2005 f40 wont rev to peak rpm under load

[Bhom1234]

I have a yamaha f40 2005 that started doing some weird stuff today. As I started to head back to the launch the motor would not rev to peak rpms under load. After shutting off and restarting several times it would rev to the same rpm each time while under load. I don't have a tachometer to see exactly the rpm value but my speed was the exact same every time. It wasn't bogging down or showing signs indicative of stalling out from lack of fuel, it would cruise very steadily at about 2/3 of peak rpms. Has anyone experienced something similar to this? I'm new to this motor and am not sure where to start other than letting it cooldown completely and seeing if the problem persists. Thanks in advance, Brian

[Bhom1234]

So called the local dealer who told me to try changing the spark plugs in case one was fouled from idling for a while. After changing the plugs and double checking the fuel lines for leaks I get the same thing as before. It has the tell tale signs of a down cylinder I just don't know how to verify this. Does anyone know how I can check this?

[DennisG01]

How did the plugs look? That can tell you a lot.

You could do a "compression test" - it's easy enough to do and the tester isn't all that expensive to buy. You may have to take it further and do a "leak down test". Both of those things... you can find lot's of info on the net about.

[99yam40]

those motors are running lean to start out with, any plugging and cylinders do not fire /burn properly

[Bhom1234]

those motors are running lean to start out with, any plugging and cylinders do not fire /burn properly

Are you referring to a plugged up carb? Would it be running as smooth as it is with a plugged carb?

[Bhom1234]

How did the plugs look? That can tell you a lot.

You could do a "compression test" - it's easy enough to do and the tester isn't all that expensive to buy. You may have to take it further and do a "leak down test". Both of those things... you can find lot's of info on the net about.

The plugs that came out were dirty but not completely black. You could still see the porcelain but it was obvious that they needed to be replaced. One other observation was they all looked the same.

[yamhoss86]

Sensors might not be working?