At idle, looking down the 2 throats with a bright light, you should see no fuel spray or dripping at all… When you quickly snap the throttle momentarily open, you should see an equal spray of atomized gasoline coming out of the center of the booster venturies along with a squirt from the accelerator pump…If only one throat is drawing fuel, then the other side is plugged up and starving 4 cylinders for fuel, causing the backfire…