My 72´ 240z is still not getting the correct tune up! I have tried almost everything but its still backfiring at the intake on both carburetors. I have watch the Ztherapy video like 10 times and followed their instructions but cannot get the job done. The Carbs were recently overhauled by them on last November.   1. Warmed up the car for 5 minutes   2. Took both of the Domes out and rinsed them throughly with Carb Cleaner   3. Cleaned the 6 spark plugs (NGK BP6ES) with brass brush and contact c