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  1. #1

    Default Fuel line air leak ANALYSIS.

    I think I will start my first thread. I posted this on another thread, got no opinions.

    OK, question here.
    Consider an air leak in the fuel line on the input side of the pump, or as d.boat says, in the incoming oil line (or cap). The fuel pump will pull air into the fuel, and pump it through the system, up to the carbs, through the needle and seat, and into the carb bowls. Right?
    The carb bowls are vented to the atmosphere. Evidence of that is the flooding over of gas if the needle sticks open.
    So, why doesn't the air pulled in by a leak simply rise to the top of the gas in the bowl and rejoin the atmosphere? How can air possibly be sucked through the high speed jet (bottom of bowl)?

    Now consider a leak on the output side of the fuel pump. I say there is NO WAY for air to enter the system there, because the fuel in the lines between the pump and carbs is under 3-7 PSI pressure. The only thing that would happen in this example is a fuel leak out of the system.


  2. #2
    Join Date
    Feb 2001
    Central West Florida

    Default Re: Fuel line air leak ANALYSIS.

    An air/fuel leak between the fuel pump and the fuel supply results in a constantly changing amount of fuel being supplied to the carburetors. This is turn results in a constant, never ending slow surge of rpms up and down. This scenario can only take place if some amount of fuel mixture is present within the float chambers which obviously has the high speed jets submerged, which in turn eliminates any possibility of having air anywhere near the HS jet.

    A carburetor which has air at the high speed jet area results in a impossibility of having any air being drawn thru it as it would be impossible to have the engine run in that situation. Air being forced into the carburetor with a fuel mixture has the air vented from the carburetor immediately, leading to the surge mentioned above.

    A air/fuel leak between the fuel pump and carburetor simply results in having the fuel mixture being sprayed around in the powerhead/inner hood area along with any air that might exist..... the air never reaches the carburetor. Eventually a spark takes place..... BOOM! Owner places want ad for new hood.

    Now, what engine do you have (year, make, hp) and what is the problem that you're having?

  3. #3

    Default Re: Fuel line air leak ANALYSIS.

    No problem, no specific engine. The discussion is intended to analyze the effects of air leaks in both the incoming fuel side circuit, and the pressurized output of the pump to carb circuit.
    I agree with your conclusions about the pump output side. No air could enter there. Fuel leaking out is the result.

    But, I am not so sure on your assessment of the fuel pump input side scenario. It has been suggested MANY times on this board that a good way to check for air leaks on the input side it to put a temporary clear fuel line in the fuel supply and watch for air bubbles.

    I say that bubble will make it's way to the carb bowl(s), and just go to the atmosphere due to the bowl venting. It couldn't go to the bottom of the bowl to be sucked up by the main jet; the idle circuits get their gasoline supply from the small brass pullover tube which sticks way down in the bottom of the bowl, so they should be "extra air free" too....

    I am having a hard time understanding how extra air actually gets thru to cause idle surging. I have an idea, but it is kind of off-the-wall, and I wanted input from the experts before I let fly with it.....

  4. #4
    Join Date
    Feb 2001
    Central West Florida

    Default Re: Fuel line air leak ANALYSIS.

    Oh, I just assumed that you had a problem with either a Evinrude or Johnson outboard as that is what this forum is all about and your questions were leading into it. There is another forum on this site entitled "Misc Marine Engine Topics" where you might encounter a better stream of traffic.

  5. #5

    Default Re: Fuel line air leak ANALYSIS.

    OH, this is definitely a Johnson/Evinrude question. To further clarify, this question does not pertain to fuel injected motors. The J/E experts here should be able to absolutely describe why an air leak will cause surging on J/E engines. I don't really care about Merc, Force, Zukes, Yamis, so that is why posting here. I wanted a J/E answer.

  6. #6
    Join Date
    May 2008
    , Wisconsin, USA

    Default Re: Fuel line air leak ANALYSIS.

    There are two major issues with an air leak in the fuel line.

    The first applies to both VRO and premix engines. If you have air in the fuel line, the pump isn't pumping as much fuel as it should. This could result in fuel starvation at high throttle settings. The symptom could be surging. If the leak is bad enough at idle, the fuel in the line could bleed back into the tank causing the pump to lose it's prime. These issues are mostly related to larger leaks. Tiny leaks will be undetectable to the premix pump.

    The second issue relates only to VRO motors. The VRO is a positive displacement pump. The fuel pump diaphram chamber is about 50 times the size of the oil pump chamber. The fuel/oil mix is dependant on the pump sucking up 50 parts fuel and 1 part oil per cycle. If there is an air leak in the fuel line, the fuel pump chamber might only fill partway with fuel. The remainder with air. If 25% of the volume is filled with air, the fuel/oil mix would be around 37:1. You get more smoke and a poor idle from the excess oil. This is the most common reason for the suggestion to check for air leaks in the fuel system.

  7. #7

    Default Re: Fuel line air leak ANALYSIS.

    Thanks for all responses.
    I guess I will throw my theory out there and see if it flies.

    Consider air leaking into the system on the suction side of the fuel pump. On that side of the pump, I can easily understand seeing bubbles in the fuel line if replaced with a temporary clear line. When those air bubbles enter the fuel pump, I contend that due to the pump action, the small passageways, the 90* turns, the splitting of the fuel into 4, 6 smaller lines on it's way to the carbs, the very small hole that it travels thru at the needle/seat, all those reasons and probably a few more will cause the fuel to become "aerated" with the bubbles that were once present. After the fuel/bubbles go thru the system, they are essentially mixed up so finely that the fuel is "aerated" and is therefore a leaner mix after metering through the jets, and entering the cylinders.
    Notice also that "aerated" fuel can and will be able to enter the main jet, while a bubble will not. A bubble will float to the top of the fuel bowl and never make it to the main jet.

    Consider putting gas in a jar, capping it, and shaking vigorously. I will bet air gets dissolved into the gasoline and results in "aerated" gasoline.

    We also know that when an engine runs out of gas, it picks up speed as it leans out, and then finally quits.

    So, since there is no specific control of how much air gets "aerated" into the incoming gas due to the leak, the engine surges first lean, and then maybe back down to normal mixture, then leaner....etc. Causes an uneven surging.
    Compound the normal mix/lean mix variation among 4 or 6 cylinders, and rpms will be varying all over the place.

    Is this even worth discussing, or are your eyes glazing over reading this?

  8. #8

    Default Re: Fuel line air leak ANALYSIS.

    I have a 1991 Baja model 270 with 454 engine, manual fuel pump with four barrel carburetor. The boat is getting air pockets in the fuel line before the carburetor. I put a clear gas line to the fuel pump to the carburetor after the boat is running about 20 minutes to haft hour you will see air pocket coming through the gas line shutting the boat down. The boat will start back up but might shut off again within 50 feet. If you let the boat set for 5 to 10 minutes the boat will run again for another 20 minutes to half hour.

  9. #9
    Join Date
    Feb 2001
    Central West Florida

    Default Re: Fuel line air leak ANALYSIS.

    This thread is going on 15 years old. Also, this is the Evinrude/Johnson outboard forum. As it stands, your post is buried under someone elses post (Hijacked).

    It would be best if you abandon this thread and forum, then go to the proper forum (Inboard/Outboard - Inboard) whatever the case may be to start a new posting under your own title so as to have it opening at the very top of the listings.
    We occasionally have questions. If you fail to answer, it may affect ours.

  10. #10
    Join Date
    Apr 2010

    Default Re: Fuel line air leak ANALYSIS.

    My eyes glazed over.

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