I had a 15 hp do the same thing and it was a cracked fuel line. Not the main line from the tank to the motor. One that run from the fuel tank fitting to the carb.

I have a little time to tinker now so will describe more fully the way it acts in detail to help pin down the problem. This is more detailed than my original description. I have appreciated all of the great suggestions.

It starts first pull every time and idles just fine. When I take off, it will run full speed perfectly for about 100 yards and then will slow down to about half throttle and run unevenly when holding it wide open. When I putt the choke lever part way out, it will run again full speed as long as I hold it there. When I throttle down it will die unless I slowly manipulate the choke to keep it running. Once slowed down to trolling speed, it is fine at that speed without the choke being out at all.
If it is a high speed jet being clogged, why will it run like a scalded cat for the first 100 yards or so?

Does this behavior help diagnose the exact problem?

X3 or a gunked up fuel inlet screen or a pinhole in a fuel pump diaphragm

It has a new fuel pump and I just checked the screen on it . I had pulled the pick up screen on the tank a while ago and checked it but will pull it and check it again.

Did it run right after you replaced the fuel pump ?

I have to ask then why will it run perfectly at high speed for about a hundred yards or so before it acts up if the high speed jet is blocked?

Yes, ran perfectly and fast as the devil. I used it all last fall and this spring and it ran perfectly then. It was only after I started using it after it sat from May untill September that I had the problem.

How about a cracked or pinhole in a vacuum line ? I am guessing that pump is activated by vacuum.

I know you said this tank is auto venting but have you tried to loosen the gas cap to ensure venting? I have one of these "auto venting" tanks and it does not vent. I just crack the gas cap while in use and do not rely on the venting cap.

Water pump……? Runs till overheat and goes to limp mode? Easy to check. Also could be rubber from previous water pump failure slowing water flow in cooling circuits.
Not sure when ECMs started monitoring temp and regarding spark. But it is a real issue with newer engines.

I will try that when I put it back on the boat. I don't remember if I have tried that yet or not.

Nope, It cools well and is an older motor without ECMs

I would think in that case, squeezing the bulb would correct the problem while running as it did when the old pump went bad. Squeezing the bulb while the issues are occurring now does absolutely nothing.
I just pulled the fuel pickup out of the tank now and checked the filter on the end of it and it had a black film on the end of it and one tiny fleck of red paint on the side. I sprayed it clean with carb cleaner and it looks good now. Maybe that will help when I put it back on the boat after trying a couple other things. I also drained the tank again and put new gas in with no additives and the exact amount of 2 cycle oil. That did not change anything but I have not tried it since I cleaned the filter in the etank yet.

Mine did about the same thing. It was a crack in a piece of fuel line that ran from the metal hook up the tank line hooks too. It seems like it was about four to five inches long and had about three turns to get around stuff. Mine would never run full throttle, but ran a lot better with the choke all the way out. Mine was sucking air, not leaking gas.

I would clean the carburetor and blow out all the orifices. At least then you would know for sure that you don't have crap in the bowl restricting the main jet. It is a process of elimination. I can speculate on your question, but it too, would be an educated guess, and not action. Good Luck!

I can't remember if these had reed valves, and that is a complicated problem to figure out how they exactly operate at varying rpms.

Sounds like it is running out of fuel. Leak in line from tank to motor. I had a cracked connector on a 40 hp Johnson a couple 10 years ago that did similar. If it’s running good for that long I don’t see it as a clogged carb but could be wrong.

Well, I pulled the fuel line off the carb, Turned the motor upside down and shook it and rolled it back and forth after running it without the tank hooked up until it ran completely out of gas. I then blew in the carb with compressor and anywhere I could including the connector into the motor. I also hooked up a line and used suction on those places and also blew air through the throat to really dry out the carb. I also put another cap on the tank. I don't plan to hook it up and run it for a day or so. I will let you know my findings then.

On a side note; I just looked down the shoreline to see about where it quits running full speed and it is much more than the 100 yards....more like 200+. It has been consistent that way so I am thinking that black film on the fuel pick up filter in the tank could be responsible because after sitting, there is time for the fuel line and carb to fill up and the motor can run fast until it has trouble sucking more from the tank as fast as it needs it. Pretty sure I am wrong again but after seeing videos of removing and disassembling the carb, I am positive I would wreck something or mess it up in the process. I am trying to avoid that until I have exhausted other options..

AS I mentioned before I put a brand new hose and bulb on it the other day. I also have looked at all other lines inside the motor and can not spot any leaks or cracks but it is hard to see because of the layout of the lines.

Could it be the exhaust port being clogged? It will go for some time until the exhaust starts hampering your fuel burning.