackpressure is doing same as having a leak

Or is the jetting wrong for my engine the carbs came with 30mm venturis and the jets were listed as 30/55/160

Any suggestions?