How are you sure it is getting spark and fuel? Did you teardown the carbs and completely rebuild and clean them?
Test the compression on both cylinders.
You need 3 things to run the engine:
I was able to pick up a $15 (20% off) compression tester at Harbor Freight: http://www.harborfreight.com/compression-test-kit-66216.html
Or it is getting way too much fuel. When I removed my carbs last weekend I found a main jet just hanging out in the float bowl. :/
I took them off to install larger jets.