Replaced the carb with a remanufactured one from Napa, stock Rochester B. The fuel tank...nope, never cleaned it. I noticed flakes of rust in the fuel filter. Looks like I have to do that next. The popping...I saw flames out the exhaust once. Never saw any come out the carb. It's a pretty distinct pop though. Timing is correct...if there was a valve problem wouldn't it idle rough? It idles and drives really nice, except for that hesitation and pop.