I test drove an M715 wih the original motor. It semed to accelerate very slowly. If I really stepped on it, it would seem to accelerate up to a certain speed, and then when it hit a certain RPM, it would "fall off" or stumble noticably...almost like it was breaking up. Then, a few times, it would backfire, until letting off the gas and getting to a lower RPM. I don't feel that I was pushing it over what it should be able to do. At no time did it go over 40 MPH. Any thoughts on what it might be? Thanks, Hal