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Thread: R90/6 idle issues

  1. #1
    God? What god? roborider's Avatar
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    R90/6 idle issues

    Next on the list of my R90/6 semi restoration is a problem with the Bings and idle.

    I have set the points and timing on the 900, including new plugs and valve adjustment. Compression test was low, but OK at about 112 PSI per jug. Next step is the carbs/idle adjustment and balancing.

    These are Bing CV carbs.

    Manual says to turn the idle mix screws out one turn, and the idle set screw (RPM) one full turn in after contacting the butterfly tab (that is, cracking the plates by one full turn). Next is to start it up and disconnect one spark plug at a time to adjust one carb at a time.

    Problem is, if I disconnect the right plug, the engine dies immediately. The cylinder is working though because if I rev the engine and then pull the plug wire, it will run on the one jug, but dies at idle. So, I conclude I have an idle circuit problem on the left cylinder. (It will idle fine if I disconnect the left spark plug, but dies if I disconnect the right spark plug.)

    I'm thinking about just rebuilding the Bings and going from there. But before I buy a kit and tear into them, I figured I'd ask if you think I'm missing something. Maybe just varnish in the idle passages or something like that.

    Thanks.
    Rob C. , Raleigh, NC
    '10 R12RT, R90/6
    2007 CBR600RR & 09 V-Star
    Suzuki DR 350

  2. #2
    Liaison 20774's Avatar
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    Rob -

    I highly, highly suggest that you don't pull the spark plug cap anymore. This can potentially do damage to the coil. Hopefully you're OK, but at the very least you may have taken some life off of the coil.

    There are three ways to balance the carbs using some "tools". Two of them operate off vacuum - either the $4 manometer or something like a TwinMax - there are other vacuum devices, but these are the most common. These don't involve shutting off the cylinders.

    The third way involves shorting the spark plug to ground. This way, the spark energy goes to ground rather than to the tip and jumping the gap to ground. Snowbum discusses this method. It involves using a spoke or extension between the end of the plug and the inside of the cap. Using the exposed metal extension, you take a plastic handled screwdriver and firmly touch one end to the engine fins and the other to the metal extension. Now that the spark energy is flowing to ground on that cylinder, the other cylinder is running and you can then note the RPM. Removing the grounding screwdriver, let the engine come back to normal, and then short the opposite cylinder. Note the RPM.

    Things you must remember to do...have about 1-2mm of slack in the throttle cables at the knurled adjuster to the carb. You mentioned the idle mixture and throttle setting screws. The synch is an iteration on three things:

    - idle mixture
    - idle speed
    - throttle cable tension

    I'd suggest reading Snowbum's page to get the details.

    http://bmwmotorcycletech.info/synchcarbs.htm
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  3. #3
    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    On the presumption that your idle jet is clean, sometimes one turn from full in on the idle mixture screw is not enough. I recommend. getting the engine started with the settings you have, and warm it up for a couple minutes. Get at least one fan and blow air on the cylinders. Then, equally adjusting the idle SPEED screws, maybe a quarter turn at a time each, raise the idle speed to ~ 2000RPM. Then adjust the idle MIXTURE screws until you see a peak on the tach. Then back the MIXTURE screws out maybe 1/8 turn further. Then reduce the engine RPM to your desired level by turning the idle SPEED screws equally and balance the carbs. You can only get a decent balance with the engine warm, doing it while warming up means you will just have to repeat the process.
    F.O.G.Rider, Rounder #6,
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  4. #4
    God? What god? roborider's Avatar
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    I have printed the pages from Snowbum, Largiader, and another source I forget, and the manual.

    I do use a fan. I did not know over ever hear that a coil could be damaged by disconnecting it, only electronic ignition. But I'll ground it from now on since that is quite simple (Actually, I usually instal an old plug and let it fire outside the engine).

    I do have a twin max that I use on the RT.

    Since I don't know anything about the carbs, and since the tank had a lot of particulate rust in it (cleaned and installed inline fuel filters now), I will rebuild the carbs and read all this material while I wait for kits and floats. I'll report back, thanks.

    Oh, my tach needle seems somewhat bouncy, not that precise. Anyone use a standard auto tach meter on theirs? I have one I use, but have not connected it yet.
    Rob C. , Raleigh, NC
    '10 R12RT, R90/6
    2007 CBR600RR & 09 V-Star
    Suzuki DR 350

  5. #5
    James.A
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    Quote Originally Posted by RoboRider View Post

    Oh, my tach needle seems somewhat bouncy, not that precise. Anyone use a standard auto tach meter on theirs? I have one I use, but have not connected it yet.

    You are riding a 2 wheeled farm tractor.
    Don't worry about it.
    If it's gonna break, it's gonna break. In 30 years it won't matter much anyway.

    You would do well to get a feel for the bike.

    " use the FORCE, Luke"

  6. #6
    God? What god? roborider's Avatar
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    Good news. I took the carbs off to rebuild, and what the heck, I swapped them to opposite sides and the problem followed the carbs. Just for fun, I messed with them and got it to idle quite well, which was nice, at about 850 RPM (using my digital auto tachometer). Then I took 'em off and tore them down. Yeah, they're dirty, so a rebuild was in order.

    Farm tractor, I like that. I had a 75/5 that ran really well. The 90/6 runs great too (didn't idle, but ran at speed nicely), it just doesn't stop worth a darn!
    Rob C. , Raleigh, NC
    '10 R12RT, R90/6
    2007 CBR600RR & 09 V-Star
    Suzuki DR 350

  7. #7
    God? What god? roborider's Avatar
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    Just FYI a carb rebuild and adjustment did the trick.
    Rob C. , Raleigh, NC
    '10 R12RT, R90/6
    2007 CBR600RR & 09 V-Star
    Suzuki DR 350

  8. #8
    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    Realize, of course, that you have some of the original anti-lock brakes.

    I like to tell folks that. if I need to make an emergency stop, I hope I've made an appointment. There are things you can do to improve your brakes, but just ride within the capabilities of what you have.
    F.O.G.Rider, Rounder #6,
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  9. #9
    God? What god? roborider's Avatar
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    That's funny, cause I told my buddy the same "anti lock" joke. Actually, they do stop, but you have to have a very strong hand. I am replacing the very worn front rotor with an EBC rotor and pads. I already have stainless lines. I will rebuild the master and caliper if needed, but neither leak and seem to work OK. I think the new pads and rotor will make it better, which will be good enough. I'm not investing in the dual rotor upgrade for this "put around town keep it under 50mph" scoot.
    Rob C. , Raleigh, NC
    '10 R12RT, R90/6
    2007 CBR600RR & 09 V-Star
    Suzuki DR 350

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