My 4-strokers did the same thing. They started out spot-on, then got lean (especially on the idle) after about 10 minutes and would often quit, making a dead-stick landing the norm. I finally figured out that these engines just didn't have the "suck" power to get the fuel into the carb at the same rate after the tank level decreased an inch or two. To compensate, I adjusted the engine to be slightly over-rich with a full tank. It then leaned out and ran perfectly after the tank level dropped. I don't think you get into this problem with a pumped engine. HTH![8D]