Pulls hard and then quits...

could be several things...i agree it's not the size of the fuel line..but perhaps a kink or a weakening fuel pump not keeping up....if it's fuel deprivation (sp?)...check the plugs to see if they appear leaned out...then pump, kink, or carb tuning is the problem.
a 600 seems slightly small..but you have a pretty mild build to be outrunning your carb CFM
ignition also feels like fuel at times...the stock box's are something that have given me problems right out of the box before, had several die at 4000 RPM like a rev limiter...junked the stock box's and ballast setup for a summit brand (MSD manufactured) stand alone digital ignition box...night and day! good luck!