Is your back fire a "pop" thru the carb..??? if all the normal tune-up issues are verified....next thing to check is compression and valve train. You could have a burnt valve, cam lobe that is starting to go or collapsed lifter....compression check is pretty simple and you can pull the valve covers and look at the rockers/pushrods in action.....if one is not moving up and down as much as the others....you need to go further inside. How amny miles on the engine..??? has it been rebuilt...??? let us know what you find.My 307 seems to run ok but has no power and is sluggish.the dwell is set at 30deg.it appears to be getting plenty of gas but, it does backfire sometimes.what could it be?
I wouldn't rush to buy a new motor until you see what's up with this one....if you are unsure about the diagnosis...why not take it to a reputable shop and get it looked at ??....the 307's were a durable and good running motor, but they did occasionally have cam issues. I worked on a lot of them in the early 70's at the neighborhood gas station.Would I be better off to put in a new engine?