Ok, I'm no mechanic and I'm jumping into the middle of the thread I know but I feel that I'm missing half the story. It seems like you have thoroughly checked the carbs and spark plugs but starting from the begining, if you will, what else have you done (failing that was there a previous thread that you explained all this and I've just missed it
As with the usual 'bike doesn't start' issues, dunno if it will help but...
Check the following:
1. Battery has juice
2. Starter turns over
3. Spark plug actually sparks (pull it out but still connected, put it next to a piece of metal - turn over the bike and look for the spark)
4. Fuel - not flooded or dry - ensure no air bubbles in fuel line
5. Air - air flow/intake is clear
Failing that very basic checklist that I'm sure you've already done. What did you do to the engine? Can you turn the engine over manually by forcing the cam-shaft to spin, some bikes can do this with rear-wheel in gear and turn the wheel, if not then another way perhaps??
Again, apologies if you've already gone through all this, just not quite sure of the full story.