Hi, I am trying to tune the carbs on my newly acquired Commando 73 750. It keeps backfiring (when the throttle twist is released after revving up). I checked the exhaust and muffler nuts, they're OK. I tried tuning the carbs but all settings to give the same results, not even sure if it's too rich or too lean... The bike was running relatively Ok when I got it, the only thing I did is add gasoline (96 octane). any idea? Thanks!