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  • new to forum, lots to learn

    I recently purchased a 2004 18.5 Center Console with a 2003 F115 Yammy. Lots of dirt bikes in my past have convinced me of the benefits of owning Japanese internal combustion engines and equipment. But this is my first 4 stroke outboard, and, even after owning and operating boats for 60 years i have a lot to learn.

    First, do 2 stroke 115s have carbs? Mine has fuel injection and is a four stroke.

    Second, on other forums I have read that older 115s like mine are pretty bullet proof but you do get the occasional fuel related problems. My Yammy has 408 hours and although I don't know it's history (it was a brokered boat) I don't think it saw a lot of use lately. When winterizing I found that the oil filter was last replaced in 2016 and the spin-on and internal fuel filters were replaced a year ago.

    Replacing the VST filter looked to be a bit of a project, so I didn't want to tackle it in the cooler weather here in Massachusetts. I'm thinking I'll do it in the spring.

    When we went for the sea trial and another "Damn, we just got a new boat and I don't care if it's 42 degrees out" ride, the boat performed well except for sustained open throttle operation. We ran max RPMs for maybe 3 or 4 minutes and it started breaking up. Lower RPMs and it was OK.

    I read that issues like this are usually caused by lack of fuel. I'm hoping that when I get a chance to run it more next spring it will sort itself out. But, if not, should I start by replacing the VST filter? Then clean the injectors? Yuck.

    Oh well, thoughts on this issue?



  • #2
    Yes, 2-stroke 115 Yamahas (USA, anyway) were all carbureted.
    AFAIK, the F115 was Yamaha's first EFI model in the USA
    and the design elements (and some parts) used on all subsequent 4-strokes.

    And yes, if they have operating problems,
    its often caused by bad or stagnated fuel.

    Originally posted by haviland View Post
    We ran max RPMs for maybe 3 or 4 minutes and it started breaking up. Lower RPMs and it was OK.

    I read that issues like this are usually caused by lack of fuel.
    One noticeable design weakness on the F115 (carried over to the original F200/F225)
    is a tiny screen inside the VST which is easily clogged

    It would certainly be sensible maintenance to open up the VST and check, clean, or replace that.

    However, the expected symptom of a clogged screen is inability to reach WOT -
    the pump can't get fuel into the rail fast enough.

    Reaching max rpm for 3-4 minutes, then dying away,
    sounds more like the other type of fuel starvation:
    inability of the low pressure ("lift") pump to keep up with demand-
    the VST is running empty.

    Theres a list of possible causes for that, such as:
    weak fuel pump (the mechanical fuel pump on the valve cover)
    clogged filters
    pinched or collapsed tubing
    clogged fuel tank pickup
    inadequate venting of fuel tank
    or an air leak somewhere on suction side.

    Generally, the primer bulb can be a helpful indicator / diagnostic.
    If the bulb is being sucked flat - the engine pump is working fine -
    the problem is with the tank, hoses, or boat mounted filter.

    If you run the boat again, try pumping the primer bulb while running at WOT
    (probably need another pair of hands!)

    if you can, or can't, keep the VST full with the primer bulb,
    and if that does, or doesn't keep the engine running strong,
    that tells you where to look for the problem.


    • #3
      You have given me lots of possibilities to think about. I will respond with more information in the morning.