Check your basic Air supply ducting for obstructions (wasp nest, etc.. if you leave the seat propped up per BRP recommendations during long storage. Other critters like tiny places too for homes ....mice, etc..) The check the Fuel supply. Age of Fuel, Quality of Fuel. Then check the Fuel FILTER inside the fuel tank for foreign particles and/or sludge (ethanol sludge separates from the gas) that will allow for starting and idle, but upon high demand suction draws the sludge against the filter membrane; restricting supply and then you feel the power drops. As the sludge moves in the sloshing tank the supply repeatedly gets interrupted creating the full power/hesitant power/full power/partial power/sputtering power cycles. Sludge getting past the filters clogs the tiny, tiny, high pressure injectors. All gas engines that are used seasonally or infrequently (boats, lawnmowers, etc..) are prone to this; also gas suppliers get junk in their tanks; etc... Filters are installed for good reasons and are relatively cheap to replace. My marine engines have two on each engine. Even Pure Gas changes over time and Fuel Preservatives have limits too. Urban myths exist about 'restoring' stale gas. Good luck with your investigation.
