Sounds to me like it's just taking a long time to get fuel up to the carb. Once you do, it runs OK. Make sure the check valve is a YS valve, not one of the Tettra brands, or whatever that other brand is. The YS valves work on an entirely different principle than the others, so if you have a different brand it might be preventing the tank from pressurizing as quickly as it's supposed to. If you aren't sure, take one off an engine that's starting easily and try it, make sure you get it installled in the right direction.