first check filter above float valve.
to do so, dismount carb, then float bowl.
remove float and float valve assembly.
if filter is stucked with dirt, clean it.
always use extra filter in between the fuel line from tank to carb.
here's a pic:
http://www.xt600.de/xt_werkstatt/+sp...immer_assy.htm
if this wasn't the problem, please ask again.
good luck
klaus