New 2.5 hp outboard motor has reduced power at full throttle

Question

I have a new 2.5 hp 4-stroke Mercury outboard motor on a 12-foot aluminum boat:

I've used the motor four times; 15 km per trip.

When I first got the motor, it seemed to have a lot more power at full throttle. Now, there seems to be significantly less power.

I don't think the difference in power is all in my head: Previously, at full power/throttle, the motor only got about 1 km to a tank (the tank is small and built into the motor). Now, at full throttle, there is less power, and I can get 10+ km to a single tank on full throttle. So the fact that I now get 10x fuel range at full throttle is evidence that it's not operating at full power.

Notes:

1. The motor has oil in it.
2. I use supreme-quality gas.
3. The weight in the boat hasn't changed and the conditions haven't changed -- I only boat on calm water, on the same lake in the same place, with minimal gear.
4. The motor's shaft is tilted so that it's perpendicular to the water (straight up and down when in operation). I believe that's the correct angle.
5. When initially breaking in the new motor, I followed the manufacturer's instructions (reduced throttling for specific durations).
6. Here's a photo of the spark plug condition (click to zoom in):

As a novice, I'm wondering why I no longer have full power/speed at full throttle.

Answer 1

As this is a from new engine:

What you have most likely done is has "broken the engine in". This means that the cylinders etc are less tight than they were at manufacture. This means that it will be more efficient as the engine uses less energy to simply move the parts of the engine around. In fact, breaking an engine in is an important part of using a new engine to ensure that you get maximum life-span and efficiency from the engine.

Suzuki has a good guide on how to do this: Suzuki breaking an engine in. Other brands have similar instructions and this period is often part of the warranty. Basically it boils down to:

1. For the first 1-2 hours (running time, don't count periods when the engine is not running; this time is the most critical for the engine) don't run the engine at full throttle. Allow engine to warm up for ~5 min before using above idle, then run at lowest gear speed for 15 min. You may go above lowest gear for short periods to get on the plane (though you won't get on this with a 2.5 hp engine anyway).
2. For the first 2-3 hours avoid going above about 4000 rpm.
3. After 2-3 hours you can run at full throttle for a max of about 5 min at the time.
4. Continue this up to 10-20 hours run time.

Failure to properly break any engine in can result in early failure of the engine.