Zedheads, Background: I’ve been having trouble getting my carbs adjusted and eliminating backfire.  I’ve rebuilt both carbs with Z-Therapy kits, adjusted both floats, used a Colortune & Float Syncs to set up carbs, and a Uni-Syn to balance them. Ignition is new Pertronix electronic setup from MotorSport with a 3.0 Ohm coil.  Still the car backfires under load when driving. So, I checked my valve clearances.  Most of them were too tight, and I’ve adjusted them all to Hayne