I havea 1980 xs650 special and no matter what i throw at it i can't seem to solve this. After warm up the left cylinder behinds backfiring and surging getting worse and worse until it's essentially running on 1 cylinder. Leave it overnight and it's back to normal until it warms up and starts over again. I've replaced the plugs, wires, caps, coil, reg/rect, had the carbs off and cleaned and synced, all the fuel lines are good, as is the petcock. The headlight went out at about the same time as it started doing this and i know i read a thread on here with someone having the same issue but he never came back to say if he ever solved it, or if the headlight was a symptom or just coincidence. I'm running out of ideas and this bike is so close to being finished i just need to get over this hill so to speak. Thanks.