I read that backfiring through the carbs is usually caused by the timing being to far advanced. It is trying to fire while the intake valve is still partially open. This is the one that will cause the most damage to an engine, since you not only have the flash, but some of the compression is being released into the intake and back through the carbs.


Im having this same precise problem, My car is a 1972 240z with a Schneider 274F Camshaft and remanufactured SU Carbs by Ztherapy!!! Also have pertronix ignitor and have set the Distributor timing to 11 BTDC. Please note i have also removed all the emmissions crap including the antibackfire valve from the balance tuve. Should i lower the timing to 10 BTDC at idle to avoid the sudden gunshot when decelerating?

 

Problem was solved! The exhaust gasket was damaged! No more backfire!