I have a mercury 115 outboard, at Neutral it runs like a champ. In gear forward, rpm hesitates but will push about 5 to 10 mph.
in reverse no issue, I think it is a lower end problem, but some people tell me prop. It runs fine. My local boat shop says lower end is 2500. I can get one off eBay for 500 to 1000,are they universal. Or am I over thinking the issue.