When i've restarted the car, i wasn't happy of how it was running, so i bought a repair kit for the carburators and started to clean them.
After that, it wasn't still perfect
When a carburetor is backfiring like that, it's because it run too lean.
Then i realised that one of the accelerator pump wasn't working properly
So again i've cleaned the accelerator pump circuit and boom, there's one of the check valves for the accelerator pump was stuck open.
After reassembly, i've used my Gunson Colortune to tune both carbs and it was way better than before.