Lower elevation jetting recommendations?

Discussion in 'Airheads' started by jebers, Aug 17, 2020.

  1. jebers

    jebers n00b

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    Hi everyone. I have a 1985 BMW R80rs with the stock Bing carbs. I just moved from the Denver area (~5,000 ft) to Seattle (~sea level). I'm looking for info on rejetting my carbs to account for the lower elevation.

    Do I need to size the main jet up or can I make the adjustment with just the needle?
    If I ride the bike now, it will be lean but I don't know how lean... How fast would I damage my engine by riding it like this?

    Thanks in advance for your help!
    #1
  2. Plaka

    Plaka Brevis illi vita est

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    The stock jetting is in your Haynes or Clymer manual. It is also in the Bing book available from Bing for cheep.
    #2
  3. tlub

    tlub Long timer Supporter

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    Your main jet really only affects things when you are running almost wide open. Your needle is a coarse adjustment. Different needle jet sizes and idle jet sizes, are the fine adjustment. It is quite possible it needs no change. When I have ridden at elevation, set for Madison, WI (elevation 900 ft) I noticed very little change in idle, or in performance, except for the normal loss of power at altitude.
    #3
  4. chasbmw

    chasbmw Long timer

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    Cv carbs are better at altitude changes than slide carbs......it is quite likely that your bike is on standard jetting
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  5. jebers

    jebers n00b

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    Thanks for the feedback guys. I've found a few places that make it sound like changing elevation with CV's is no big deal but they're always talking about going from low to high. Effectively running more rich. I'm going down in elevation and running more lean which scares me a little more. The bike seems to run fine but I don't want to do any damage running lean. Am I overthinking it?
    #5
  6. tlub

    tlub Long timer Supporter

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    Maybe. If after about a minute of running (start the bike, pull to half choke, put on helmet and gloves, pull out driveway, pull off choke within a half mile) you can run fine, you are not too lean. If it takes several minutes to get to the 'pull off choke completely' stage, go one size larger in the needle jet (like 2.68 to 2.70) and that should be all. Heck, because of alcohol gas, you may want to do that anyway. But for the most part, I doubt you will have any real issues. You'll notice it in flat spots long before you damage anything. This will be more of a rideability thing than anything else.
    #6
  7. jebers

    jebers n00b

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    That's a good tip! I rode it this weekend and from what I remember, it didn't take too long to let off the choke. I'll have to pay closer attention next time. I was also going to check my plugs and see if that can tell me anything.
    #7
  8. tlub

    tlub Long timer Supporter

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    Plugs are harder to read these days with the oxygenated fuels. They stay white a long, long time, unless you are running really rich.
    #8
  9. Plaka

    Plaka Brevis illi vita est

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    Look at the jets that are in it. Compare to the stock jetting for your model. Set up to stock jetting. The CV is good at compensating for higher altitude making them popular with the small aircraft crowd. But for best performance, run the stock jetting for the altitude and temperature you ride in the most..
    #9
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  10. Solo Lobo

    Solo Lobo airhead or nothing Supporter

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    I ran stock jetting in my R80G/S in Seattle for a decade with no issues
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  11. ccmickelson

    ccmickelson MonoMania Supporter

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    Agreed. I've had stock jetting in all of my airheads and ridden from sea level to as high as 13,000 feet. It only requires adjusting the mixture and idle to compensate for the change. The bike will tell you when it needs adjustment.

    Solo Lobo's R80 g/s just recently made the opposite journey from Seattle to it's new home here in Santa Fe, wheezing and refusing to idle from 7,000 feet elevation gain. Just required some minor tweaking to lean the mixture and raise the idle and it's once again a happy airhead (if a bit homesick).
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  12. 190e

    190e Long timer

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    It's mostly true that CV carbs compensate for altitude changes but only when they are operating in the Constant Velocity part of their range. That excludes idle which is why idle mixture settings are often mentioned as requiring adjustment. Ultimately it excludes full throttle too so main jet sizes might need changing in some cases.
    #12
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