Tuning an airhead - science or art? Help appreciated

Discussion in 'Airheads' started by bikecat, Oct 5, 2010.

  1. bikecat

    bikecat Long timer

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    Please bear with me, gents.

    As some of you may know, I had a surging problem on my R100R that seemed eliminated with a carb rebuild. However, a rebuild meant setting up the carbs and tuning the bike, and I think I found more wisdom on the net than I can manage. :huh

    The often quoted sources are:

    http://bmwmotorcycletech.info/synchcarbs.htm

    http://wisconsin.airheads.org/content/view/33/51/

    I found this one particularly simple and appealing, however it requires a good "ear" which is not what I have:

    http://rossmz.blogspot.com/2008/12/tuning-40mm-bing-cv-carburetors-for-bmw.html

    Being a recent airhead owner, I certainly do not have the experience to know what is the “right” setting or “sound” to expect from the boxer. I have a few questions on which I hope your collective wisdom can shed some light:

    (1) Do the mixture screws have a huge effect on the idling or are the idle screws sole masters of the idling of the bike? I managed to get a balanced idle using only the idle screws.

    (2) Leading from that, do I have to tamper with the mixture screws’ stock settings (3/4 turns out) if the carbs can be balanced with a TwinMax? Any benefits to doing so? Or should I monitor the plugs before attacking the mixture screws?

    (3) Backfiring – is that an evil thing? Is it an indication of too “lean”? Do I adjust the mixture out to rectify that?

    I do apologise if my questions are basic and the answers seemed obvious, but I think it is important to get the basic rights and my trawl through the internet unfortunately turned up more information than what is useful to me. Finally, I drew some inspiration and comfort from Duane at http://w6rec.com/duane/bmw/tuning.htm when he said:

    “A good BMW twin tuner with a lot of experience will almost never find a perfectly tuned engine. Each of my mechanics went through a "crazy period" when they were learning to tune carbs. While riding their own bikes, every few minutes they would reach down and do a slight tweak on a carb. Eventually, they learned to accept that it would never be perfect. The very best that you will ever achieve is a workable compromise. Be happy.”​
    #1
  2. SUVslayer

    SUVslayer Long timer

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    I'm certain there are lots of folk out here with way more knowledge than I'll ever have on this topic. But from my experience, to get it running smoothly, you should get it warm, then adjust the idle mixture. Takes some patience, and some redoing. Then adjust the idle screws. Then adjust the throttle cables at just off idle, and then at 3000rpm (balancing), which is what the twin max is for. Repeat all as necessary, and don't let the motor get too hot.

    Mixture screws and balancing (with the twin max) are apples and oranges, so you need to do both. As for backfiring, I don't know if you mean the little pop-pop-popping that happens when you cut the throttle, or a bonafide backfire. Backfires are no good. The popping could be mixture, could be an exhaust leak. Could be old plugs.

    The only other thing I can suggest is watch for pinched throttle cables. Despite many attempts at re-routing and un-pinching my cables, if the handlebars are turned all the way left or right, the cables pull and I'll get a different balance. therefore, I try to keep them straight when balancing the carbs, since that's how it'll spend most of it's time.
    #2
  3. bikecat

    bikecat Long timer

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    I appreciate the input, SUVSlayer.

    Yes, I adjusted the engine after a 10-mile ride; the part that I did not do is the off-idle throttle cables adjust since the play on both seemed equal.

    The backfiring is the type that manifest during close-throttle; not a loud bang but a series of putts putts putts, louder than a 2-stoke Vespa tho.

    Plugs are new when I set the carbs - I used new plugs to eliminate the foul plugs variable.
    #3
  4. Country Doc

    Country Doc Wanderer

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    The last comment from Duane is definitely right-on. However, let me quickly address your questions.

    1) The mixture screws have a huge effect on idle speed (and thus idle balance)

    2) You cannot balance the idle without adjusting both mixture screws and idle stop screws. I set the idle as low as it comfortably can go, then adjust the mixture screws (as it's easier to find the "fastest" rpm on the mixture screw when you've deliberately set the idle too low), then I go back and re-set the idle stop screws to a balanced happy faster idle, then I go back and re-check the mixture screws which inevitably need a bit of tweaking, then re-balance again with the stop screws... :1drink I wouldn't worry about the plugs, reading plugs is fraught with error.

    3) These bikes definitely should not backfire. If they are backfiring then something is amiss. Could be too rich or too lean, depends...!! If the jetting specs is correct as per the Bing book, carb passageways are all clean and open, and the mixture/idle screws set properly, then a BMW airhead will almost NEVER backfire even under high rpm closed throttle/high vacuum conditions, IMHO. (unlike my highly-strung dirtbikes, which crack and fart like crazy on closed throttle, even when set up perfectly).

    dc
    #4
  5. Grider Pirate

    Grider Pirate Long timer

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    Even those with the smog equipment? My '95 PD did it all the time till I removed the smog stuff. Same with the '88 RS.

    Answer to OP's subject question. It SHOULD be science. However, your bike is a screaming minimum of 15 years old, and you are likely not the original owner. It is likely not to have been perfectly maintained for all of those years. Get the Bing Book. Set the carbs up IAW your tech data, but remember that the ignition timing, valve lash, compression, and general condition of the engine affect tuning.
    People who have been tuning for a while have no trouble getting an airhead set up 'by ear', but that's not something you learn overnight.
    #5
  6. P B G

    P B G Long timer

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    Even then...

    One of the websites covering tuning these things has a comment along the lines of (paraphrasing). That even if both sides were 100% identical and brand new, and you went for a ride with a slight cross wind, from that time forwards one cylinder experienced a different environment than the other, and its tuning will never be exactly the same again.



    I chased all sorts of mystery cold start issues around this summer. New battery solved it. My bike has never cranked like this before. I adjusted my valves, syncd the carbs, cleaned the chokes (swapped the chokes to make sure) and anything else from moving my timing back and forth trying to get a happy start medium. Ends up the best is just to set the timing where it should be, make sure the carbs are decently in tune, and give it a good battery.
    #6
  7. bikecat

    bikecat Long timer

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    Thanks for the input.

    I have a good battery (about 6 months old) with Enduralast. I also made sure that valves are within specs and that fuel bowl levels are right. Also did the 3/4 turns off the mixture (as per BMW manual), did the paper feeler thingy for the idle screws and added the 1/2 turn stock setting (as per BMW manual). The difficult part for me is how to interplay the mixture and idle after that; which to me is akin to a dark art for which there seems no universally correct answer.

    I understand that only experience will tell me what is the "right" note that engine makes (the "art"); I hope to eliminate possible errors (on my part) through the prescribed specs (the "science").

    Is cold starting (without using the choke and an incorrect high idle) an unreachable myth? :)

    Cheers
    #7
  8. advNZer?

    advNZer? Long timer

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    i cant tell if you are asking a real question here,but i am going to counter with another.Why?
    Surely if you can cold start without choke the mixture MUST be too rich
    #8
  9. bikecat

    bikecat Long timer

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    Thanks, it sets my dull mind thinking.

    It dawned on dumb me that I have been spoilt by the lack of the usage of a choke on my other machines; which are scooters with "auto-chokes" or EFI motorcycles - they don't need chokes! :doh :baldy

    In other words, use of a choke for a cold-start is a norm for airheads, and nothing to be worried about? That, ironically is a relief for me :)
    #9
  10. rufusswan

    rufusswan Been here awhile

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    Well ... choke at 'cold start' will vary for a given bike. Mine starts with one little flick of the kickstart and just a little throttle. But that is true only in the summer. In the winter it appreciates about 1/4 choke and no throttle. Once it's warmed up, if I don't give it a chance to cool off, it can be harder to kick so I sometimes push the start button.

    For my bike, a bit of after-fire is an indication of incorrect mix screw setting, once that is corrected cold starting is far easier. Bad mix is also indicated by not immediately going to correct idle speed when I throttle to a stop.

    Duane is correct. These things breath living air of varied moisture content at whatever air pressure on "any given day" at gawd knows what ambient air temp and wind. Everyone of these old bikes is different, every day.
    #10
  11. P B G

    P B G Long timer

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    Mine tends to fire up after just about a second cranking, but I turn the choke on as I crank, so it pops as I come through about 1/4 or 1/2 choke. If she's cold sometimes she cranks a bit longer and I get more choke or crack the throttle slightly.

    When you're used to old stuff you're used to finding the happy place.
    #11
  12. kraven

    kraven Hegelian Scum

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    Yep.

    And don't let it turn into some artsy fartsy thing. It's all logical scientific stuff.
    You just have to figure out, through trial and error, what your bike likes.

    This is why no two used ariheads are the same, functionally. Some only need the choke on long enough to get them running. Any more than that and they foul the plugs.
    Some need the choke on til they get some temp in the cylinders and heads.

    If you want to learn about airheads and tuning, you'll learn. 1 perfectly dialed in set of bings come from 1000 days of having them wrong, or not letting a pro do it while you learn the theory.
    #12
  13. Big John Sny

    Big John Sny Been here awhile

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    If you don't have a lot of experience balancing carbs, getting a vacuum measuring device is the right way to go. Something like the twin max. You should check the vacuum off idle. Small changes will affect the balance quite a bit. You can't really eye ball the amount of play, so the balance will be offjust from slight cable adjustments. At idle set the idle speed screws like you did so they are even. Then adjust out the mixture screw. It will increase the rpms at first and then start to make the cylinder run worse. As soon as the the idle first starts to drop, turn the screw back in a quarter turn or so. Do that for both sides. Then hook up the twin max. and equalize the balance with the idle speed screws (not mixture screws). Do this with your throttle cable way backed off so that you dont run out of free play and adjust the throttle just to have it held in place by the cable. Once you have both sides balanced with the idle around 1000-1100, then adjust the cables down close to where you want them. Leave enough freeplay that you can turn handle bars side to side without affecting idle. Then do the same vacuum test off idle. I do it around 2500 to 3000. It doesnt matter exactly where as long as it is a bit off idle. Of course all of this is done after the bike has been riden to warm it up. This should give you good results even if you have never balanced carbs before. If you are still having problems after that, good things to check are whether or not both needles are really at the same height and at the right level (I found mine had been moved up by the previous owner for some reason on my bike, at least they were even). also you can check the fuel level in each bowl by shutting off the fuel, removing the bowls and measuring the fuel depth to make sure they are equal. This will check the float height and make sure that one of them isn't sinking some. Make sure the choke is fully turning off on both sides.
    #13
  14. mykill

    mykill odd

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    They are carbs and are never perfect. The best you can hope for is getting them to run their best at your most common elevation, temperature and humidity. You will notice the deviations the most at idle, one needs to learn not to let it drive them nuts. You can adjust the speed to compensate for a bad mixture setting but this will cause popping on decel or lumpy transitions from idle to mid throttle.
    Getting the idle speed/ mixture right requires patience, a keen ear and a sense of rhythm. Best results are obtained after at least 15 minutes of riding and then keeping a fan blowing during the balancing. Slacken your cables and hook up your balancer, start and adjust the speeds to match vacuum, better a little fast at first, then work the mixture. You are looking for the fastest/ smoothest idle and you obtain this by slowly moving the mixture screw in or out, it will stumble either way, the happy spot is in the middle. After you are smooth on both sides then adjust the speed to your desired RPM, keeping the vacuum even. Then try the mixture again. If the factory calls for 3/4 turns out and you end up far from that then something is wrong. When you are happy with the idle you can adjust the cables and balance at part throttle.
    Do make sure that timing, valves, etc are all set properly with no intake or exhaust leaks and that cables are in good shape and chokes are properly adjusted. Needles and needle jets wear out and cause a rich condition off of idle and kill mileage and make for boggy acceleration.
    #14
  15. coastranger

    coastranger Been here awhile

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    I have ued the twin max for years, recently got the carb mate. better. bike runs even better then zero in with a twin max. hose connections to the unit are poor, but, when on, its right on
    and even with all the advice about shorting method, this is far better, for me anyway
    twisting wrenches since 1957
    #15
  16. Steptoe

    Steptoe steptoe

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    I haven't used gauges to balance an airhead for nearly twenty years. I used to balance two or three almost every day for a few years and you soon learn how to do it by ear and feel.

    On my own airhead i keep a screwdriver and 10mm spanner permanantly in the pannier and usually end up adjusting it every week as i feel it fall slightly out while riding it. It only takes a minute :D And i'm always fiddling/tweeking with friends airhead carbs whenever i see them.
    #16
  17. Rapid_Roy

    Rapid_Roy I'm tired.

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    I thought backfiring also meant the carbs could be out of sync, but I may be wrong.
    Ear tuner here.
    Last time I rode my '84 RT I increased one throttle cable gap to match the gap size on the other side and it ran awesome.I knew it was out of sync.:lol3

    I do everything at the beginning of summer, and by the end of summer, I am lazy.
    :evil
    #17
  18. supershaft

    supershaft because I can

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    Mixture screw settings greatly effect idle and just off idle. It also greatly effects off throttle cackling I call it whether or not the bike has EPA air pumps or not. All BMW's can backfire on deceleration via the mixture screw. Some of them IMO run slightly better when they do cackle. If you don't like it, just a tad bit richer than optimum will stop it. BMW spec turns are JUST a starting point. With every thing being near perfect, the optimum setting can be far from the recommended setting and the other carb can end up with a very different setting. Sure, different settings from side to side can be a sign of something not being right but not necessarily so. A lot of times air leaks from loose carb spigots, sucked in enricher body gaskets, or leaking valves cause idle issues. Float level also greatly effects idle as do a lot of other things.

    I think setting the mixture screw half way in between the stumble points works best. I call that a neutral setting. One carb might have a full turn from stumble point to stumble point and one carb might have one half a turn. This is common. An equally bit richer from both carbs' neutral setting would be 1 hour out on the carb with a full turn and one half hour out on the carb with a half turn. Does that make sense? Personally, I turn the screws quickly to help identify the stumbling points.

    Very often the stumbling point on the rich side (out) is hard to detect. Sometimes it seems like the mixture screw has little effect at all. It's usually a sign of leaking valves that can be accurately diagnosed with a leak down test. In these cases, I resort to the fastest idle setting or one half turn out from the lean stumbling point until something further can be done.

    Ear versus manometer or a gadget that mimics a manometer? A manometer is the ears BEST teacher. Period. (Even though mixture screws are best adjusted by ear.) The problem with a lot of manometers is that they are not damped properly. Yes, the damping on a manometer is adjustable just like on a Twinmax via different size holed plugs in the manometer's vacuum lines. Read the instructions!
    #18
  19. bikecat

    bikecat Long timer

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    Thanks for the overwhelming response and inputs.

    As a guideline, how much deviation will be reasonable from the 3/4 turns for the mixture settings? Just so that if I get to that point I know that either I'm doing wrong or that there is something else to look at.

    So my checklist:

    1. Ensure fuel level in carb bowls correct; valves, timing etc good.
    2. Start with stock settings, adjust after full warm up
    3. Adjust and balance idle via idle screws first, throttle cables slacken
    4. Adjust and balance via mixture screw, in-between stumble points
    5. Adjust and balance off-idle via throttle cable adjusters.

    Will go meddle and see how it goes this weekend. :)
    #19
  20. Beemerboff

    Beemerboff Long timer

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    When you check the fuel level take one bowl off when the other side is in place and full - that way the fuel doesnt drain out of the hose as much and you get a more accurate reading.

    I use a colortune to set the mixture screw - the best mixture seems just out from inwards stumble, but mid point as previously suggested will be better that just in from out.

    The popping could be a air leak in the joints in the headers, some have an aluminium gasket which eventually vanishes down the pipe.

    The simplest balance check is to raise each throttle cable 1/16 th of an inch - the change in revs should be the same each side.

    If the mixture is correct there should be just the slightest odelay before the revs move up too.
    #20