Blowback reminds me of my wifes 71 Dodge Dart slant 6. It was idling rough as heck one time. I was a youngster still learning. A former Dodge mechanic at work said bring it to work. He switched around two plug wires. Said crank it. It backfired through the carb. Ok. He swapped the two wires back to the right place and it idled smooth as silk. Have to do that every now and then he said. Carbon. But of course those were the days of leaded fuel.