Problems with yamaha 115 4 stroke - 2CoolFishing
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Old 08-09-2010, 08:26 PM   #1
railbird
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Problems with yamaha 115 4 stroke

I have a buddy that is having problems with is 115 yamaha 4 stroke. He says when he doesn't run his boat for a week or more this problem shows up. He says if he fishes every day it never does this....
Here is what is happening. It gets on plane no problem and sounds just fine until he throttles thru about 3500rpm's, it begins to bog down if he tries to get to 3800, it will spit and sputter, so he will back it off and slowly throttle up 100 rpm's at a time it still spits and sputters, but it will slowly sound better and over about 20 minutes it will slowly get to near WOT. What do you guys think? Any of you every encountered this? Mechanics please weigh in. Thanks in advance.

chuck
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Old 08-10-2010, 09:03 AM   #2
Pocketfisherman
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Dirty jets in the carbs would give these symptoms. With the gas sold today with the ethanol content, I'm surprised this does not happen to everyone these days.
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Old 08-10-2010, 03:44 PM   #3
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I agree with the carbs/needles, but another possibility {esp. if it is EFI ** is 1) dirty fuel filter and/or 2) failing fuel pump. When he is running it more, it could keep the fuel system pressurized just enough so that enough fuel continues to get through.

I had similar problems recently with a Merc 115 4s (aren't they the same as Yammi?) - engine would run fine at times and at other times sputter and idle roughly; turned out to be a fuel pump dying a slow death - would run for a while and then peter out. It happened moreso on hot days, or later in the day when temps got hotter, with vapor lock compounding the problem.

As an aside, has he had any problems with the engine making oil? Seems to be a pretty common problem on the 115 4s Yamahas and Mercs. Just curious...
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Old 08-10-2010, 05:50 PM   #4
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I have a 2005 Mercury 115 HP 4-stroke EFI, which has same power pack as your Yammie. I had similar problem.

Here are steps I would do.

1) Take the water separator filter off fuel line and dump its content into a glass jar. Let it sit for couple of hours with a lid on the jar. If there is water you would see it in bottom layer along with other contaminants. You need to empty your fuel tank, put in new fuel mixed with Startron stabilizer. Stabil would be good also.

2) Check the second fuel filter on the fuel line under cowling. If dirty, replace it.

3) Take out all four spark plugs and check if there is black carbon deposit. If yes, note the plug number. Clean all plugs or replace them. You can buy them at Autozone for less than half of what boat store would charge you.

4) Hook up water hose to lower unit and start engine. Let it run for five minutes. Now, pull the plug wire off spark plug number one. If engine rpm drops, that is good sign. If rpm does not change, that cylinder has problem. Make a note of that cylinder. Repeat with the next three plugs.

5) Do compression check on motor to ensure all four cylinders/pistons are in good shape

6) Now, here is where the problem most likely locates.....the fuel injector and/or fuel pump. Connect pressure gauge to Shrader valve on top of fuel rail. Turn on engine (don't forget to connect and turn on water hose to lower unit) and watch reading. If it reads 40 psi +- 5, your high pressure fuel pump is good. That is good news. If not, you are looking at a new fuel pump...about $800 part.

7) Now, pull fuel injector number one off while engine is running. If no change in rpm is noted, that means that fuel injector is not working. Repeat same process with the other three injectors. Make note of which injector is not working.

8) Take all four injectors out and number them with a felt tip pen. Put them in a plastic bag and take it to a shop specialized in fuel injector cleaning using bench flow and ultrasonic cleaning. Cost to clean runs from $17 to $22 per injector. This is the only method to reliably clean a dirty clogged up injector. When the shop returns the injectors to you, they also include a computer readout of each injector BEFORE and AFTER cleaning. Compare that to your previous notes on which injector was bad in step 7.

9) When you get the injectors back from shop, replace old O-rings with new ones and install them into fuel rail.

You would be surprised how good your boat run after injectors are ultrasonically cleaned. My boat was not able to make any higher than 1000 rpm after about ten to twenty minutes running. It was running on only one cylinder due to dirty injectors. After the cleaning described above, it now runs 5,000 rpm without hickup.
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Old 08-10-2010, 08:09 PM   #5
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Pretty luck guy to need to run his boat every day to keep it from breaking, at least that is what I would tell me significant other and employer!
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Old 08-10-2010, 09:12 PM   #6
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Thanks guys I will forward to my buddy.
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