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Thread: Air heads running at above 7,000 feet elevation

  1. #1
    Registered User DonTom's Avatar
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    Air heads running at above 7,000 feet elevation

    Is there anything that can be done to get better performance on my 1971 BMW R75/5 at high elevations--such as riding up hill on a freeway at above 6,000 feet or so?

    I guess something is needed to advance ignition timing for low pressure. My 1984 Yamaha Venture runs very well at high elevations, as it has a "pressure sensor" to advance the ignition timing whenever it can, such as higher elevations or lighter loads, etc. It can barely tell the difference at 7,000 feet. Any thing like that pressure sensor available for airheads to work with any electronic ignition? Or any other ideas?

    My 71 BMW runs great at sea level, does great even up the hills, but I can barely climb a hill at 45 MPH when above 6,000 feet elevation--in any gear.

    What I have now on my BMW are Mukuni Carbs, dual plugs on each cylinder (with two six volt Dyna dual output coils in series--still has waste spark) Dyna electronic ignition.

    I assume all airheads run very lousy above 6,000 feet elevation and not much can be done about it--but I figure it won't hurt to ask.

    -Don- Reno, NV
    Original owner of:
    1971 BMW R75/5, 1984 Yamaha Venture
    2002 Suzuki DR200SE, 2013 Triumph Trophy SE

  2. #2
    Sir Darby Darryl Cainey's Avatar
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    That's strange!
    I rode my 1977 R100RS up to the top of Mount Evans, CO 14,160' with no problems.
    Stock carbs.
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    Administrator 20774's Avatar
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    Don -

    If you have the CV carbs on the bike, it might run a bit better. The CVs have a limited ability to adjust for the lower atmospheric pressure at the higher altitudes. People who run nominally at lower altitudes can ride to higher altitudes such as traveling cross country and needing to climb through passes.

    But if you plan to spend any length of time at that attitude, you probably need to adjust the carbs. At higher altitudes, there is less air. But if your carbs are adjusted to run with the air at lower altitudes, it will then be too rich at the higher altitude. You will need to drop the needle in the carb down one notice and maybe consider some jetting changes.
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    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    R100GS, '89 Guenther's Avatar
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    I live at 6,800ft and any trip west is UP and sometimes over 12,000ft passes. I had this feeling this summer in St. Paul at 700ft that my R100GS showed a bit more power. I can accelerate with two-up in the mountains depending on how steep the road is and how fast I am going. YMMV!

    And btw. I did not change anything from stock setup.

    Maybe the Mikuni carbs are to blame. Or the 750cc motor.

    /Guenther

  5. #5
    Beemerphan Radar41's Avatar
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    Cool High altitude

    Same here, live at 5,000' and regularly ride up to 12,000' with no discernible power loss.

    Both the R75/6 and the R100RS are equipped with CV carbs, perhaps that IS the difference.
    Don "Radar" Wreyford
    00 K1200LT, 98 R1100GS AE, 84 R100RS, 76 R75/6 (rebuild complete)
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    Registered User toooldtocare's Avatar
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    I have noticed some power loss at elevations while riding double, but not so much it is not fun to run, and only at altitudes of 7K or more. (88 R100RS, 95 R100RT)

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    Curmudgeon nrpetersen's Avatar
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    I had my '75 R90/6 with low compression cyl base shims to top of Mt Evans two up with stuff. I recall leaning out the idle mixture at high elevations. Otherwise the variable venturi carbs seemed to pretty much compensate for most of the altitude effects. Then it was back to Minnesota.......
    Retired w 2005 K1200LT, 2000 R1100RT, & 1975 R90/6

  8. #8
    Registered User DonTom's Avatar
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    Quote Originally Posted by 20774 View Post
    Don -

    If you have the CV carbs on the bike, it might run a bit better. The CVs have a limited ability to adjust for the lower atmospheric pressure at the higher altitudes. People who run nominally at lower altitudes can ride to higher altitudes such as traveling cross country and needing to climb through passes.

    But if you plan to spend any length of time at that attitude, you probably need to adjust the carbs. At higher altitudes, there is less air. But if your carbs are adjusted to run with the air at lower altitudes, it will then be too rich at the higher altitude. You will need to drop the needle in the carb down one notice and maybe consider some jetting changes.
    The Bings from 1971 were pure junk, and wouldn't do any better at high elevations, IIRC.

    I have tried re-jetting the Munki carbs for higher elevations and I couldn't even notice the difference.

    Therefore, I assumed it was more than a fuel-mixture problem.

    Back in the early 1970's even very few cars would run well above 7,000 feet that had lots of guts at sea level. My 71 BMW was no exception for those days. But things have changed a lot since then, so I wondered if there was anything can be done to make a noticeable improvement.

    -Don- Reno, NV
    Original owner of:
    1971 BMW R75/5, 1984 Yamaha Venture
    2002 Suzuki DR200SE, 2013 Triumph Trophy SE

  9. #9
    Administrator 20774's Avatar
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    You can certainly check the timing, but correct me if I'm wrong, timing is just timing, right? The spark needs to happen at a specific point in advance of the piston position in order to have burned the fuel properly before top dead center. Sure, there's volatility of fuel and what happens at altitude, but I would think that is a lesser problem...could be wrong. You need to use the right fuel at altitude also. IIRC places at higher elevation don't sell the highest octane, like 93, and it's usually a lower number. That's because of the altitude.

    I'm surprised a good set of CV Bings wouldn't work well after rejetting a good carb synch (yes, setting mixture). The slide Bings are the same animal as the Mikunis...ie, slide carbs.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  10. #10
    Nick Kennedy
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    Hey Don
    I have a 1978 R80/7 and I live above Telluride Colorado at 9,750' and have for 36 years now. I'm up on Red Mt., Molas and Coalbank passes frequently at over 11,000', its our local gentleman road racing course when there is no traffic. Our bikes run good up there.

    The problem with increasing timing advance with altitude is that when you go into the lower elevations your going to detonate, and that is very bad. But I don't think that is your problem.

    That said We do increase the advance while zooming around up here, about 10 degrees for me. You have to keep a eye on your plugs and know how to read the color to see what is going on in there. And Lower octane gas up high is fine because higher altitude= lower compression = less detonation chances. They don't even sell much high octane gas up here.
    A very clear explanation of the relationship between Altitude / ignition advance / octane is in the Classic John Muir VW book below; This is a really well written, Hilarius Manuel " for us complete idiots"

    http://www.amazon.com/Keep-Volkswage.../dp/1566913101

    Here is another link on the relationship of HP loss due to altitude increasing:


    http://www.wallaceracing.com/braking-hp.php

    Our bikes run fine up here, a bit low on power but no problem, they haul right along, even with full fairings on them.
    It sounds like you might want to put your bike in a reputable airhead shop like Ted Porter's and have him go over it and do some tests. It needs something or maybe several things fixed for sure.
    Good Luck let us know what you find.

  11. #11
    Registered User DonTom's Avatar
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    Quote Originally Posted by nickrides View Post
    The problem with increasing timing advance with altitude is that when you go into the lower elevations your going to detonate, and that is very bad. But I don't think that is your problem.
    That's why it has to be automatic, like the pressure sensor in my 84 Venture. I could be wrong, but I always assumed that is what makes such bikes run so well at the higher elevations. Low pressure advances timing, high pressure retards the ignition timing.

    Do the later airheads have some type of pressure sensor?

    -Don- Reno, NV
    Original owner of:
    1971 BMW R75/5, 1984 Yamaha Venture
    2002 Suzuki DR200SE, 2013 Triumph Trophy SE

  12. #12
    Registered User DonTom's Avatar
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    Quote Originally Posted by 20774 View Post
    You can certainly check the timing, but correct me if I'm wrong, timing is just timing, right? The spark needs to happen at a specific point in advance of the piston position in order to have burned the fuel properly before top dead center. Sure, there's volatility of fuel and what happens at altitude, but I would think that is a lesser problem...could be wrong. You need to use the right fuel at altitude also. IIRC places at higher elevation don't sell the highest octane, like 93, and it's usually a lower number. That's because of the altitude.
    I have always used 87 octane everywhere on my R75/5. Here in Reno, some stations have 85, which is fine too.

    But my understanding (which could be wrong) is that advancing ignition timing simulates higher compression. And that seems to be the way my 84 Yamaha Venture works with its pressure sensor that controls the timing advance. It works very well.

    Quote Originally Posted by 20774 View Post
    I'm surprised a good set of CV Bings wouldn't work well after rejetting a good carb synch (yes, setting mixture). The slide Bings are the same animal as the Mikunis...ie, slide carbs.
    The 1971 R75/5 CV Bings (black background with a silver "Bing"-changed to the opposite perhaps in 1972, which were much better carburetors), main problem was when the bike got just a little warm, the idle would be 3,000 RPM's and would not set lower without dying. Perfect idle when cold. This warm carb problem every BMW shop knew about back then--and there were no real fixes other than putting on a later Bing Carb or some other brand. 99% of my problems went away with the Mukuni carbs, but IIRC, it make no real difference at 6 to 7,000 feet elevation. In theory, the Bings with the CV should be a bit better, but I don't recall any noticeable difference between the two at higher elevations.

    The smaller CC BMW engines of 1971 (R50/5, R60/5 )had a slide carb that worked a lot better than the CV Bings they put on the 1971 R75/5.

    Back in those days even most cars would be slow in the hills at high elevations. I don't think my 71 BMW was an exception. By 1974, some cars ran much better at higher elevations--and perhaps the same with cycles. These days with EFI, (but IMO, it's better mostly because of the "E", not the "FI" )it seems everything runs great at high elevations, even the very small engines, thanks to those countless sensors.

    Anybody else here with a 70 or 71 BMW R75/5 so we can do a better comparison? I don't know what they changed in later models.

    -Don- Reno, NV
    Original owner of:
    1971 BMW R75/5, 1984 Yamaha Venture
    2002 Suzuki DR200SE, 2013 Triumph Trophy SE

  13. #13
    Day Dreaming ... happy wanderer's Avatar
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    Quote Originally Posted by DonTom View Post
    That's why it has to be automatic, like the pressure sensor in my 84 Venture. I could be wrong, but I always assumed that is what makes such bikes run so well at the higher elevations. Low pressure advances timing, high pressure retards the ignition timing.

    Do the later airheads have some type of pressure sensor?

    -Don- Reno, NV
    I do not think any carbureted airheads had pressure sensors. Not sure about K bikes but Oilheads starting with the 1100 had one in the Motronic V2.2 engine controller.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  14. #14
    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    Airheads have not had vacuum/pressure operated advance mechanisms, only centrifugal.
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  15. #15
    Benchwrenching PGlaves's Avatar
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    Timing is a minor part of the issue. Mixture is the major part of the issue. At higher elevations the air is "thinner" containing less oxygen. So carbureted engines tend to run richer. The /5 carburetors are crude fuel mixers at best and when adjusted to run well at low elevations will be too rich at high elevations. This is better than set to run well at high elevations and then forced to try to perform at low elevations where the mixtures are too lean and things (like valves) burn.

    At a minimum, drop the needles one notch. Don't forget to put it back when you go to sea level.
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