You gotta start with the basics. It takes three things to make an engine run. Compression, ignition and carburetion. Do a compression check. If it's got lousey compression nothing will help until that's fixed. 145 to 165 PSI, maybe some higher on the SS? Even as low as 100 would most likely run, but it's important they be within 10% of each other. If they are 20 % apart you have a serious issue. It sounds like you should adjust the valves, but loose valves will be OK.... for a while. (on the other hand if you can hear the tapping over the straight pipes, maybe you should adjust the valves) Tight valves are bad new fast. Backfiring through the exhaust is a bad first sign. If the compression is OK spark is next. Does it have electronic ignition? It's not a bad way to go. If it were 1974 I'd say go back to points, but today? I'd go with electronic unless you know and understand points ignition and want to stay on top of them. If it's got good spark, then you must verify the timing. If timing it correct, now you can finally get to the reason we came here. In todays world there's a good chance it has "green slime syndrome". Old gas can frig a carb up. Disassemble, make sure every port, hole, jet and crevice is clean. One partially clogged hole can cause stumbling as RPM's increase. Has it ever run right since the cam & pipe? If it ran right before the cam you gotta go with bigger jets. I can't tell you what size jets as I'm not familiar with the SS motor....or even Harleys in particular, but when you add cams or pipes things need more gas. At least at this point you'll know everything is OK but the jetting. Don't forget to check float level as the carb goes back together.