Gasoline forklift losing power

I just got a komatsu 15 gasoline forklift. It starts up and has plenty of power. After it warms up it starts to die out. I van be full throttle and it gains and loses power. Is it bad rings? The line from the valve cover to the top of the carb. It blows out "air" when idling from the valve cover. I disconnected the tube to show you where I mean.