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27-08-2014, 11:09 AM
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#1
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Registered User
Join Date: Oct 2013
Posts: 45
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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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27-08-2014, 12:41 PM
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#2
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Registered User
Country: UK
Location: Surrey
Occupation: Motorsport Development inc Engine Managment
Boat make: 21ft
Engines: Various
Cruising area: Solent / Kent / Cornwall
Join Date: Oct 2011
Location: Surrey
Posts: 375
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Compression test and visual on plugs/pistons is needed.
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27-08-2014, 06:17 PM
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#3
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Registered User
Country: Guernsey
Occupation: Postie
Interests: Heavy Heavy Metal
Boat name: Slayer II, Slayer, Pen15
Boat make: Phantom 19G-R, Bernico F2 Extreme, Ring 15
Engines: Evinrude 115HO, Yam 90
Cruising area: Channel Islands
Join Date: Aug 2009
Posts: 1,460
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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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28-08-2014, 01:20 AM
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#4
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Registered User
Country: Ireland
Location: Dublin
Occupation: Boatbuilder
Boat make: Hydrostream V-king, 650SS OCR ,Ring 21, Ring 18, Phantom 18.
Engines: 300Hp Mercury 2.4, 130 Yamaha, Bridgeport EFI, XR6, Merc 200.
Cruising area: Malahide, Dublin
Join Date: May 2004
Location: Dublin
Posts: 1,803
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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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28-08-2014, 02:25 AM
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#5
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Registered User
Country: Ireland
Occupation: Burning Petrol
Interests: Boats & Bikes
Boat name: The Doc
Boat make: Ring 21C
Engines: Yam proV 200
Cruising area: River Shannon
Join Date: Jun 2014
Posts: 33
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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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29-08-2014, 12:22 PM
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#6
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Registered User
Join Date: Oct 2013
Posts: 45
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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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29-08-2014, 12:25 PM
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#7
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Registered User
Country: UK
Location: Weston Super Mare
Occupation: Electrical Engineer
Join Date: Mar 2004
Location: Weston Super Mare
Posts: 6,351
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Quote:
Originally Posted by eshroom
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?
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Anti foul will slow it down loads! I'd say that's your problem and the motor is fine
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29-08-2014, 12:40 PM
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#8
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Registered User
Country: UK
Location: london
Occupation: Powerboat Skipper
Interests: Boats , bikes!
Boat name: Renegade
Boat make: GPV-RENEGADE
Engines: 150 HO etec
Cruising area: Thames, south coast, anywhere!
Join Date: Dec 2008
Location: london
Posts: 2,330
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It could also be a bush spinning
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29-08-2014, 04:02 PM
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#9
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Registered User
Country: Guernsey
Occupation: Postie
Interests: Heavy Heavy Metal
Boat name: Slayer II, Slayer, Pen15
Boat make: Phantom 19G-R, Bernico F2 Extreme, Ring 15
Engines: Evinrude 115HO, Yam 90
Cruising area: Channel Islands
Join Date: Aug 2009
Posts: 1,460
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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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30-08-2014, 10:36 PM
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#10
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Registered User
Join Date: Oct 2013
Posts: 45
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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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01-09-2014, 07:08 PM
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#11
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Registered User
Country: UK
Location: london
Occupation: Powerboat Skipper
Interests: Boats , bikes!
Boat name: Renegade
Boat make: GPV-RENEGADE
Engines: 150 HO etec
Cruising area: Thames, south coast, anywhere!
Join Date: Dec 2008
Location: london
Posts: 2,330
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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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Im re branding pepper spray as Arsehole Repellant.im going to make a fortune.
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03-09-2014, 04:19 PM
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#12
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Registered User
Country: In the UK, in my office, working, waiting to go boating.
Occupation: Chartered Engineer
Interests: boats, water, snow, football
Boat name: “DoubleSix”; “Blue Lias”
Boat make: Ring 21c; Pacemaker 21
Engines: Opti 200; twin Volvo AQ110s
Cruising area: Lyme Bay
Join Date: Aug 2012
Posts: 138
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Quote:
Originally Posted by eshroom
....
I'm taking her out tomorrow for the winter.....
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That's criminal - its only the 3rd of September ;-)
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10-09-2014, 12:05 AM
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#13
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Registered User
Join Date: Oct 2013
Posts: 45
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Quote:
Originally Posted by DoubleSix
That's criminal - its only the 3rd of September ;-)
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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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