when i took my carbs off to clean them i ran into a similar problem putting everything back together. for me, it all depended on positioning of the cables, it had to be wrapped perfectly around everything in order to get the appropriate throttle response. too little or too much slack both caused problems. if it works without the tank and doesnt work when its on then it is probably pinching the line somewhere.

 

as stated above, its pinching the line. take off tank, and air box if you have to and check periodically when puting everything back to see what restricts the cable