I don't think it has anything to do with the design of the engine. My Tower .61 cuts off when I close the throttle all the way without hesitation. As far as I know, the bore and stroke are the only actual differences between these two engines. If that's true, then it must be due to variations in parts fit, i.e., a QC issue. Either that, or something is not assembled correctly, which I doubt, because Frank has checked just about everything. Maybe try replacing the length of fuel tubing that runs from the remote NV to the carb?