First post for a while, got a GMC running Like a bag of nails, new headgasket fitted, four new push-rods, valves and springs on no 1+2 cylinders, carb + fuel pump overhauled, LT and HT system changed With new coil..enging timing set up and strobed, fins well fire 10-15 mins then starts to miss on 1+2 again, got 9.5 bar compression through all 6 cylinders, good sparks, checked, rechecked, valve clearances, CBs, etc, all good...