I have a 93 Johnson 15 horse tiller that shuts off when i try to let the throttle down. Runs great anywhere from 25% throttle to wide open. Any lower it just shuts off. Weather in gear or out. Also has to have the choke on to start every time then must quickly kill choke and crank throttle to keep running. Basically have to slam in gear to keep running. Any tips would be great! Thanks.