First and foremost (and most importantly) stay calm... If you get all worked up you will most likely miss something simple and then be kicking yourself in the ass later for it.
Check all the "common sense" things first; (even if you think they are correct!)
>Carb slides in correct? (slanted cutout on the slide should be showing out the backside of the carb (intake)
>Carbs in sync?
>Airflow restrictions?
>Fuel restrictions?
>Fuel on, good gas, full gas
>Plug wires loose or bad?
>Clamps on front of carbs (at the intake boots) tight?
>Intakes/Reeds tight?
Can't think of any others off the top of my head right now. If these are all good, you might need to go deeper and do some of the following;
>Check Compression
>Clean Carbs out real good
>Pull your reeds and make sure there aren't any broken or badly gapped ones. (this can sometimes cause bad running like you are)
>Pull stator cover and make sure flywheel and pickup coil are in good condition and tight.
>Get it running and have someone keep on the gas to keep it running, spray carb cleaner around the motor in places like the head, reeds, etc. anywhere you could have an air leak. If the motor stumbles when you spray you have an air leak there.
Hopefully that helps get you moving in the right direction.
Tate