416 sb been running same combo for a few years.
no previous poblems maybe 1500 miles on motor.

has developed a high speed hesitation and backfire out exhaust.
pulls good threw the gears but once in high gear at full throttle for a few seconds, feels like key is shut off then backfires out exhuast. then recovers as long as you get out of the throttle and runs fine at part throttle.

plugs/wires/cap seem good no sign of a lean cylinder.
mech advance not stuck, cant find vacuum leak.
new carter hi-po mechan pump, fuel pressure/float set ok.
carb cleaned.
havent pulled valve cover to look for broke spring but wouldnt expect that with low miles.

got me stumped.....mallory ready to run dizzy and msd coil.
only does it after going into high gear(4 spd tranny) at full throttle, i dont feel it 1st threw 3rd.