DF-140A not reaching full throttle

[ShellKnobCat]

2019 DF-140A with 520 hours (it was a rental) on a 23' Landau tritoon. With a 19" prop it has always reached 6100 RPM and 35 GPS MPH with the wife and I aboard. Last spring I discovered a birds nest in the motor (the boats sits on a lift at a marina) but there had been no issues with the engine. But around August full throttle RPM and speed started to drop to about 5500 and 30 MPH. The engine runs great and at less than full throttle but RPMs and speed have not changed. This spring I put new plugs in it and the first couple of times we took it out it ran right up to 6100 and 35 mph. Then I discovered ANOTHER bird nest and cleaned it out as well as I could including vacuuming everywhere I could reach. Now it's back to about 5400 - 5500 at full throttles and 30 MPH. It feels as if the throttle cable has slipped or stretched because it runs so well up to full throttle - and it runs great at full throttle but just doesn't reach the RPMs it did before Doesn't miss, doesn't use oil and there have been no alarms. My mechanic looked at it and tells me all seems fine. He is going to look at it again and take off all the air intake to see if it's plugged up with bird nest crap. That seems to be the obvious problem but I'm not sure. So I think it could be:

1. Bird nest material clogging up the air intake or maybe the throttle linkage though what I can see of the linkage looks fine.
2. Stretched or slipped throttle cable.
3. Something else hopefully not exensive

[briscoe]

With cowling off, does the cable move the gas all the way? Disconnect the linkage at motor and see how far it moves, then move it at the helm (have wife move it for you, while watching). Not running the engine, of course. Just a guess on my part. The other thing could have tondobwith something blocking the fuel line or flow? I had some stuff in the gas tank, plug a couple of my injectors and it would only get on plane ~4k rpms.

[ShellKnobCat]

Turned out the problem was number 1 - bird nest material clogging the air intake. Mechanic took the airbox off and found it full of stuff and the spark arrestor was clogged too. Once cleaned it runs as it should right up to 6100 RPM. Lesson learned - buy and use a motor cover!