91' Johnson 15hp

Discussion in 'Bubba's Outboards' started by ArkansasCat, Oct 30, 2007.

  1. ArkansasCat

    ArkansasCat Member

    Messages:
    143
    State:
    Batesville Arkansas
    I just purchased a used 91 model Johnson 15hp motor. I didn't get an owners manual with it so I am needing some help.

    What is the plug gap?

    What is the best plugs to run in this motor?

    How long does a fuel pump last before the diaframe goes bad and will I get a warning before it goes out completly? (I sure don't want to be stranded at dark on a cold night :smile2:

    I'm also plannig on changing the lower unit oil. Anyone that has knowledge on these small horse power Johnson/Evinrude motors know of anything else I should look at before running it this winter? Thanks
     
  2. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    For that engine use a champion plug QL77JC4 it says to gap them at 30 but I would gap at 40 if I did a lot of slow trolling or no wake running. This helps keep down fouling at slow speed.
    As for the fuel pump I have seen them 35 years old and still working and I have encountered them a month old and shot. No warning come for the failure either. On the VRO systems it does but on the fuel pump alone it doesn't.
    On the lower unit Just take the lower drain and fill screw ou and when empty refill with 85-90 marine lower unit oil. If the motor is new to you then by all means change out the whole water pump so you will know that it is in good working order.
    When you change the lower oil get the two gasket that are located behind the screws and replace them. These can be bought at about any marine supply store or repair shop.
    Check for matal chips when draining the lower unit. The black graish powder form is common chips aren't.
    Do a compression test and this will tell you how good of health the engine is in.
    Do a good decarb on it. You can look in the library on here and it has some good info on pump changes and carb rebuilds with pictures.
     

  3. ArkansasCat

    ArkansasCat Member

    Messages:
    143
    State:
    Batesville Arkansas
    Willard, that's exactly the kind of info I was hoping to get. I didn't know that about gaping the plugs a little more for slow running. I haven't ever heard of decarbing before. I'll start doing some research on how to do that.

    Thank You very much for taking time to help me on this.
     
  4. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    This will work for you.



    Decarbing is a 2 part process. The first part is preventative, done by using a fuel additive to prevent carbon build up. SeaFoam is one such fuel additive and can be found at most auto parts stores. It also works as a fuel system cleaner and should be used as a preventative maintenance procedure.If Your intent is to REMOVE the existing carbon buildup from the cylinders, heads, and rings, a spray decarb solvent product, such as SeaFoam Deep Creep, Power Tune, or Ring Free, is needed to do the decarb process. Free moving rings are what seals your cylinders and gives you compression. Compressed and stuck rings means loss of compression, broken rings, damaged pistons and heads, and eventually, engine failure.Run engine at fast idle, with engine running and warm, slowly spray liberal amount into each carb. Its gonna smoke up the place. Spray for a couple minutes, now spray a larger amount into the carb(s) until engine chokes out and stops.Remove spark plugs and spray the decarb product liberally into each cylinder, install the spark plugs, let it soak for an hour or more. Start the engine and run at medium throttle, or if at the lake, run it at full throttle. It won't hurt to spray some more through the carbs. Run it for atleast 10 minutes to flush the crud out of your engine. Now remove and clean, or replace the spark plugs. It works well to do the spraying, the night before you go to the lake. This way you can let it soak overnight, and run at full throttle at the lake.Don't do this in front of the garage door or the house, unless you want it covered with greasy black crud.
     
  5. AwShucks

    AwShucks New Member

    Messages:
    4,532
    State:
    Guthrie, Oklaho
    Just got some of those plastic washers Bubbakat referred to in his first post. Closest marine store to me is about 20 miles, so I went to NAPA. Got the washers for .62 cents each. Ordered 10 just to make it worth my while. LOL
     
  6. ArkansasCat

    ArkansasCat Member

    Messages:
    143
    State:
    Batesville Arkansas
    Hey guys thanks again for the great info. I stopped by the local marine dealer here and he was out of those washers. I will give NAPA a try today. Never would have thought of trying there.

    I have run "Stabil" in my big motor for years. Should I switch to "Sea Foam" or is this a good fuel additive to help prevent the carbon build-up?
     
  7. AwShucks

    AwShucks New Member

    Messages:
    4,532
    State:
    Guthrie, Oklaho
    Brad, I think I would use both. Each has slightly different properties but when used together should cover the entire spectrum of preventative maintenance.
     
  8. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Stabil is good but seafoam is a fuel system cleaner and works best for me.
     
  9. ArkansasCat

    ArkansasCat Member

    Messages:
    143
    State:
    Batesville Arkansas
    Thanks Guys! I'll be picking-up some Seafoam tomorrow.