I recently bought a 1996 Formula III which had a 617 motor put in it. It works best kind when the rpms are up but when the ski doo is stopped or moving slowly and you give it a lot of gas it boggs out with a heavy miss....i have cleaned the carbs and changed the plugs and this dont fix anything! when i am moving at a decent speed anywhere about 20 mph it works perfect! Any ideas what this can be? Thanks