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lean running carb?

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  • lean running carb?

    What exactly is a carb that runs lean? I keep reading how a piston is melted due to a carb running lean. How do you know if yours is running lean? How do you adjust?


  • #2
    Re: lean running carb?

    Well, Noah, your 55 is a special case but before I get into it a little education is in order.

    Gasoline burns best and produces the most efficient power at a ratio of 16 parts air to 1 part gasoline by weight. In an internal combustion engine at full throttle, the mixture is richened to about 14 to 1 for maximum power. This is because the limiting factor is the ability of the engine to take in air. Since a slightly rich mixture loses less power than an equally lean mixture, the engineers err on the rich side to ensure that all oxygen available in the air is burned.

    However, gasoline will burn at mixtures as rich as 12 to 1 and as lean as 18 to 1. On a cold start, not enough gasoline vaporises to give a burnable gas/air mixture, so we "choke" the engine to get more gasoline into it and thus get more vapors. HOWEVER, that is also why, when you overchoke an engine, it will not start (flooded). The mixture is too rich to burn (say 10 to 1). Similarly, if there is not enough vaporized gasoline mixed with the air (say 20 to 1), it is too lean and will not burn. That is why the engine stalls when you screw in the idle mixture needle too far.

    At the far end of the burning spectrum, 18 to 1, the gasoline burns hotter and tends to auto ignite before the sparkplug fires it. This results in too rapid a rise in pressure and the very quick pulse puts extreme pressure on the top of the piston. Additionally, since the sparkplug has also fired the mixture in a different place, when the flame fronts collide, they "hammer" the piston. This is known as detonation and often (not easily on outboards) can be heard as "pinging." The combination of extreme heat and pressure is extremely destructive, melting the top of the piston and often shattering the rings along with melting the exhaust side.

    OK! Now back to your 55. Normally you would expect a simple 2 cylinder engine with only one carb servicing both cylinders to lose power if run too lean. Since both cylinders run lean, the engine loses power, you back off, and no damage is done. HOWEVER: because of the geometry of the reeds and the carb, that 55 always runs a bit leaner on the top piston.

    So: if you set the idle mixture needle on the front of the carb a little too lean, the bottom cylinder will run well while the top runs lean. With your particular engine, setting a little too lean results in a BEAUTIFUL idle and part throttle cruise. However, even at part throttle, it may result in a melted piston; almost always the top one, and if not damaged at full throttle, usually a hole about 1/4 inch in the top of the piston, right at the sparkplug.

    You set that carb the same way the carbs on the larger multi carb, 3 and 4 cylinder engines are set.

    Start with the idle mixture needle set to 1 1/4 turns out from lightly seated. In 1/8 turn increments screw it in. At first, speed will increase as you turn it in (lean). Continue until the engine slows, barks, or stalls. Note that setting. Now, starting at 1 1/4 again screw out the needle until the engine "sags" slows, or stalls. Note that setting. Reset the needle to the average between those points. HOWEVER: under no circumstances set the needle to less than 3/4 turn out from lightly seated. I prefer 7/8 even if the idle suffers a bit.

    Next, take the engine out on the water. Put it in gear and rapidly accelerate (hole shot). If the engine sags lean, then picks up and accelerates, then it is too lean. Screw out the needle until it accelerates with no hesitation. If it sags rich, coughs, or runs rough then clears itself and accelerates, then it is too rich. Lean the needle 1/8 turn until it accelerates normally without hesitation EXCEPT IF THIS SETTING IS LESS THAN 3/4 turns out.

    At first, it will be difficult for you to recognise these sounds, but with practice you will notice them and the difference right away.

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    • #3
      Re: lean running carb?

      Thanks Frank. I am waiting for my carb kit and should have it by this weekend. I will adjust it then and let you know what the result it. Again thank you for another lesson.

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