On any engine when you engage the blades it should Rev UP not DOWN and not stumble and falter and finally get going. What does it do once you're mowing in high grass? Does the governor kick it up when you hit high grass or does it lay down and lose rpm? Either it's not getting enough gas to handle the increased sudden load of the blades or the governor is out of adjustment or both. I had a friend with an older FORD with a K-series engine that would drag along and the governor not kick in. It needed more fuel and the governor adjusted, then ran fine.
