I just swapd my old d16y8 w/ a new one...When I start it, it runs really ruff until I start going...and when I take off from a dead stop it goes really slow until it hits about 3k RPMs then it runs normal and every other gear runs normal... I thought it would be spark plugs but when I put those in they lookd fine besides them being black but I scrubd them w/ a wire brush until they were close to new