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

enter image description here

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.


  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): enter image description here

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

  • The spark plug is probably not the issue. If it is malfunctioning, you'll normally have big problems starting the engine or it stops randomly. All you can do there is check the distance between the contacts. The right distance must be somewhere in the manuals and is usually in the range of 0.5-0.8mm.
    – PMF
    Commented Sep 4, 2023 at 8:12

1 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.

  • Thanks. Does breaking in the engine usually result in less power/slower boat speed?
    – User1974
    Commented Sep 2, 2023 at 8:24
  • @User1974 No, however, without a measurement of the power output (you could roughly do this as distance over time (i.e. max speed) for a set distance) before and now, you can't say that you have lost power - perhaps all you have got is improved efficiency. If you didn't break in the engine correctly, you might have lost power.
    – bob1
    Commented Sep 2, 2023 at 9:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.