After reading all of this there are two things that tend to stand out in my mind right off the bat... you said your slides are in the right way but please double check this because they are VERY easy to put in wrong and think you are correct...
There's two ways to make sure,
1. on the RIGHT side of each carb there is a tiny window, push the throttle untill you see a ping mark appear in the window, if you don't see one they are wrong.
2. pull the airbox and look into the intake side of the carbs, there should be an angled edge showing toward the rear of the slides, if it is just a straight cut bottom edge of the slide they are in wrong.
Having slides in backwards makes it REALLY hard to start and then run like shit.
If this is all correct then my other thought like others have said, do a leakdown test OR if you can get it to run at a decent steady RPM, go around the motor, intake boots, cylinder base, etc. with a can of carb cleaner and as you spray these areas listen for the motor to hesitate or drop RPM's then you will have found your air leak. Otherwise you'll need to build a leakdown tester.
Tate