Welcome! Need support, you got it. Or share you ideas and experiences.
I have a old 1969 Evinrude 18 horse Fastwin electric start outboard. I know the thing can't be more simple to work on. I replaced the lower unit by myself and I never had any mechanical education other than a small engines course in high school. The thing runs like a charm, but I'm not getting full speed out of it.
When I had the old lower unit in it, I got about 28 mph out of it. Now I only get about 15. I doubt that the lower unit replacement has anything to do with the present power loss. When I did the lower unit switch, I didn't touch the cable adjustments, the control box or the carb and settings.
If anyone can give me some assistance, please get back or e-mail me.
The first things that come to mind are the gear ratio of the lower unit and the prop. If the gear ratio of the new lower unit is different. That could be the problem. If you also changed the prop at the same time, not unusual if you have hit a rock or somethin'. You may have used a prop with a different pitch or diameter.
If you still have the old parts, check then against the replacements. Now, just correct as necessary and you should get your old zip back, if that was the problem. ;-)
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