The 401 is not all that hard to work on but is sure is time consuming to get it right. The first one took me over a year to get right. As far as the motor goes check with an adjuster's manual or service manual and it will show it all. There is a lot of info on this board hunting it up is part of the process - you will learn more that way than if we point you to it. I think the 403 is a much better machine.