Airhead Air Fuel ratios

Discussion in 'Airheads' started by Rob Farmer, Feb 6, 2011.

  1. Rob Farmer

    Rob Farmer Long timer

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    Yesterday's weather was perfect for a spot of engine tuning so I rigged up my PD with my air fuel ratio meter. There's a stretch of motorway near me that has a long steady climb that is perfect for load testing.

    After a few runs and a couple jet changes and needle adjustments I ended up with what appears to be the perfect set up. This is a full throttle run in fifth gear from around 60 Mph. The spike at 12 seconds is me rolling the throttle off.

    [​IMG]

    It looks nigh on perfect and holds this ratio all the way through the rev range, the bikes flying and will pull into the red line in top gear on a flat road. I have noticed though that there's a tendency at around 2-3k to run slightly rich if you back the throttle off for a split second and then nail it - the gauge shows it dipping to 10.2:1

    Anybody got any thoughts on the ratio I'm running. I've always taken 11.5:1 as being ideal for power but slightly rich.

    The bikes a UK 100PD with 40mm Carbs

    I'm running :-
    2.66 needle jet
    3rd clip from the top on the needle
    158 Main jet
    The bikes fitted with the early clam style airbox
    #1
  2. supershaft

    supershaft because I can

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    My airhead makes the most power at about 13.5. Most all do. I would shoot for at least 13. I knew 130 mains made the most power jetting my bike by my seat of the pants so I went back up to 135's for safety's sake. The 135's read around and a little above 12.5 on the dyno. 130's got close to 13.5 with more than one more pony. It won't pull past about 5000rpm in top gear with 125's in it. I am guessing 140's had me around 12 and also guessing that 140's to 130's got me almost three horsepower.

    I would try 150's and 145's.

    That's with 38mm Dells but my bike has late model small port heads with raised port floors too. Sport cam. Reshaped ports. Stock 8.7:1 CR. Sport Staintunes. Stock square airbox minus the airpumps and the crankcase breathers.
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  3. Rob Farmer

    Rob Farmer Long timer

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    Thanks SS. I'm struggling to find main jets. The standard 150 is too lean and the next available size if 158. Do you know if jets from other manufacturers fit?
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  4. supershaft

    supershaft because I can

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    That surprises me. What reading are you getting with the 150's?

    I get my jets either from a dealership or Bing USA. I keep 170's down to 125s in steps of five for all occasions. Bing makes 260 and 262 needle jets but you can't get them through BMW. You can get the biggest ones Bing makes (264's through 274's) through BMW.
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  5. Rob Farmer

    Rob Farmer Long timer

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    150's were giving around 13.8 - 14

    I'll try BMW for the main jets later this morning.

    I take it you think high 11's are too rich? Maybe a k&n will move it up into the 12s

    I've heard that a disk with a 3cm hole in the middle inserted half way along the early filters gives a boost. Could be a good opportunity to try it.
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  6. pommie john

    pommie john Long timer

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    I'd be very interested to find out what A/F ratio is considered best for an airhead.

    The dyno operator I used thought mid to high 12s is about right for an aircooled motor. I've heard it said that 13.1 is right for a modern watercooled bike.
    I recently bought an A/F meter and found my race bike is getting around 12.5-12.9 on full throttle ( when I have time to look at the gauge).
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  7. supershaft

    supershaft because I can

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    Well I'll be. They jet out about right with 135's in 32mm Bings. I guess that 40mm Bing needs bigger jets for slower velocity. I will re-guess that you can get some 155's and call it done? I still think the stock mains would ping and melt stuff running at almost 14 but . . . .

    11's is way too rich. Most look for between 12.5 and 13.5. Personally I think they like a little below 13.5. Just like jets, my life is too short to split main jet sizes or air/fuel ratios by more than five or point five!:norton

    I don't know about the air filter mod.

    I have never noticed any difference with K+N's jetting wise but I have never done back to back dyno runs with them either. I use them because they filter way better then paper.

    We've got to remember as far as jet numbers are concerned that your fuel might have a different specific gravity. Big differences in float height can change things too!
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  8. DaveBall

    DaveBall Long timer

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    You guys are getting way to technical for me. I have been riding airheads for a long time and one of the reasons I do is because I like to keep it as simple as possible. If I have to start using a computer to tune the damn thing, forget it. Might as well as buy one of those new fangled models with all the computerized bells and whistles.

    Give me the old fashioned tune by ear and throw a Carb Balancer on once in a while. But then, I am not looking for the ultimate fine tuning that you guys probably are.
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  9. Rob Farmer

    Rob Farmer Long timer

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    Dave,

    If my bike was a standard machine when i bought it i'd have been quite happy to just ride it but it isn't so I'm prepared to spend some time sorting it out. Fuels the equivalent of $9.20 a gallon so along with the pleasure of riding a well sorted machine there's a financial incentive to get it right.
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  10. supershaft

    supershaft because I can

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    There is nothing like more power AND better fuel economy! It's been going on for well over 100 years so why stop now! More power for better mileage IS keeping it simple on a number of different levels and I did tune mine without any computers. I just doubled checked it with a computer.

    After reading a lot of reasons why people ride airheads I can't figure out why at least half of us aren't riding EX500's. There are simple and a lot cheaper.
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  11. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Ah, A/F ratios. Fiddled with that a lot in the mid-'70's with expensive and finicky equipment.

    What measuring equipment are you using? I did a quick Google on "logworks 3
    dashboard" which led to the LogWorks Data Logging software:

    http://www.innovatemotorsports.com/products/logworks.php

    and thence to a site with wideband AFR guages:

    http://prosportgauges.com/wideband_AFR_Gauge.aspx?gclid=CPPHptqn96YCFcpQ2godVEtWDQ

    It looks like there is a lot more available these days than there was "way back when". Going a little OT, but can you point us in the direction of what you're using?
    #11
  12. pommie john

    pommie john Long timer

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    I use one of these

    http://www.aemelectronics.com/wideband-o2-air-fuel-uego-gauge-kit-745


    It's cheap and easy to install. If you just want to look at it ( as opposed to data log) it's a two wire installation, and a weld-on bung in the exhaust.
    #12
  13. Rob Farmer

    Rob Farmer Long timer

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    Bill,

    The logworks 3 software is free --> http://www.innovatemotorsports.com/support.php

    I have the LM2 from Innovate http://www.innovatemotorsports.com/products/lm2.php

    It uses the same VW/Audi wideband sensor as the expensive dynos and logs to an SD card for uploading to the logworks 3 software or you can plug straight into a laptop and view data live for stationary or rolling road work. The plus side is you can simply ride the bike, evaluate the data, make adjustments/swap jets at the side of the road and see the effect straight away. The downside is tuning at high RPM on the road can be downright dangerous - I'd sooner use a dyno for this.
    I use an adapter that pokes into the exhaust pipe instead of a welded bung in the exhaust. Innovate to a cheaper alternative to the LM2 if a permanent install is required.

    The frustration with the jetting on the later 40mm equipped airheads is the carbs are unique to the 100R and the 100GS and are different to the earlier 40mm carbs, there's only one needle available for them and no jets between 150 and 158 readily available. I'm either going to have to flow more air or start drilling jets out to get sizes that work.
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  14. Lornce

    Lornce Lost In Place

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    That's a little more informative.

    Out of curiosity, what's the stock rwhp from a small port, low compression 32mm Binged airhead? Does it break 45hp?

    :ear
    #14
  15. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Thanks for the pointer on the equipment. It's truly become sophisticated these days-- back when, the several-hundred-dollar AFR meter was simply a pair of thermistors in a Wheatstone bridge with a microammeter measuring the thermal conductivity of the exhaust gas at the tailpipe.

    I'm a tool and equipment junkie, so gadgets like this have always fascinated me.
    #15
  16. El Hombre

    El Hombre Banned

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    I think at $9.20 a gallon, I'd be trying to lean it out; with acceptable drive ability. It's only!! $3.50 a gallon for the 91 octane stuff around here.
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  17. Rob Farmer

    Rob Farmer Long timer

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    Had a chat with Richie Moore today. Lots of pointers from him. It an absolute pleasure to talk with someone who's so clued up and open with his knowledge.

    Nice guy :thumb
    #17
  18. supershaft

    supershaft because I can

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    So what did he say?

    I was looking at my dyno chart and thinking back and I might have been stretching 13.5 a bit. My bike is probably around 13.25 the most and it is probably below that more than above it. Still, from my experience around more than a couple of dyno's. I would call 12.5 good for power but slightly rich. Slightly rich is never best for power on top. It can really help you get off bottom and through the lower midrange but other than that IMO rich and best power almost never go together. One thing is sure. More than just slightly rich is a power killer anywhere you look at it. I would call 11.5 too rich and costing you power and efficiency. Remember that between two and three HP on our bikes is a 5% difference in power! It all adds up!
    #18
  19. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    You could drill out smaller jets to get the exact size you need. Since you want what BMW doesn't sell.

    There are precise drill bits to do the job.
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  20. Lornce

    Lornce Lost In Place

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    This.

    :deal
    #20