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Thread: Idle RPM's

  1. #1

    Idle RPM's

    What does everyone set their idle speed? Is there a problem with too much speed/rpm's? (I set mine a little high, like around 1,900 instead of the recommended 1,100.)


  2. #2

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    There are 2 problems with higher idle speeds:
    1. High idle rpm makes it harder to manuver around the dock and such because the speed is higher, harder to coast up to another boat as well.
    2. High in-water idle can set you up for potential lean runaway when started out of the water due to the large opening of the throttle.

  3. #3
    How does higher idle speed effect lean runaway again? In a carb'd ski, if the low-speed circuit (or high) is set correctly, then the opening of the idle/throttle plate simply uncovers the appropritate carb signal holes right? Maybe I'm missing something...

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    Yamaha artisan Cutlass's Avatar
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    I can't 100% explain it but its happened around here a few times. It could lean runaway due to that lack of load and lack of exhaust backpressure (from the water) with the idle cranked up. If its not exactly a lean runaway, a high in water idle speed could rev extremely high on the trailer. People really underestimate the load the impeller in the water has. Its a completely different world running these things on the trailer.

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    Quote Originally Posted by jim mcgreehan View Post
    How does higher idle speed effect lean runaway again? In a carb'd ski, if the low-speed circuit (or high) is set correctly, then the opening of the idle/throttle plate simply uncovers the appropritate carb signal holes right? Maybe I'm missing something...
    In a high idle speed setting the throttle blades are past the ideal carb signal holes so the low speed setting is no longer correct. Another player in the runaway is with no load it doesn't take as much power to turn the engine over, and its already spinning 2600-3200 anyway, so the heat of combustion(heat because the mixture is a bit lean from extra air from the throttle being open but not enough to pull fuel in adequate amount from the main jet so its lean) is enough to ignite the fuel mixture. Basically its like an older carbed car that diesel's when you shut it off when the idle is set too high. Remember those days? If you leave the car in gear it would not do it long because it was loaded enough to stop it.
    What the guys here call lean runaway is basically dieseling (self ignition), but on a two stroke every stroke is a power stroke so it revs way higher than a car engine that diesels, cause car engines have an exhaust stroke that slows the process down.

  6. #6
    cmgww's Avatar
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    Quote Originally Posted by LT1GMC View Post
    Quote Originally Posted by jim mcgreehan View Post
    How does higher idle speed effect lean runaway again? In a carb'd ski, if the low-speed circuit (or high) is set correctly, then the opening of the idle/throttle plate simply uncovers the appropritate carb signal holes right? Maybe I'm missing something...
    In a high idle speed setting the throttle blades are past the ideal carb signal holes so the low speed setting is no longer correct. Another player in the runaway is with no load it doesn't take as much power to turn the engine over, and its already spinning 2600-3200 anyway, so the heat of combustion(heat because the mixture is a bit lean from extra air from the throttle being open but not enough to pull fuel in adequate amount from the main jet so its lean) is enough to ignite the fuel mixture. Basically its like an older carbed car that diesel's when you shut it off when the idle is set too high. Remember those days? If you leave the car in gear it would not do it long because it was loaded enough to stop it.
    What the guys here call lean runaway is basically dieseling (self ignition), but on a two stroke every stroke is a power stroke so it revs way higher than a car engine that diesels, cause car engines have an exhaust stroke that slows the process down.
    Listen to this guy. He knows his stuff.. esp. w this topic

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