2010 f75 loss of rpm, mph - A great (not really good or fun) puzzle

I'd be really interested and eternally grateful if someone on this forum can get to the bottom of a mechanical issue with the above motor - which 3 different certified mechanics and and a service ticket with Yamaha themselves cannot figure out. <br>I am the second owner of a f75 4 stroke tiller, 2010 motor. I have it inspected and test ran 2 times before buying the rig from an individual who bought this rig new and took really great care of it - or so it seemed. The motor is on an Alumacraft Navigator 175 powermatched rig. It has on the original prop and it is in great condition.<br>When I first bought it, the boat ran at 5500 rpm and about 31-32.5 mph trimmed up (depending on weight in boat at full throttle.) It did so for the first 2.5 years I owned the boat. Then last fall, it stated to run slightly slower and less rpm - like 5300 one day, then the next it was 5500. This really never fluctuated during the same outing and never ran poor overall. I changed plugs my last time out last fall and it ran at 5500. That may have been coincidence, however.<br>This spring it did the same from oner outing to the next and actually stared to drop rpm more more frequently - until a trip in early June this year it just maxed out at 5000 rpm and 27 mph at full throttle and never went back to "normal." In the process of trying to figure it out on my own, I noticed that I could actually get the rpms up to 52-5300 if I backed off the throttle in very small increments and found a "sweet spot" (an estimated 85-90% throttle).<br>I always had the certified shop winterized the rig and always have used stabilizer and Yamalube Ringfree as recommended. So, I called my usual mechanic and he recommended almost emptying the tank and refueling with 91 non-oxy. I usually used 87 10 ethanol. That didn't work, so I took it in and they adjusted the throttle and put new fuel filters in. That did nothing. I lost a bit of confidence there, so I took it to another shop and they test ran it with diagnostics and felt strongly it was fuel injectors. I decided to have them cleaned for a pretty penny and then took it out and it still ran the same -EXCEPT for a moment again, when I backed off the throttle it raced to 5800 rpm. It never has done that since.<br>So, I did some research and talked to another shop in Mpls and they did the following:<br>1. Ran diagnostics on dynamometer, did compression and leak down tests and inspected the VST screen and spark plugs and chamber. All returned "normal" except plugs showed rich and VST screen was dirty. So, I had them clean VST, and changed plugs and they again ran diagnostics and cited that the motor was running "in the shop tank" as it should given specs for age and output. I did not have decarbonization done since they believed the VST was the problem. I was really hopeful - but the first time out after, NO CHANGE. Actually, top rpm was 4950 and I really could not get it above 5200 rpm by backing off slightly. It still did make a small difference backing off slightly, but not as much as earlier in the season.<br>So, in a frustrated effort, I asked the shop to contact Yamaha and open a ticket. They did and I brought it back a few weeks ago. The mechanic ran it on the lake with instruments and confirmed 5000 rpm was highest he could get and I am not sure he was able or took the time to back off and find slight increase in rpm. Again, diagnostics were done in the shop were done and sent to Yamaha. Yamaha cited it was 1. Bad injectors, 2. Overweight boat, 3. Damaged hull.<br>2,3 were not the case, so I spent the $$ for new, factory injectors and IT STILL RUNS the SAME. So frustrated and completely at a loss. I reviewed the owner's manual for power loss troubleshooting and nothing there has helped. I feel given that motor is acting differently under load on the water than in the shop that has to be telling, also as this problem has developed the motor seems more "rpm stressed" by additional boat weight like trolling motors and people than before. Is there something I along with all the so-called experts are missing?
b.t.w. revving motor above 6000+ rpm in neutral is no problem. I have only done this briefly.

Any similar experiences, recommendations would really be appreciated. I have a seemingly separate issue with the lower unit and a squealing bushing I am posting on this board as well.
<br><br>Much thanks,<br>-AdamG

Looking at that model boat, I believe it has an internal fuel tank. Have you checked to see if the fuel pick-up line that runs into that tank to its bottom may have a filter on the end of it? If it does it may be gradually clogging over time. All the engine diagnostics in the world won't find it because no one ever thinks about that hidden filter that some manufacturers put on that pick-up line and it should absolutely NEVER have been put there for this very reason. If you find one on it, pull it off or cut it off. There is no reason to have a filter on the end of a fuel pick-up line in the tank as most boats have in-line combination fuel filter/water separators and then that runs to the engine where there is another fuel filter/eater separator that then goes to the VST tank where there is that other nasty filter that then goes to the injectors where each has yet another filter. I had a friend who chased a ghost like this for months and it turned out to be the manufacturer put a damn filter on that pick-up line that got clogged over time that absolutely no one ever thought of until he found an old grey-hair somewhere who had seen this before. $1000's of dollars spent thanks to a $2 filter that never should have been put there. The symptoms you describe are a close match to his. One of the tell-tail signs of this while running underway is backing off the throttle that lets the fuel pump catch up and suck up enough fuel through that restriction that when full power is applied again you get it for a little bit until the restriction hits again and the fuel pump just can't suck enough. Also, after a full shut down sometimes the fuel sloshing around in the tank will break some of that crap lose on that filter and then after running a bit it finds it's way right back to that filter clogging it again and the situation repeats over and over and over.

Thanks Deepsea21, I think a lot of what you shared makes sense. I do have the feeling something is being overlooked and it may be simple in a sense. Right now, my rig is at the shop troubleshooting the squealing noise from the lower unit (which appears to be another 'ghost') then it is getting winterized as the lakes are locking up here. All along this process with the speed and RPM I have felt like something is suffocating either the fuel/air pressure systems and I absolutely know that the backing off the throttle for a slight increase in speed/rpm is a big tell when it comes to the problem. Thus far, the mechanics haven't either felt it was a factor or could not get it to reproduce the backing off for slight (100) rpm increase. Even then, it never has gone back to the 5500 I am used to - and it now more many uses and months later, the motor never gets near original rpm max at almost full throttle. Everything you posted makes sense - so I will check to see if there are any additional hidden filters they may be missing that are clogged or compromised. I do however believe that they did run my boat on the water with a portable shop fuel tank and line to see if that made a difference and it didn't. That makes me wonder if a clog either developed and traveled somewhere into the short fuel line within the motor or like you said it is another hidden filter. What appears different from the experience you shared is that I never can regain full rpm and it always goes down slightly at full throttle.
The deterioration of rpm has been a slow, and not up and down process and the rig seems more affected by boat weight than ever before. Stripping the weight down does make a difference but the boat still does not return to close to "normal" rpm.
In looking at some other similar posts I am wondering if there is a possibility that the squealing noise is the water pump impeller that under load and sustained runs shifts within the housing and rubs wrong - affecting the motor's performance and that the motor self regulates and does not allow to run at normal rpm because of this. I have never had as sensor go off however and it seems to be cooling fine. The current impeller is 2.5 years old with 100 hrs on it. The one before was installed wrong and crooked within the housing and did make a noise. There is also the chance the rpm loss and squealing noise are unrelated. I am working on uploading a video with the motor making the noise to this post and the other squealing one.
I really do appreciate all the input.

A bearing in lower unit locking up will cause RPM loss and a squeal