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Old 27-08-2014, 11:09 AM   #1
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2-stroke loss of power

Hi all

I have a 2-stroke '98 mercury 125hp 2+2 motor in great condition and serviced at the beginning of the season. It sits on a P18. I have probably put 10 hours on it since then, but did run out of fuel a couple of times.

The first time I took the boat out top speed was 50 nautical miles per hour. I left it for a month and since then it tops out at 39 nautical MPH.

There is no stuttering or sudden loss of power to indicate a blocked filter or carb, it idles fine, delivery is smooth, it just doesn't go as fast.

Any ideas?

On a side note, when people on this forum talk about the speed of their boat, are they talking land MPH or nautical?
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Old 27-08-2014, 12:41 PM   #2
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Compression test and visual on plugs/pistons is needed.
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Old 27-08-2014, 06:17 PM   #3
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Is the boat kept in the water or on a trailer? If it's in the water then I'd say it's due to a dirty weedy running surface, not the motor.

Nautical miles per hour is knots, most speeds you'll see are normal miles per hour because the number is higher
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Old 28-08-2014, 01:20 AM   #4
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Check spark on all 4 and pull plug wires to each cylinder under load to isolate which cyl is misfiring.
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Old 28-08-2014, 02:25 AM   #5
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I'ld clean the carbs too, are you still making the same rpm's? The 2+2 runs a vacuum to draw air/fuel to the last 2 cyl above 2,5 - 2,800rpm
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Old 29-08-2014, 12:22 PM   #6
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Thanks for all the replies!

It is kept in the water. I feel a bit bad as the hull was perfect before I dropped it in, had never been left in the sea. I've had it in a small, stale harbour since April. It got filthy within a month so I took it out and had anti-foul paint put on. But surely anti foul won't slow it down?

My RPM gauge is broken, but the engine doesn't sound any different...

I'm taking her out tomorrow for the winter, so will get compressions checked and new plugs and service before dropping her in next summer. Hopefully local mercury dealer will get to the bottom of it.

In the meantime will save for a new mercury 4-stroke 150 - or should that be an Opti XS?
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Old 29-08-2014, 12:25 PM   #7
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Quote:
Originally Posted by eshroom View Post
Thanks for all the replies!

I've had it in a small, stale harbour since April. It got filthy within a month so I took it out and had anti-foul paint put on. But surely anti foul won't slow it down?
Anti foul will slow it down loads! I'd say that's your problem and the motor is fine
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Old 29-08-2014, 12:40 PM   #8
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It could also be a bush spinning
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Old 29-08-2014, 04:02 PM   #9
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Yep I'd say the anti-foul is your problem, you'll never see an anti-fouled race boat

Even anti-fouled boats can develop a layer of slimy weed in the summer months which will lose you significant speed
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Old 30-08-2014, 10:36 PM   #10
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I'm sure you lot know better than me, but 20% drop in power from antifoul paint?

Took the boat out today and it looks pretty clean underneath.

Any alternatives to anti foul if I want to leave it in the water for 5 months a year?
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Old 01-09-2014, 07:08 PM   #11
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Get a rev counter sortd mate that way you get more idea the motor is on song.
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Old 03-09-2014, 04:19 PM   #12
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....

I'm taking her out tomorrow for the winter.....
That's criminal - its only the 3rd of September ;-)
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Old 10-09-2014, 12:05 AM   #13
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That's criminal - its only the 3rd of September ;-)
She lives in Greece, been in the water since March, but I won't be going back now until Christmas.

Mediterranean waters are perfect for these boats, the lack of shade got some people though.
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