Hi Mark, Probably teaching granny all about eggs here...
Could it be water in your fuel tank? do you have a drain cock on the tank? The symptoms would seem to fit with water in the system, as water is heavier than fuel it tends to sit in the bottom of any in-line filter, float bowl etc and go unnoticed in warm up, only tending to get sucked into the carb at high power or in certain attitudes. A 4mm bubble of water sucked through the carb will cause just the sort of stumble you experienced, just long enough to give you heart palpitations, but not long enough to stop the engine spinning. You don't need to put water in the tank to get water in the tank. When the tank is near to empty, say after a hot day's flying it will be full of warm air/fuel vapour. As the sun sets, the tank cools and the air inside contracts sucking in moist air through the breather, the moisure condenses out on the cold tank surfaces and runs down to the bottom of the tank. Repeat this every day for 2 months and you can have a surprising amount of water in the bottom of the tank, I have heard of 2 galls being decanted from the wings of a Cessna. Pete
