Older Outboard Losing RPM at WOT

[corruptibleseed]

Hi, I have a 2011 Yamaha F60T.

The outboard runs great with no issues. I am just curious about a minor loss of rpm I have noticed:-

1. At around 900 hrs, the outboard would do about 5500+ rpm at WOT.
2. At around 1200 hrs, the outboard would do about 5400 rpm at WOT.
3. Currently, at around 1700 hrs, the outboard does about 5300 rpm at WOT

The above readings were all done with the same prop on the same hull. The engine will happily hold the WOT rpm steady for as long as conditions allow. The weight of the boat is roughly the same and was probably heaviest at around 1200 hrs. Currently probably lighter than at the 900hr mark because I've upgraded the *****ing motor batteries to lithium.

My aluminum Yamaha prop hasn't been changed since probably around the 300hr mark but hasn't been too badly banged up either. Just a few nicks over the years. I did think about trying out a new prop but the old one does work well enough with no vibration.

I suppose it is also possible that the boat has gained some weight from being slightly waterlogged as it does have a few bits on it that's made of wood encased in glass.

Would the loss of 100 rpm every 300-500 hrs after the 1000 hr point be fairly normal? Would this be caused by fair wear and tear on the pistons/cylinders fit?

Most of the time, I run at under 5,000 rpm so the loss at WOT doesn't really have much of an impact. Just something I have noticed and was wondering about.

Thanks!

[99yam40]

I would not think it would be normal.
damaged or wore down prop would cause an increase in RPM due to not biting as well.
compression or even a leak down test should show if rings and valves are sealing off well.
have the plugs looked OK when you changed them in the past?

[DennisG01]

Agree - definitely wouldn't be the prop.

Tune up items are a good start - and certainly worthwhile to do if they haven't been done in a while, regardless.

Leak down would be a fair test to do. I'm not sure if compression test would be helpful in THIS case wince there's no baseline compression test to compare it to. If a compression test had been done yearly, then that would probably have been helpful. It's probably not worthwhile to compare to any specs for the engine (comp #'s) either since there are differences in compression gauges - and even a brand new engine can vary slightly from "specs".

Is there anything going on with the hull that could help explain this?

[DennisG01]

Oh... maybe another possibility could be a stretched throttle cable where it's not fully opening the throttle? I think that MIGHT be able to be adjusted? Not entirely sure on your engine, though.

[corruptibleseed]

Thanks guys. Guess I can rule out the prop.
99yam40 - yes, the plugs are changed out regularly. Just did them last month and they look decent. Never needed to change them, but I just do it every 400 hrs or so for peace of mind.
Guess I'll also see about doing a leakdown test to find out what's going on.
DennisG01 - actually, you're right - it may possibly be the throttle linkage. From memory, it seemed to be opening fully, but I guess I should put the time in and see if any adjustments are required. It's not hard to check or adjust on my engine so I have no excuse.
No changes to the hull that would explain this. If anything, I removed a thru-hull transducer a year or so back. Guessing that should have reduced drag a bit and increased the rpms but there was no change!
I guess the problem is that the engine has no problems and I don't run at WOT much so this issue never really bothers me! That said, it is getting along in hours so I guess I should start paying attention and try to identify issues before they become problems.
Thanks so much!

[99yam40]

If rail pressure is staying in spec, maybe having the injectors tested and cleaned might help if you cannot find anything else wrong.

[corruptibleseed]

Thanks so much 99yam40.

So I had a closer look at the throttle linkage over the weekend and it isn't being fully opened! Adjusted the linkage a bit and I noticed a slight improvement when I ran the boat this morning (almost made it past 5400!) so I think this may be the right track. I think I'll need to adjust both ends of the cable to get enough travel. I have had to change the control cables a number of times so it looks like I didn't spend enough care getting the linkage length correct.

Funny thing is this is one the first things I checked when I used to fly giant scale model aircraft years ago! Feel rather silly that it just didn't occur to me to look at it sooner.

Thanks everyone!