Assuming you do have good spark at the plugs, and the timing is correct, I think this is fuel related. Make sure there is gas getting into the throat of the carb when you work the throttle. I had an issue with my carb rebuild (my error) where I forgot to install the tiny check balls correctly, plus I had forgot to replace the accelerator pump. It ran "ok" but something was missing when I accelerated. I also had whiteish smoke. My accelerator pump was stuck in the carb and it almost looked like it was a pressed fitting. Come to find out you need to pry it out of there with needle nose and install a new one. Again, this was my rookie error.
Just a few things to try as you are getting fuel to the carb, and you are getting some sort of spark.
