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Discussion Starter · #1 ·
Been working on a 2006 Mercury 15 HP 4 stroke that I recently got from someone that hadn't used it much in 2 years. Motor starts well every time, idles well, and runs well until around 1/2 throttle. After that, the RPMs seem to increase, but the speed doesn't. The motor also seems to shake substantially more. Pumping the bulb while running doesn't help.

Maintenance was long overdue, so I changed the fuel filter, oil filter, oil, and ran some Sea Foam through it. It seems to sound much better at idle and lower speeds, so I was optimistic when I took it out tonight for a water test. However the problem persisted. I think it could be a fuel issue, so considering changing the fuel line, but wondering if anyone had any opinions.

Also worth noting- it ran fine around three trips ago (the first two times I took it out). Reached much faster speeds with an extra body on board.

I'll see if I can post a video. The issue begins around when I reposition my hand on the throttle.

Many thanks, CP.
 

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Discussion Starter · #3 ·
Sounds like you damaged your prop and it’s now hitting the rev limiter
Interesting... hadn't thought of that. Haven't taken it near shallow enough but there was one time when my buddy (won't name names) changed gears pretty fast and made me squirm. Not sure if that would do it, but will check it out.
 

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Zephyr Cove is on FIRE!
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Got me thinking that it only happens when I take it out by myself. Maybe less weight is making it hit the rev limiter?
Yes could be for sure. Sounds like rev limiter which could be wrong prop, outboard mounted too high, damaged prop hub or clutch dog. You can get a tiny tach to monitor actual RPM and compare to the outboard’s max RPM rating.
 

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I had the same issue on my Gheenoe. It turned out I had the outboard tilted on slightly too high of a setting and had too much weight on the front of the boat, would ventilate just like that, hit the Rev limiter, and start slowing down.
 
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