2001 Optimax 225 Performance Issues

[lewis_is_bassin]

I recently purchased a boat equipped with a 2001 Mercury Optimax 225. The previous owner indicated that maintenance, including spark plug replacement, was performed in February 2024. During a lake test, the outboard operated smoothly.

However, on my initial outing, I experienced some stuttering after starting the outboard. Never cut out just a bit of rough idling. While heading to my first fishing spot, I noticed that the outboard would reach approximately 2500 RPM, maintain that speed for about 10-15 seconds, and then suddenly accelerate to 3500 RPM. These seems to happen every time run from spot to spot.

Upon arriving at my first fishing spot, I shifted the outboard into neutral to turn it off and noticed a slight high idle. Rather than just turning off, I shifted back to forward and then back to neutral, the high idle subsided.

For peace of mind, I conducted a compression test and found that all six cylinders registered between 100-110 psi. The outboard has approximately 450 hours of use. During the compression test, I also observed that the spark plugs were quite fouled see pic.

I assume the plugs are contributing to the issue(s). However, given the significant amount of oil present on the plugs, are there any other components I should examine?

[Princeton_Man]

My first thought is that it's a fuel issue. Contaminated or old fuel (especially if it's ethanol gas). I'd pull the fuel filter, dump it in a clear glass jar and let it set for a bit. Contaminates, water, etc will settle at different levels. Even if nothing shows, I'd siphon the tank/s, replace the fuel filter and plugs, and give it another test.

Keep in mind, contaminated fuel can also clog injectors and burn pistons.

[FD]

Optimaxs are notorious for ghost issues mostly related to the cranking battery.

Sounds like a fuel problem, but if not, put a new, high amp battery in it.

[J.W.]

are there any other components I should examine?

I'm not saying it's necessarily related to your issue at all, but just to give you a heads up, a known issue with those Optimax engines is the air compressor. Some motors never have an issue, but it's common enough you want to check it and then keep an eye one it. A friend of mine just had to replace his last summer. When they start to go bad there is usually some intermittent power loss.