1985 K5 Silverado Blazer backfiring issues

Hey guys


Been having a horrible week with my new blazer i bought. Its a 1985 K5 Silverado Blazer, auto (3 speed auto w/ od), 350 (not original motor).

When the truck starts it idles fine at around 750, and when you put it into gear the passenger side exhaust pops/backfires. It only occurs on the passenger side not the driver side at all.

I have done to try and fix it
-new plugs
-new wires
-rebuilt carb
-timing is set
-firing order is correct
-vaccume is at 24 '' Hg in P/N, 20 with brakes on, and around 14''Hg in R/D/1/2 with brakes on

I do have a leaking brake booster, but i had redone the header gaskets and put them back on the truck and now it is all messed up again. It was not backfiring at all before i took off the headers to put in the new gasket

Any info would be greatly appericiated

Dibsing