As mentioned, your problems could be related to several different things (ignition, carbs), but I would definitely have a look inside your carbs to check their jetting and verify proper float level setting right away. There's no telling what the P.O. did, if anything, to tune the carbs for the mods done. For the typical "pods and pipes" type mods you have, some re-jetting is usually required. For most of the carb sets, the increase needed isn't huge, maybe one or two up on the pilots, same for the mains, but your '76-'77 carb set can be an exception to this. It came with the smallest mains (122.5) ever put in the 650. While fine for a totally stock bike, those small mains are quickly outpaced when you start doing mods. Your carb set may require 3 or 4 steps up on the mains, to a size in the low 130's. But, before you get started on jetting changes, you'll want to positively I.D. the year of your carb set. You need to know this because jetting changes are based on the year of the carb set, not the year of the bike, and carb sets are easily swapped on these. The one pic you posted isn't detailed enough to I.D. the carb set 100%. It does appear to be a linked set meaning it could be the original '76-'77 set but it might also be a '78-'79 set. To positively I.D. them, remove one of the pods and look at the intake bell. Both sets would have the oval slot on top but a '78-'79 set will have 4 additional round holes around the edges. A '76-'77 set would only have 2 additional holes .....