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Thread: Synching Carbs with Smartphone

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    Administrator 20774's Avatar
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    Synching Carbs with Smartphone

    I ran across this on another forum. We know about various ways and tools to help synch the carbs. The other forum poster likes to use the shorting method to synch the carbs. This requires looking at the RPM of the running cylinder and comparing that to the RPM when the opposite cylinder is shorted. You make adjustments to get the RPMs the same when one cylinder is shorted.

    He said that he uses an app on his phone that gives BPM or beats per minute. Something he uses for his music. BPM can essentially RPM and might be another way of comparing the performance of each cylinder during this synching process. This might be especially useful for older bikes with no tachometer.

    The Android app is called "BPM Tap". I haven't tried it myself, but have downloaded the app. I'll be interested to "calibrate" it with my /7 and see what it tells me about RPM on my R69S.
    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!

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    Quote Originally Posted by 20774 View Post
    when the opposite cylinder is shorted. .
    What does that mean, Kurt?
    Is the other cylinder cut off from igintion?
    If yes, it would run as an air compressor.
    I would have my doubt, that this is a good way to sync....?? In effect, you are putting a load on the running cylinder. I would not expect it to idle at the same rpm as under no load???

    I have the feeling that playing with apps and motorcycling don't mix

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    #4869 DennisDarrow's Avatar
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    The "shorting" method is a VERY commonly used method by LOTS/MANY folks without a tach. Even with a tach..../5 on it is a totally valid and REAL method.
    Am NOT gonna get into the actual "shorting" methods as there are many and some folks say one must do certain ways to avoid burning out coils or electronic ignitions............THE FACTORY owner's manual for /2, /5, and /6 outline the method as a matter of fact........

    Let's say that you have the bike idling normally at 900 RPM. When you short out the plug on one side the engine goes chug a couple of times and then dies.............When you do the other cylinder it keeps running at let's say 400 RPM.............The second cylinder needs to be lightened a bit or the first one increased until you get both sides doing the same thing and the idle when both are running is acceptable.

    The FACTORY recommended that shorting and chug chug die is what you are looking for............God bless........Denns

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    Sorry, but this reads to me like the English assembly instructions of an early 60s Japanese model kit. I still don't know what "shorting" means.

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    Quote Originally Posted by EMSimon View Post
    Sorry, but this reads to me like the English assembly instructions of an early 60s Japanese model kit. I still don't know what "shorting" means.
    Shorting means giving the spark energy an alternate way to find ground other than running through the plug and across the gap. You're basically causing that cylinder to stop working for a short period of time. In the /2 days, you could pull a spark plug cap...you could do this because there was a safety gap in the magneto system which gave the spark energy another way to jump to ground. For the /5-on bikes, this is not a good thing to do as there is no way to "release" the energy and it reverberates within the wiring and can harm the coils.

    Shorting is done by providing an extension to the outward part of the spark plug...usually a small rod or even an old spoke...on the spark plug side you can thread on the small removable caps that come on some spark plugs. With engine running and this extension in place with the high tension cap on the other end, you can get a plastic handle screwdriver and securely put one end on the engine fins and bring the shaft of the screwdriver up against the extension. Perfectly fine to do...the spark just takes the path of least resistance to ground. It's important to firmly and quickly move the screwdriver on and off the extension...thus no or limited changes for the spark to actually jump.

    Since the Airhead engine is essentially two separate motors connected by a common crank, the idea is to short one cylinder and see how well it runs on its own. You check the running RPM of one cylinder. Release the short, let the engine clear itself, then short the opposite side. Note the RPMs. Then you go about adjust the carb settings to get the RPMs to match. I think one should slow down the fast running cylinder rather than to try and speed up the slower cylinder. The reason for this is that by speeding up the slower cylinder, you take any slack out of the initial setup which might affect basic idle. When you're done doing the idle speed, the resulting idle might be too high or two low (should be somewhere near 1000 to 1100), you can easily turn each idle speed screw a tad either direction the same amount. If necessary, you can go back and reshort to see if the one-cylinder only RPMs are still the same.

    This shorting approach is used to set idle speeds as well as throttle cable tension. The idle mixture, the third part of synching, is to just get the best running RPM for the cylinder under adjustment, plus a tad richer.

    I was suggesting that if you didn't have a tach, you could use the smartphone app to indicate the "beats per minute" of the engine sound...it might not be actual RPM, but is a relative number that you can compare left to right.
    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!

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    Quote Originally Posted by 20774 View Post
    I was suggesting that if you didn't have a tach, you could use the smartphone app to indicate the "beats per minute" of the engine sound...it might not be actual RPM, but is a relative number that you can compare left to right.
    How does the phone app "hear" the "beats per minute?" Just through standard speaker/microphone? If so, is it simply acting as a tach? Counting RPM's? One "beat" equals 2 RPM's?
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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    Quote Originally Posted by jimmylee View Post
    How does the phone app "hear" the "beats per minute?" Just through standard speaker/microphone? If so, is it simply acting as a tach? Counting RPM's? One "beat" equals 2 RPM's?
    Jimmy -

    I haven't even used it yet...just downloaded it. I'm sure it uses the microphone to count the beats...I've heard of cellphones being used as medical devices in far corners of the world...I'm sure they're used to get a person's pulse in the same way.

    Edit: It might actually use the flash/light to go through the skin to monitor blood pulses.

    When I get the chance, I'll fire up the /7 and use the app to count the "beats"...then I'll look at the tach and see what kind of number I'm getting. On the one hand, for using it to synch the carbs, the actual basis for the number if irrelevant...it's just something to compare with. But if there seems to be a relationship between beats and RPM, I could use it to get a sense of what the idle RPM is on my other two bikes.
    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!

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    Quote Originally Posted by 20774 View Post
    Jimmy -

    I haven't even used it yet...just downloaded it. I'm sure it uses the microphone to count the beats...I've heard of cellphones being used as medical devices in far corners of the world...I'm sure they're used to get a person's pulse in the same way.
    What the technical world can do now? WOW Quite remarkable.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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    JoeDabbs
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    I've always used the shorting method (actually, I just unhook one of the plug wires at a time) since my /6 was new. I do have a tach. It's never goofed up anything on my bike as far as electrical concerns, though I've heard that other years require care and use the plug with the added post on top to be safe.
    Joe Dabbs
    2011 RT
    1975 R60/6

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    O.K., now I see what you are doing. You comparing the rpm of the running cylinders which each other. When I read it the frist time, I thought you were trying to match the rpm of the running cylinder before and after "shorting" the other.
    That makes more sense.

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    shorting method

    The shorting method is well described in an article on my website,
    http://bmwmotorcycletech.info.
    In something like 40 years or so of using this method (I also use other methods, including vacuum gauges, depends on the situation and the bike) I have NEVER EVER looked at the tach, or wanted to know the rpm difference between cylinders.
    All you need is your EARS, unless you are totally deaf.
    The shorting method is THE best method for all the Airheads.
    You must do it correctly, however.
    http://bmwmotorcycletech.info/synchcarbs.htm

    Use the tachometer to check on the final rpm, before synchronizing the length of the throttle cables.
    Don't use too low an rpm on /5 and later Airheads.
    snowbum

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    Quote Originally Posted by snowbum1 View Post
    All you need is your EARS, unless you are totally deaf.
    I've heard both you and Duane Ausherman the same thing...maybe I am deaf because I'd rather look at the tach or my TwinMax to know that it's balanced. My ears haven't been around these bikes long enough to hear the subtle differences.
    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!

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    Registered User JGREGOR1.JG's Avatar
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    I use stix. works fine. Before that a row of vacuum gauges for multi's and shorting for twins.
    Smart phone should work. Android ears!
    Jim Gregory
    92 R100RS
    05 Sportster Roadster

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    Quote Originally Posted by snowbum1 View Post
    All you need is your EARS, unless you are totally deaf.
    The shorting method is THE best method for all the Airheads.
    This is the method I learned.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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    Nick Kennedy
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    Gents
    With no disrespect to anyone, I offer a challenge to the shorting method.
    Balance your carbs with the shorting method, go ride a few miles.
    Check it and maybe redo it, again.
    Go ride a few more miles and check out the feeling, throttle response and vibration.
    Then,
    Hook up a mercury or hydraulic fluid stick or vacuum gauge and see you how you did.
    If needed readjust the idle mixture, cable length, etc. and go ride.

    One of my main riding partners is a airhead techno guru from the early 60's, and owns a mercury stick carb balancer.
    He taught me to very carefully adjust my valves to match exhaust to exhaust, intake to intake exactly and exactly balance the carbs, using a Instrument you can read exactly.
    Airheads are really rough as guts and will run wildly out of tune, but getting the intake balanced precisely is a good thing IMHO and I don't feel the shorting method, while it is fair baseline, is all that great.

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