Drew.
When you cleaned the carbs, did you remove the pilot jet and make sure you can see through the center of it? If not, you may want to do it again. Also, check the intake manifold for cracks. Over time, these harden and crack or seperate from the aluminum flange. The cracks are hard to see but, will make the bike hard to start.
Another little tip. Unscrew the sparkplug cap from the coil wire and take a screwdriver and unscrew the slotted brass insert where the spark plug goes. Under there you wil find a little spring and a resistor. Take the resistor and throw it as far as you can and replace it with a piece of copper rod the same length and diameter as the resistor. Those resistors go bad after time and will cause the bike to be very hard to start.
If after all of that the bike still pops back, you may need a valve job.
Good luck. :cool: