I'm having the same problem with my 660 but the only difference is it runs great with no air box connected to the carb but as soon as I put the air box on and connect it It will run but when I give it gas it dies So I replaced the air filter,cleaned the carb checked all the spark plugs and checked the cap and rotor and cleaned and replaced the fuel filter. So I'm not sure what it is.
did you clean the carb internally (take it apart and blow out jets etc)...sounds like a clogged jet...what did the plugs look like when you changed them?
This problem with Honda's has come up before. Read these. http://www.minitrucktalk.com/showthread.php?t=2274 http://www.minitrucktalk.com/showthread.php?t=6493
