I have a 25 hp 4-stroke outboard. To explain my problem the…

Hello, welcome, my name is ***** ***** do I have the pleasure of speaking with? I have been in the marine industry for 25+ years have been a certified marine technician since 2006 and I will be happy to help you.
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I am sorry to hear you are having these problems with your 25 horse 4 stroke outboard. I know that can be frustrating. Hopefully, we can get you back on the water quickly.

Your question was - Why does my outboard not make full power?

I would be happy to help you out with that.

The engines need 3 things to run properly. Compression, times spark ignition, and the proper ratio of fuel to air. If it is missing any one or any combination of the 3 the engine will either run poorly or not at all. If you brought this to me here is how I would go about troubleshooting it. We also do it in this specific order. If you jump around you are likely to miss things or spend money on parts you don't need. There is also no rule that says you have 1 problem and 1 problem only, and once you find that 1 problem everything will be fixed. In most cases there is usually 2 or 3 different things that are wrong, and you will notice small improvements when going through all the different steps.

1. As simple as this sounds you always start by running it off of a different fuel tank, like a small plastic portable tank. Half the time there isn't anything wrong the motor itself, but rather something wrong with the boats fuel system. This can be a pinched or collapsing fuel line, bad or old gas, or a bad anti siphon valve. Marine engines are very sensitive as to the quality of the fuel, and if any part of it is over 90 days, the fuel itself is always going to be suspect. This is a pass or fail test, and the engine is either going to run better or it is not. If it does run better, on this portable tank, first change the anti siphon valve and retest with the engine hooked back up to the boats fuel tank. If it goes back to running poor on the boats fuel tank, that tank will need to be pumped out. Bad anti siphon valves are a common problem, when they go back they create a blockage and restrict the amount of fuel that flows to the engine. If the engine does not get enough fuel it obviously will not make full power. Click this link, this explains more on the anti siphon valve and why they go bad. http://www.sterndrives.com/anti-syphon-valve.html. On this anti siphon valve there is an inlet side and an outlet side. The inlet side faces the fuel tank, the outlet side faces the fuel line. When the engine runs, the fuel pump creates suction. This suction pulls the check ball off of it's seat and fuel is allowed to flow around it. The problem with this is over time the spring in the valve will get weak or simply break. When that happens the check ball will be free to jump to the outlet side of the valve and create a blockage/restriction on the outlet side of the valve. If the spring is simply weak and not broken, this will more or less happen at random. Click this link for a picture, it will make more sense once you see it http://imgur.com/a/tpq4H. The anti siphon valve itself is located at the fuel tank, where the fuel line connects. If you pull the fuel line off the tank you will see a hose barb, that hose barb is the anti siphon valve. As part of this test replace all fuel filters. Also, as part of this step check the air filter/spark arrestor on top of the carb and make sure that is not overly dirty.

2. If no change, then compression testing the engine is next. Your results need be about 130 psi or better on that engine, all cylinders should be within 10-15% of eachother. If you get below 130, then the engine is worn out or has mechanical problems, and you might as well stop your troubleshooting here, as there is nothing you can do to an engine with low compression to make it run better. If you do get low compression, the next step is to check the valve adjustment.
3. Spark test. Before doing the actual test, install new spark plugs. Next, get a timing light and run the engine and hook the timing light up to each plug wire one at a time and see if the timing light flashes or not. Flash means the engine is sparking when running, no flash means no spark. Again this is a pass or fail test. If there is no spark on an cylinder, swap the coils around and see if the lack of spark follows the coil or stays on the same cylinder. If it follows the coil, then there is a bad coil, if it stays on the same cylinder, you likely have a bad CDI box. There is a catch on this test, on this test it needs to be done with the boat in the water and under a load. Often times the engine will show good spark on land, but the spark will actually break down and go away when the engine is under a load. So it always need to be done in the water.
4. Hook up a fuel pressure gauge between the pump and carb, measure fuel pressure with the boat going as fast as it can. Pressure needs to be at least 3 psi. If it's low, the fuel pump is weak. Make sure you do this after you do #1. If that anti siphon valve is bad that will cause low fuel pressure because the fuel pump would be struggling to pull through that restriction/blockage that a bad anti siphon valve creates. If fuel pump pressure is low often times what happens is the diaphragm will get a tear in it, and the pump will leak fuel directly into the engines crankcase, causing the engine to flood out at idle/low speeds.
5. Lastly, if you do all of the above steps and still do not get any kind of result, the problem is going to be in the carb. It is either dirty on the inside, or has a sticking float not allowing fuel into the engine. in any event if you get to this point the carb would need to come off and be professionally rebuilt.

If any part of my answer is confusing, incomplete, or you need more detail on the issue. Please reply and let me know. I am not done until you are happy. If I have saved you considerable money or a trip to the dealer. Please consider leaving me a bonus tip for my efforts. You will be brought to the tipping screen after you select a rating for my service.
Thanks for letting me assist you,
Jason.