1980 Mercury 25HP Losing Power

Ok, this motor is running fine and has not let me down this season. I have noticed though, that when running at wot
I lose a little rpm after motor is fully warmed up. After starting motor I idle for a bit, then cruise slowly for a bit, then wot.
I will hit max speed for a few minutes then slowly lose rpms and lose 1.5 mph. Then stays steady for the remainder of the journey.
Get to my fishing spot fish for an hour or so and the same thing happens after start up. Any ideas?

You have to start testing things like the coil and performance of the fuel pump.

Thanks ppine, I have new coils installed and rebuilt fuel pump at the beginning of the season. This did not seem to happen when
water temperatures were low in the spring, now temps are in the low 80's.

Pappy, I was hoping that you would chime in. Pumping bulb does not make a difference. This year motor had 4 different sizes of high
speed jets available at that time. I live in Coloradp at 6500 ft and sometimes operate the motor in high mountain lakes up to 9400ft in elevation. I experience a definate loss power at these high elevations due to lack of oxygen. I have tried lower pitch props with a little sucsess. New jets for this motor are not redily available. I did however locate a smaller high speed jet at a shop in Delaware and purchased it about a month ago. I have not installed it yet. I will try that, although I will not be comparing apples to apples. Jet sizes that were available are .070, .067, .065, .063.
I assume that the .070 would be the standard jet for use at sea level. The new one I have is .063. My concern is if I install the smaller jet
I will run too lean at lakes at lower elevations. I run this motor between 4500ft and 9400ft as mentioned before. Would adding additional oil to my fuel counter balance this lean condition?

Was this a test??
lol......you have a problem! An enviable one though. Would love to fish in some of these areas you are talking about.
For your normal high elevation I would run the 65 and check it at WOT. How to check? You can quickly blip the choke on/off and monitor RPM. If it drops with the choke you are okay. If the RPM goes up then you need to add jet. Try at a high mid-range RPM at first.
You can go up or down from there. Sounds like a lot of work to change for all the altitudes you fish at so maybe pick a happy medium and remember to not run hard and run extra oil at the lower altitudes or....change jets for your locale every time.
Definitely run additional oil when jetting an engine leaner until you are sure of your jetting.

You telling me that you run the engine at altitude let me know that we are on the right track with jets at least.
If you ask around your area and find an old timer with local knowledge he may be able to pin-point your jet selection a bit better than I although I know I can get you in the ballpark from here.

Thanks Pappy, I will try this next time I go out.

Well folks, I installed the NEW smaller jet .063, and took her out yesterday. I had the same problem loss of rpm and speed after a minute or so of running at wot. Smaller jet did not seem to change anything compared to the old one. Oh well. I think I will reinstall the larger jet once again so as not to worry about running too lean. Thank you all for your responses. Carphunter