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  1. #1

    Oil and Fuel and Lubrication I'm a little confused.

    After the last few days of reading through these forums I am still confused slightly.

    I am trying to understand how lubrication gets through a 2 stroke motor and what can cause lubrication problems.

    I understand that there is an Oil bath in the crank that keeps the lower 1/2 of the engine happy and oiled and the top half gets oil (mixed with the fuel) after the carbs (or in the fuel before the carbs) and this oils the top 1/2 of the engine.

    From my understanding a few things can cause heat / lack of lubrication problems.

    • Loss of oil (leak in the crank) or (no oil in fuel mix)
      • Bad gaskets / Cracked lower 1/2 for crank
      • Bad Oil Pump, Fail to premix fuel properly
    • Carbon buildup
      • Using low quality (87 octane) fuels / old fuel (not winterizing properly) allows carbon to buildup
    • Carburetor Miss-alignment
      • If engine runs with not enough fuel, that also means not enough oil to lubricate things properly
    Are any of these completely misguided. Just rebuilt the top 1/2 of the engine (787 Type) and am starting the break in later today, its running low RPM wise but is running smooth at 3k rpm (out of water) just have to adjust the throttle cable so the idle is up higher.

    Thanks in advance
    ~Karl


  2. #2
    RumRunnerRon's Avatar
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    Crank bearings are lubed from the oil fuel mix entering the crankcase from the carbs. If you premix well that is pretty self explanatory. Oil injection means the oil is injected into the air/fuel stream at the base of the carbs as it enters the crankcase. From the crankcase it is pulled into the cylinders through passages by the vacuum the pistons make as they go up and down. On a 951 the oil bath you are referring to is for the counter balance shaft gears. If memory serves me (and I could be wrong) the rotary valve gear ( say on a 787) is bathed in oil too but, I think it gets its oil from the oil tank(two stroke oil) fed to the cavity by a hose. Not positive on that so if I'm wrong someone will chime in and correct me. Hope that helps explain things some.



    Quote Originally Posted by blackley1 View Post
    After the last few days of reading through these forums I am still confused slightly.

    I am trying to understand how lubrication gets through a 2 stroke motor and what can cause lubrication problems.

    I understand that there is an Oil bath in the crank that keeps the lower 1/2 of the engine happy and oiled and the top half gets oil (mixed with the fuel) after the carbs (or in the fuel before the carbs) and this oils the top 1/2 of the engine.

    From my understanding a few things can cause heat / lack of lubrication problems.

    • Loss of oil (leak in the crank) or (no oil in fuel mix)
      • Bad gaskets / Cracked lower 1/2 for crank
      • Bad Oil Pump, Fail to premix fuel properly
    • Carbon buildup
      • Using low quality (87 octane) fuels / old fuel (not winterizing properly) allows carbon to buildup
    • Carburetor Miss-alignment
      • If engine runs with not enough fuel, that also means not enough oil to lubricate things properly
    Are any of these completely misguided. Just rebuilt the top 1/2 of the engine (787 Type) and am starting the break in later today, its running low RPM wise but is running smooth at 3k rpm (out of water) just have to adjust the throttle cable so the idle is up higher.

    Thanks in advance
    ~Karl

  3. #3
    Blackhawk36's Avatar
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    Apr 2012
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    Maryland, USA
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    Karl, there is no oil bath in the bottom of the engine. The engine is lubricated by the oil and fuel mixture. The mixture comes in through the carb with air/fuel/oil and is atomized into a mist. The mist goes into the bottom or the engine and lubricates the crankshaft, crankshaft bearings, and connecting rod bearings, and the bottom of the cylinder. As the piston comes down in the cylinder, it pressurizes the crankcase, forces the reed valves closed and the intake ports are uncovered in the cylinder. The pressure in the crankcase forces the same air/fuel/oil mixture into the combustion chamber, lubricating the upper part of the cylinder. The mixture is then compressed and ignited producing power.

    Heat becomes a problem when your carbs are out of adjustment or you fuel lines/carb filters are clogged with debris. This creates a lean condition and the result is engine damaging detonation of the fuel mixture and high heat. The detonation from the lean condition will cause damage far more quickly than lack of lube due to not enough fuel.

  4. #4

    Join Date
    May 2012
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    Northern MN
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    The biggest causes of piston/cylinder scuffing with a 2-stroke is running the motor lean (gummed up carbs or an air leak either in the case or carb boots), this will melt pistons and a lot of other things. And lack of oil, this can be from a bad oil injection pump, or if you pre-mix your gas and oil, it can happen when you run lean or run out of gas. Another one that a lot of people over look with mixed gas is that most of the gas available out there has ethanol in it, the ethanol will absorb moisture from the air and actually cause the oil to seperate from the gas. In extreme cases I have seen gas absorb enough water to the point that water was gathering in the bottom of the tank (water provides no lubrication).
    All 2 strokes use some form of roller bearings in their lower ends, the good news here is that they usually survive the first lean seize/dry seize as they tend to hold up better with no lube than the top end will.

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