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Thread: Rough running 2 stroke 40

  1. #1

    Default Rough running 2 stroke 40

    Hi, we've just started running an older 2 stroke Johnson after sitting for a few years. Runs okay at higher RPM's but wants to stall when going into neutral or idling down just above neutral. Other than working on the carbs, jets, whatever, is there some kind of additive (seafoam, etc.) that would be good to run in the fuel to help clean things up? We ran it for about 4 hours on Saturday, mostly trolling and wide open on the way in to the ramp.

  2. #2


    Yamaha Ring Free - follow the directions on the bottle starting with the shock treatment. Install new plugs after the shock treatment and it'll run like a sewing machine.

    I have never been an additive guy, I had a Johnson 150 on a bass boat with those symptoms and put up with it for a couple of years because everyone told me those old two strokes are like that, you just have to clean them out often at full power.

    When the Ring Free was recommended by a dealer in Eastern KY I laughed, agreed to try it because I was offered my money back if I followed direction and it didn't work. I went back for a case and use it in everything.

  3. #3
    Join Date
    Nov 2009

    Default Sveral things can cause rough running

    My boss was not completely sold on additives. We had a rental fleet of johnson out boards 6hp on up to 200hp.We used ,engine tuner,(a johnson product) for removing some of the carbon from the exhaust tube. Under extreme conditions it was a waste of time. We always used the 2+4 , fuel additive in winterizations and in fuel systems suspect of water( small amounts of water).
    Annalisis on any engine started with a leak down test , Not a compression test, but similliar to it . It would, using an air compressor and a special metering device and gauge, expose where and how much pressure in the cylinder is lost during the compression stroke (in a fixed position).
    The carb / fuel system includes the pick up in the tank , fuel pump, carbs, reed block, and seals on the crank shaft.
    On winterization the engines were run with conditioned fuel and fogged to protect the crank and seals during the winter
    Test you spark on each cylinder should be at least 1/4"- 3/8" or better.
    If the additive you try doesn't work ,let me know ,and tell me what hp and year model. I still have a few old johnson books.

  4. #4
    Member marshall's Avatar
    Join Date
    Nov 2007
    Near Phoenix, AZ


    I'm not familiar with that carb but if it were a bike or sled I would say soak the pilot jet and air metering screw in carb cleaner and spray some up in the orifices that they seat into. After an hour or so spray those orifices again and blow out with air.

    If your carb is one of those that has a low and high speed adjustment then your low speed is blocked. Either way very easy to clean and adjust.

    Just adding chemicals to your fuel is not going to suck it up through your low speed circuits and fix your problems because there is no flow through a blocked jet.

    The low speed pick ups are generally clogged in storage situations because they have smaller holes. The main circuits have large holes and even though they get contaminated they generally don't get blocked. If you do have a partial blockage of a high speed circuit or main jet you will have a lean condition at wide open throttle because the blockage acts on the jet as if it is a smaller jet.

    Chemicals are more successful in cleaning partially blocks jets because there is flow through them and the chemical cleans it as it passes through.

    Focus on your low speed adjustment or pilot jet circuit which ever applies to your situation.

  5. #5


    My Johnson does exactly the same thing if I sync it by the book. What I've found is increasing the idle speed slightly does the trick. This is done by slightly advancing the timing - a half or full turn on the advance trunion is all that's needed. Older 2 strokes seem to need such coaxing, or maybe that's the black magic part of keeping a 2 stroke running well.

  6. #6
    Member Dupont Spinner's Avatar
    Join Date
    Jun 2007


    Try the Seafoam seems to be the new wonder product. I know a bunch of folks that had some rough running equipment from chainsaws to V-8's and all had much succes with using the Seafoam. It's a cheaper and quicker fix(if it works) then all the alternatives.

  7. #7
    Member Sobie2's Avatar
    Join Date
    Jul 2006


    Sounds to me like a hole in the fuel system. Maybe a clog around the float bowl. I'd start by draining the float bowl.



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