No power under load

Honda gxv670 on a hustler zero turn. Starts, runs fine, engage blades wants to die. I can increase throttle stays running but when under load it chugs. I've checked air filter, fuel filter, an gas flow to fuel pump, it's fine. Don't know how to check fuel pump. I believe this is a vacuum fuel pump. Can they act this way. The pump is my nxt step when parts house opens tomorrow. My reasoning on why its fuel related is the filter has barely any gas in it when engine is running. Any suggestions wd b appreciated. Matt