When I bought my 03 z400 afew months ago it was running bad. It had the same symptoms that you have..stalling at idle...lack of power...etc. i took it in for a carb service where they took the carb out and cleaned everything. When I got it back it ran great. It must have had some gunk in it somewhere.

 

this may seem stupid but it could be bad gas. even if it is "fresh" it could be from the bottom of the tank at the station and have crap in it. clean the carb and put new gas in it.

 

hey check out z400central.com