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  1. #1
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    Air temp and RPM's

    How much can air temperature/humidity affect peak engine RPM's? Earlier I was getting 6700 out of my 700 twin, and now I was only seeing 6350 today consistently. The only thing different was the air temperature outside. It was near 90 today, and when I was getting the higher RPM's it was maybe in the low 60's. The only other thing I can think of thats different, I am no longer running premix along with the oil injection, I'm just running off the oil injection now.


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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Im trying to decipher all of this. What I am guessing that as air gets warmer, your mixture gets richer? Is this correct? I did notice my idle mixture was richer than the last time out under much cooler conditions.

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    I think it is that when the air is colder it expands faster there for giving you more rpms its kinda of like how a car runs better when the air is colder outside

  6. #6
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    Quote Originally Posted by BryanP View Post
    How much can air temperature/humidity affect peak engine RPM's? Earlier I was getting 6700 out of my 700 twin, and now I was only seeing 6350 today consistently. The only thing different was the air temperature outside. It was near 90 today, and when I was getting the higher RPM's it was maybe in the low 60's. The only other thing I can think of thats different, I am no longer running premix along with the oil injection, I'm just running off the oil injection now.
    During the course of a day,My SLX rpm's will be down 60-80 during the day,as soon as it cools off in the late afternoon ( about 10-15 degrees ) my 80 rpm's come back.
    I would guess 100-150 rpm could be typical when going from 60 to 90.

    Running extra oil will diplace some fuel ( leaner)

    Start with a compression test.

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    I can check compression, however both motors I am dealing with are almost new with about 10 hours each on them. I'm going to guess the warmer temps along withno more premix in the tank richened me up enough to lose some rpms. If I'm slightly too rich I will leave it alone. I just don't want to go too lean. Plugs were a nice dark brown when I last looked at them. Maybe I will pull the heads tonight and have a look. Nothing its doing while running tells me its lean. I don't have any drop in rpms at full throttle, just maintains a steady rpm.

  8. #8
    PolarisNut's Avatar
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    A 30* temp swing is significant. My boats are easily down 100-150 rpm in 90* humid weather, vs fall weather.

    Cool, dry air is more dense, and contains more oxygen. The more oxygen, the more fuel you can burn, and the more power you will make. As the air heats up, it becomes thinner, and carries less O2. Not a whole lot you can do, unfortunately. That's why I don't like to pitch my props so steep in the spring/fall...I let the motor rev a little more than normal, so when the hot weather sets in, the RPM will still be close to where it should be.

    If you really want to get technical, look up or calculate your "density altitude"...this factors in humidity, temp, and barometric pressure. This way, you can use that info to compare how your ski runs from day to day or week to week.

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    Im not too concerned about losing the RPM's as long as the warm weather isn't making me go lean.

  10. #10
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    With a 350 rpm drop, I would take a good look to make sure everything is OK. Not saying anything is bad- just worth a look. Having a recent compression number doesn't hurt.

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