After a few outings with a new to me Z1 it acts like its out of gas after 2-3 hours of run time. It will only take 12-15 gallons each time. So I removed the floor thinking there was something wrong with the pickup. With the floor removed and a full tank I ran it for about 15-20 min to get the fuel below the fitting on the tank. I stopped in the middle of the lake and removed the pickup line. Instantly the tank took in air. I checked the vent line and it will allow air to go out of the tank but not into the tank. Should the vent line not allow air to enter the tank via check valve? I believe it is creating a vacuum and the fuel pump cannot pull fuel past the vacuum pressure after a couple hours running. Is that possible? Any insight is greatly appreciated. TIA!