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Thread: AFR (Air Fuel Ratio )meter to tune carbs

  1. #1

    AFR (Air Fuel Ratio )meter to tune carbs

    I have been away from BMWs for a long while and just restored my '87 R-80.

    I need to tune my carbs and wonder if an AFR meter is ever used by the folks here.

    For my Porsche Speedster I use an AFR meter but would like advice on using it with the Bing carbs on my R-80. There is a senser that reads the air-fuel ratio and it is shown on a meter. I put the senser into a bung in my exhaust on the car, but guess I'd just somehow stick the senser up the exhaust to get a reading for the bike.

    I did a search but can't find anyrthing on this subject and would appreciate any advice.
    Last edited by hsbroker; 07-25-2011 at 07:15 PM. Reason: spelling error

  2. #2
    Registered User
    Join Date
    Jul 2009
    Topeka, KS
    I built a diverter out of 1/2" copper and just made an "L" shape about 12" x 8" that can be placed inside the muffler openings. Just solder the 90 degree fitting and you're ready to go. Just remember that a little too rich is always better than a little too lean.

  3. #3
    Dean--what AFR do you shoot for at say, 1,100 RPM idle and at a cruising RPM of say, 3,000 RPM?

    This is an'87 R-80


  4. #4
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    I poked around the Airlist and found some posts where the ratio of about 14.1 to 14.7 was needed for gasoline but for E85, the stoichiometric ratio was 66% less or down to around 9.7.

    Another learned poster said stoichiometer was only a setting you should look for at idle...having the same 14.1 at load would result in a dangerous situation and could hole a piston.

    Seems like a lot of trouble to go to...unless you were in one of those states that uses sniffers as part of state inspections...
    Kurt -- Forum Liaison ---> 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!

  5. #5
    You could try to find the arse who stole my CO meter and SnapOn timing light a month or so ago.

    The only thing I currently see available for CO exhaust meters are made by Gaston. I have had both their analog and digital meters and they are fragile and crap out in short order.

    You can get the pro gas meters from the 60's to 80's for a few hundred, but they will take up as much space in your garage as your Airhead.

    If you find a CO meter, you will find the proper CO setting in your manual or on the sticker under the rear fender. Typically, with current panther pee, you want to set the mixture at the high end of spec. For my 79 RS that is 3.0 percent CO.

    CO meters designed for HVAC measure of heating equipment stacks only go up to 2000ppm, which is 2 percent, so those won't do.

    I searched exhaust gas meters recently and came up with nothing less than current 4 or 5 gas meters starting around 3 grand.

  6. #6
    Grammarian no, Rider yes ISAMEMON's Avatar
    Join Date
    Jun 2006
    Kings Valley Oregon
    color tune spark plugs
    find em many places, e-bay, bing

  7. #7
    The AFR unit I found is made by PLX Devices and was $316. Totally plug and play
    with an easy to read meter. I'll see what it shows my R-80 is doing.

  8. #8
    Quote Originally Posted by hsbroker View Post
    The AFR unit I found is made by PLX Devices and was $316. Totally plug and play
    with an easy to read meter. I'll see what it shows my R-80 is doing.
    Looks to me that those are O2 types that require install of a sensor in the exhaust. That will work, but you need 2 sensors. Not a tube up the pipe type for shop adjustments. Oxy sensors just will not work at the muffler outlet. They must be near the exhaust port or not far down the head pipe to heat up and work. But I don't consider install of an oxy sensor in the head pipes a plug and play solution. Nothing like it. You won't see that of any use at the Airheads Tent.

    Oxy sensors are very nice for monitoring and adjusting systems when they are available and a perfect device for feedback control of computer control of injection systems. Most all the newer bikes have them.

    Carbs need the exhaust measure for precise confirmation of mixture. But tuning by ear and feel and reading plugs will get you there just fine once you know how to do that. The CO meter just eliminates the need of practiced skills many DIY'er just don't have. One major reason the Airheads Tent is of great value. You will always find a few million mile Airheads with these skills available to show you how to get the carbs set.

  9. #9
    John. jstrube's Avatar
    Join Date
    Nov 2009
    Atwater, CA
    A Company called Innovate Motorsports makes an O2 meter that is a wideband unit that can be used for tuning throughout the RPM range. They sell an exhaust probe style clamp that may or may not work in an exhaust outlet. The opening may be a bit small for it, I don't know, I do not own the probe, just an early version of the data logger.

    The Innovate O2 sensor & meter/data logger uses a 5 wire HEATED WIDEBAND O2 sensor that will give you A/F ratio, or Lamda readings. It can be used for Gas, Alcohol & anything in between. With the data logging function, you can plot RPM vs. O2 & with a little work, rig up a sensor to plot throttle opening as well for load info.

    This is a very accurate unit & their technology is used by a few of the other manufactuers making data loggers for racing purposes like RacePak.

    The only problems I see using it on an Airhead are these 2:

    1. Does the probe fit in the tail pipe?

    2. Does the front crossover tube affect readings (yes it will, but how much)

    By welding a bung in each tube ahead of the crossover, or blocking off the crossover, a very accurate reading & therefore, good tuning data can be found. This will be my plan when I get there.

    Oh yeah, here is a link to the company, they have quite a few different units:

    What I do when tuning is drive & gather data over various conditions, like high RPM, high load, steady throttle, etc., then review the plot on the computer. You will never tune a carb as exactly as you can EFI, but you can find a happy place.

  10. #10
    Registered User lkchris's Avatar
    Join Date
    Mar 2003
    Albuquerque, NM
    If you have original compression, original carbs, original valves and original camshaft, all you need are factory carb specs. The work you anticipate has already been done.

    It's fun to mess around, but you won't improve anything.
    Kent Christensen
    '12 R1200RT, '02 R1100S, '84 R80G/S

  11. #11
    I wasn't thinking straight-of course the crossover tube will mix exhaust from both cylinders so you wouldn't know what each carb was doing on it's own. Duuh.

    I ordered a meter to synch the carbs which seems to be the proven way---except for doing it by voodo through sensing what the carbs are doing. Don't think I'll ever be there---or live long enough to finish wading through Snowbum's treatese on synching carbs.
    Last edited by hsbroker; 07-27-2011 at 08:34 PM. Reason: grammar correction.

  12. #12
    Registered User
    Join Date
    Dec 2007
    Reno, NV
    Having cut my teeth on 4 and 3 cylinder Japanese bikes, synching Airehead carbs is EASY!

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