need help ym 1900 23 hp,loss of power

[ttrap]

well i have a ym1900 23hp 2wd, I PUT NEW PISTON RINGS,HG, ROd BRD,and had the rad rod and flushed,here by problem. it starts and runs real good for about 20 to 30 min,then under load it starts smoking black smoke,wants to die. after a little bit it starts right up ,not sure if its overheating or mybe
govner or injetors or fuel pump,its seem real strong for about 20 min,just under load,it want to do this, any help would be nice

 

[California]

One thing that will kill it after a while is a fuel tank cap that is airtight. Unscrew the cap a little and try that.