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I have a 15hp mercury 2cy that will go to high speed

I have a 15hp...

I have a 15hp mercury 2cy that will go to high speed intermittently. It runs great otherwise. Low speed is smooth and so is high when it decides to go there

Mechanic's Assistant: What is the model/year of your Mercury?

I believe it's 2003

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

Yes. Haven't tried anything yet. Don't know where to start

Mechanic's Assistant: Have you checked the battery, particularly the terminal connections? Are they corroded?

They are fine

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Answered in 34 minutes by:
1/18/2018
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,085
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
Verified

Hello my name is ***** ***** am I speaking with? I read what you wrote, is there any kind of story or history to this problem, have you ever personally seen this engine run correctly? What kind of fuel tank are you running the engine off of, is it a plastic portable type or is it built into the boat? How old is the fuel itself?

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Customer reply replied 4 months ago
The motor ran fine last winter spring. The fuel is fresh. It has a plastic portable tank. It will go to high speed some times and when it does it runs great

G​ot it. I guess the next question is this. I do not know what you have for tools or knowledge. Did you have any questions that you specifically wanted me to answer? Or would you prefer if I explained what we would do in the shop if you were to bring it in?

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Customer reply replied 4 months ago
Explain what you would do. I have tools but need pointed in the right direction. Is it fuel, linkage, adjustments to idle ?

T​he first thing that needs to be figured out is if it is a fuel issue or a spark issue, and the engine is lacking spark on 1 cylinder. The first step is to get a timing light, run the engine, hook up the timing light to each plug wire and see if that timing light is flashing or not. If it is flashing, that means that the cylinder is sparking. If it is not flashing, then there is no spark on that cylinder. If there is no spark, start with new plugs first. If still no spark, the ignition system will need further troubleshooting with a specialized multimeter.

Now if it has spark on both then it is going to be a fuel issue. The fuel is either no good, isn't making it from the tank to the carb, or isn't making it from the carb and into the engine. The next step would be trying brand new fuel and seeing what happens. If no change, open the fill cap on the tank to make sure it's venting properly. If it runs better with the fill cap open, then the fuel tanks vent system is not working properly or is clogged. If there is no difference, pump the primer bulb over and over to force fuel to the engine. If the engine runs better, the fuel pump is weak and on it's way out. If the primer bulb is sucking flat and will not pump/reinflate on it's own, then you have either a bad bulb, or your fuel tank has an anti siphon valve on it that is sticking. Click this link, this explains more on the anti siphon valve and why they go bad. http://www.sterndrives.com/anti-syphon-valve.html. On this anti siphon valve there is an inlet side and an outlet side. The inlet side faces the fuel tank, the outlet side faces the fuel line. When the engine runs, the fuel pump creates suction. This suction pulls the check ball off of it's seat and fuel is allowed to flow around it. The problem with this is over time the spring in the valve will get weak or simply break. When that happens the check ball will be free to jump to the outlet side of the valve and create a blockage/restriction on the outlet side of the valve. If the spring is simply weak and not broken, this will more or less happen at random. Click this link for a picture, it will make more sense once you see it http://imgur.com/a/tpq4H The anti siphon valve itself, if you have one, is located at the fuel tank, where the fuel line connects. If you pull the fuel line off the tank you will see a hose barb, that hose barb is the anti siphon valve. The reason I asked what kind of tank you were using is because those anti siphon valves are legally required on all permanantely mounted tanks. So if you were using a boats fuel tank, by law, it should have one and would have one unless it was removed by somebody. On plastic tanks, they are not legally required, but they are a good idea to have, so some tanks will have them while others will not. You have to look and see. If you go through the above steps, and nothing stands out and the engine is still running poorly, the problem is going to be inside of the carb. It either has a sticking float and is not allowing fuel in to it, or there is some junk floating around inside of it clogging up the fuel jets. In either event, if you get this far, the carb would need to come off and be properly rebuilt. Does that all make sense. I think that should do it. But I work for tips so I do want to make sure you are happy with my service before you go. If you had a further or more specific question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you
Jason

Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,085
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
Verified
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Satisfied Customers: 17,085
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Experience: Degree in Marine Technology. Gas and diesel marine mechanic.

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