As crazy as it sounds; since i cannot physically see the bike, i would recc that u rem the pipe, and the airbox.
Leave them off, and try out the bike in your garage.
(The pipe could be faulty/obstructed, and the airbox can be collapsing under a vacuum, when u open the throttle, or someone left a shop rag in the airboot...)
If neither of these help, then verify:
1. piston not installed backwards
2. proper base gasket was used (ie, not obstructing any transfer ports, etc.)
3. your reeds are not installed upside down/incorrectly.
ps---did this bike run when u bought it?