[Jac Mac]

Remember with the Vac secondaries there is no accelerator pump to the rear barrels, you might find that you need a longer duration pump shot ( slightly smaller shooter or larger 50cc pump & cam) to cover the point where it backfires. We used to pull the secondary idle tubes out of 3310's fitted to Big Port 351c Falcon GTHO etc & enlarge them slightly to richen up at the point where the secondaries started to open to get rid of a similar problem on those cars where a spring change didnt work... simply enlarging the rear main jet size might help, but I suspect its happening at the point where the secondaries open & or transfer from their idle to main circuits, therefore would probably be too rich in the overall picture..
One other trick that might help pin down the problem area... Cut a piece of 0.040 SS wire@ wrap it round the aircleaner attach stud, then fed each end into the idle air bleeds inside the air horn on the secondary side, then test drive, if that makes no change then try the main air bleeds & test drive....This would increase the fuel flow/richen either circuit and possibly help point you in the right direction without making any 'non-reversable changes..