2009 40 hp Mercury 4-stroke low RPMs

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Gotem
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2009 40 hp Mercury 4-stroke low RPMs

Post by Gotem » 03 Jul 2016, 08:37

My cousin just bought a 2009ish Tracker 175 TX aluminum bass boat which has a 40 hp Mercury 4-stroke. The boat had sat outside for years unused. He got the boat from the original owner who he knew. All the gas was changed. The boat starts good and runs good except he can not seem to get over 4500 RPMs with two people in the boat and a speed of 18 mph. With one person the speed increases to 20 mph. A new 175 TXW with a 40 hp should go about 27-29 mph according to the Tracker website and I am assuming the 40 hp should have an operational range between 5000-6000 RPM's. So what could be slowing down the RPM's? Spark plugs? Tilt and Trim (although I told him to make sure he trims out the motor)?

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Ebug
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Re: 2009 40 hp Mercury 4-stroke low RPMs

Post by Ebug » 03 Jul 2016, 09:25

Start by asking the original owner if that is how it always performed. If the answer is no start troubleshooting the fuel system. If the answer was yes look into changing prop pitch.

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gunz
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2009 40 hp Mercury 4-stroke low RPMs

Post by gunz » 03 Jul 2016, 11:01

pics of the boat?

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momule
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2009 40 hp Mercury 4-stroke low RPMs

Post by momule » 04 Jul 2016, 01:10

With the motor sitting unused for some years it is likely to never run properly until all of the rubber parts in the engine are replaced. Fuel lines and vacuum lines are subject to rotting just due to temp changes. Even if the motor was prepared for storage this will probably have to be done. Included parts to be concerned with are all fuel hoses, gaskets, a carburator cleaning and rebuild, fuel filter, oil, oil filter, etc. I would look in the gas tank to make sure it's clean and replace the spark plugs. It's also a good idea (although too late in your case) to not start the engine until you make sure that the moving bits in the engine aren't froze up. The reason for replacing these rubber parts is that they deteriorate with time and no use, especially if they have had gas evaporate from them in storage. Even if the motor starts the stiff rubber will quickly start breaking down causing problems that you don't currently have.

Since you didn't know to do these things it may be a good idea to take it to a certified mechanic to have these things done. You'll spend several hundred dollars but you'll be able to use the motor without a string of mechanical problems happening during the rest of the season. You also won't suffer the joys of having a fishing trip ruined because the motor refuses to run right when you're a half hour out on a lake somewhere.
Current Craft: Lowe 1448 Big Jon, all aluminum transom, decks, console w/ Yamaha 40/30.
Previous: Cajun 185 w/150 Johnson, Tracker TX17 w/60 Mariner, 24' Pontoon w/60 Merc, Ozark Tri Hull F/S w/120 Evinrude, 16' Lowe Olympic Jon w/ 40 Mariner, 12' Lowe w/ 4.5 Merc, Coleman Scanoe, Ouachita alum canoe

turbotodd
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2009 40 hp Mercury 4-stroke low RPMs

Post by turbotodd » 04 Jul 2016, 16:51

Is that an EFI motor or carb?

If EFI, the fuel pump filter is probably plugged/restricted. Also, the injectors themselves can get plugged by dirty/staled fuel. The downside to EFI is that old fuel still does the same thing as a carb'd motor, however, things get real expensive once it happens, where cleaning 3 carburetors is a little cheaper. Always a trade off......

nowgrn4
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2009 40 hp Mercury 4-stroke low RPMs

Post by nowgrn4 » 05 Jul 2016, 05:24

If it's EFI I would run a can of Sea Foam to 10 Gal of gas through it first. Sitting for years? Do a water pump service and try some new plugs too.
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dootech
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2009 40 hp Mercury 4-stroke low RPMs

Post by dootech » 10 Jul 2016, 22:04

turbotodd wrote:Is that an EFI motor or carb?

If EFI, the fuel pump filter is probably plugged/restricted. Also, the injectors themselves can get plugged by dirty/staled fuel. The downside to EFI is that old fuel still does the same thing as a carb'd motor, however, things get real expensive once it happens, where cleaning 3 carburetors is a little cheaper. Always a trade off......
Turbo is headed in the right direction, if efi odds are the motor is fine. If carbed you need to rule out a carburetor issue. LOOK at the prop and lower unit, show us a picture. What prop is on it? What condition is it in?

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momule
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2009 40 hp Mercury 4-stroke low RPMs

Post by momule » 10 Jul 2016, 23:55

You guys are probably right in suggesting SeaFoam as a first option. Seems like I never get lucky enough to have a can of additive take care of mechanical problems though. I use Stabil in my fuel for ethanol and SeaFoam as a preventative just in case. I also run an external fuel/water filter just in case. The last time I put a used motor into service that had been sitting idle for several years it required a carb rebuild, impeller, new hoses, etc etc etc... But there's always a chance that the cheap option might work. And low RPM's beats the hell out of lots of other problems that an outboard can put ya through.
Current Craft: Lowe 1448 Big Jon, all aluminum transom, decks, console w/ Yamaha 40/30.
Previous: Cajun 185 w/150 Johnson, Tracker TX17 w/60 Mariner, 24' Pontoon w/60 Merc, Ozark Tri Hull F/S w/120 Evinrude, 16' Lowe Olympic Jon w/ 40 Mariner, 12' Lowe w/ 4.5 Merc, Coleman Scanoe, Ouachita alum canoe

turbotodd
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2009 40 hp Mercury 4-stroke low RPMs

Post by turbotodd » 12 Jul 2016, 21:58

If for some reason the motor is starving for fuel, and continued to be run under load, it's running lean...and can kill itself. Have seen this on an F115. Sat for a couple years. Fuel pump was weak. Owner didn't want to fork out the money to replace it. Month or so later, it was back with zero compression on #2 and #3 cylinder.

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