I'm going to be the one who asks the dumbest, freshest and most personal question of all.... :wink:
Are you kicking it "through"? If you go at it half hearted and wait for it to kick back, it will sense your lack of determination and give you one back just for good measure. I walked "the walk" for about two years, till I figured out that fear of kickback was the reason for all the pain. I just wasn't kicking it through the compression with enough power to keep it going forward, I was so busy waiting for it to hurt me that naturally it kicked me back, just for good measure..
Indeed the battery might be bad, but a weak battery won't cause the motor to fire at the wrong time, just cause it not to fire at all and the carbs have nothing to do with a kickback, as pointed out above. A sticking valve, would cause it to blow back through the carb, or out the pipes but unless the valve was sticking closed; a practical impossibility with out having a bunch of bent valve train; not cause much of a kickback at the kick start lever, as all the power would be going out the carb/pipe instead of against the piston.
It is either out of time, got a broken wire that makes contact at bad times, or now that it has done the kickback to you a few times, you have gotten over-cautious and you are not kicking with determination...
Recheck the wiring, reset the carbs for a good idle when you get it running, and think on whether the leg pain, is causing the kickback...that's what caused mine..