If it will jump start, it is possibly a bad battery regardless of it " checking " good. That starter requires a good bit of amperage, not just voltage.
Lat summer on our Wyoming trip my riding Buddy Ray's 1800 started acting up. Sometimes it would start fine and other times we'd have to Jump start it. Finally in Powell, Wy. he had enough and bought a new battery Miracle of Miracle the problem went away along with a crappy 3 year old battery.
Tried jumping it with a car, got the same results. Guess it goes into the shop this week. I’m at the end of my mechanical abilities. Thank you! For all the helpful suggestions! Nice to be a part of this community!