Mix screw adjustment

Discussion in 'Airheads' started by headtube, Oct 2, 2012.

  1. headtube

    headtube 6 mesas de invierno!

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    As I'm waiting on the Clymer manual to arrive I was wondering what the setting on the mix screw is for a 79 R100 RS? Any tips on syncing the carbs without special tools? Thanks.
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  2. bmwrench

    bmwrench Long timer

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    Start at 3/4 turn from the seat. Move the srew in and out until you find the sweet spot-where the engine idles fastest. Synching can be done without vaccum tools or shorting devoces-you can make your own from an old spoke or two-but it takes practice.
    #2
  3. disston

    disston ShadeTreeExpert

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    Does your bike have 40 mm carbs? Are there any numbers on the sides of the carbs? What are the numbers on the carbs, please?

    40 mm Bings all start the mixture screw 1.25 turns out.

    Is this bike new to you? Do you know if the ignition is stock or it's running an after market conversion?
    #3
  4. headtube

    headtube 6 mesas de invierno!

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    This bike is new to me; a first time owner of BMW. My carbs are 40mm. However, the previous owner molested the bike... ALOT. Hence the carb numbers 94/40/107 and 94/40/114. Not the best combo but the bike does run and idle fine.

    The ignition is electronic. Again... according to the PO, it was stock as of 79. I have not looked into this yet.
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  5. Airhead Wrangler

    Airhead Wrangler Long timer

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    '79 had points ignition in a "beancan." The electronic beancan didn't come around until '81. You're PO is a bit off. I'd pull the front cover and see exactly what you have. The points can is easy to recognize as it has a condensor mounted on the outside of it and spade connection with a single wire running to it. The later electronic can has a two-conductor wire running directly out of the can to a connector block abount 6" away. Also, the points can has plastic breather tube running about 8 or 10 inches up the left side of the alternatory. These are often missing though, so don't go solely by that.
    #5
  6. disston

    disston ShadeTreeExpert

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    They are both 40mm carbs. The 114 is an RS carb with a needle jet size 2.66 ; the 107 is an S or RT carb with a needle jet size 2.64 ; Other than that difference the two carbs appear to be identical. That is comparing the jet and other sizes listed in the Bing manual. In operation this small a difference in needle jet size would not be noticeable, I think.

    Both jet needles and needle jets are wear items. The needle jet is the jet that sits on top of the stack in the center of the carb. The jet needle hangs into this jet. I know it's confusing but we are talking about a needle called a jet needle and a jet called a needle jet. The jet needle rubs a bit against the needle jet and this causes wear as the needle is rubbing up and down. Bing recommends changing these two parts at only 25K. Some of the mechanics have much higher mileage expectations from these things. I have changed the two pieces in my R90/6 and with, maybe, 35K on them I thought there was a small improvement. But really it's something that is hard to feel with the Butt Accelerometer.

    The PO may have already done this. If not just get both sets so you have matching carbs. On most parts fisch you can find the numbers for the size jets you want. The needles BTW are the same here and they are both on notch #2.

    Some other numbers you may want to check;

    Main jet 160
    Idle jet 45
    Idle mix starting point is 1.25 turns out
    Clip position is #2

    There are other things mentioned in Bings manual but these items are sometimes messed with by owners. The items above, like I said, are the same in both carbs, or should be.
    #6
  7. disston

    disston ShadeTreeExpert

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    We are concerned with what kind of ignition you have but the balancing the carbs basic procedure with out any "expensive" tools is the same whether it's electronic or ignition points.

    Just a little note to begin with. You should do the entire tune up procedure. (I suppose you have changed all the oils and fluids?) The tune up starts with setting the valves. The numbers are 0.006 intake and 0.008 exhaust. Set cold. (I use 0.10mm intake and 0.20mm exhaust)(I don't know why. Everybody else likes the .006 & .008 numbers)

    Next is the points dwell or gap and timing. Timing should be set for idle and then corrected for full advance. There is a dot on the flywheel for the full advance setting. You need a timing light to do this. Or you can get by with a test light to set the ignition for idle.

    After these items are attended to or you are satisfied that it's really time to just balance the carbs anyway you will need one cheap other tool. These are a set, pair, of small, 4mm, studs used for shorting out the plugs and setting balance. You can either make this tool for about $3.00 or you can buy it at Northwoods Airheads for $13.00

    http://www.northwoodsairheads.com/Tools.html

    I think you'll find the tool on that page. Here is what it looks like;

    [​IMG]

    At your local neighborhood hardware store they have a section of nuts and bolts. They should have this small 4mm screw there. I found two sizes. One was 60mm long and one(for a lamp or something) was 70mm long. The longer one is better but a little shorter will still work. Take the ferrule that came with a new set of spark plugs with you to the hardware store to help find the screw. This is the thread size of the end of the spark plugs that the wire caps clip onto. Get two screws and two nuts. Cut the head off the screws, use the nuts to lock the ferrules to one end of the remaining stud and you now have a pair of shorting rods to balance your carbs.

    Or just buy them. I think half the people that I give this info to just buy them any way. For some reason I think you are going to make yours. I'll try to be back later to do the stand up routine of using your balancing tools but right now I have to go out side and rebuild the emergency brakes on my car.
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  8. headtube

    headtube 6 mesas de invierno!

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    Great info guys. I set the mix screw to 1.5 turns. I have yet to give the bike a good ride. Up and down my street is all. Just off idle the bike seems sluggish, but when it gets to 3000 RPM's it goes well. Almost feels like a turbo kicking in?? Blipping the throttle produces low end vibration/shaking. I'm thinking this is not normal.

    I will indeed make that shorting tool. Thanks Disston.
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  9. Solo Lobo

    Solo Lobo airhead or nothing

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    Sounds like it may be only running on one unil you get up into to rev's a bit and the second chimes in.
    #9
  10. Uncle Pollo

    Uncle Pollo Long timer

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    Sluggish from bottom ... define sluggish.
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  11. headtube

    headtube 6 mesas de invierno!

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    Difficult to define. A kind of bogging down but not so much that it stalls. I don't believe it's running on one cylinder momentarily until it reaches higher RPM's. It may be a carb issue. If I have (as disston says) two different Needle Jets than that may be it, because it only occurs at the lower end of throttle. I will order new ones of the same size, plus new Needles. My next move is to adjust the valves.

    BTW... Main Jets are 165 - not 160 as they should be.

    On a side note. I bought this bike "uncertified" for what I thought was a decent price. Why do previous owners always bugger things up by adding OR subtracting from a perfectly good motorcycle? Today I discovered that I have the wrong center stand installed. :becca
    #11
  12. disston

    disston ShadeTreeExpert

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    So now you know the PO did fool with stuff. He may have known what he was doing and he may not. With the larger main? Something else needs to be lessened usually. I'd check all the jets.

    I don't have any advice really on which jets you should use. Some mechanics do play with this stuff a lot more than I have and they have experience with more than one bike. My only real concern is that what ever is done to one cylinder, the same should be done to the other, carb.

    I'll try to post a tutorial for you about the shorting method of balancing the carbs. May be what you need, can't hurt. I'm going to sleep soon though so I'll get to it tomorrow I think.
    #12
  13. disston

    disston ShadeTreeExpert

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    I'll try to make this as basic as I can but I get sidetracked.

    Motorcycle on center stand.

    In the old days we had a simple method of balancing the carbs on an Airhead. We pulled the plug wires off the plugs and rested them on top of the plugs so they still made contact but could be lifted to break contact. What we are going to do will have the same result but we are no longer allowed to have an open secondary in the ignition circuit. If the bike has electronic ignition this can and probably will cause several hundred dollars in damage but even if the bike has the old points and condenser we are told it may cause damage. Therefor we have made the shorting rods and with their use either side can be stopped from firing but instead of an open circuit the secondary fires into ground and no damage is done.

    The ferrule from the original plug package is threaded onto the end of the screw and held in place by an extra small nut. Enough threads in the ferrule are left so this can be screwed onto the spark plug. It should be tightened a little because the bikes vibration can cause it to lose contact or fall off. The plug wire can be attached to the other end of the small threaded rod. The bike will run like this on the center stand rather well. During the process if you want to take a spin or something I recommend taking the rods off because they can become loosened on the road and are hard to deal with while negotiating traffic (don't ask how I know this).

    The bike was previously warmed up with a 10 to 20 minute ride. With the engine idling a screwdriver is used to short either cylinder's plug to ground and the running cylinder on the other side is noted. The bike may continue to idle on one cylinder or it may die. It should sound the same, same rpm, same number of thumps before dying. To balance the carbs use the idle speed screw. You are looking for balance.

    Total rpm or idle rpm is only a function of both carburetors working together. If you have the carbs balanced but idle to low then both need to be raised.

    Idle mix is much harder to explain and I have trouble with this. I use the starting number of turns listed in the Bing book. If I am having a racing idle that is too fast I probably have them set too lean. Very small increments of movement seem to be a lot of change sometimes. I prefer to err on the side of too rich rather than too lean.

    The shorting rods are used for setting idle balance. The carb cables need to be balanced also. This is balance for everything above idle. Idle is only when the throttle is completely at rest and there is no play of the throttle cables. Once the throttle is twisted and the cables engage idle is over and has no bearing on what the engine does. To balance the cables I watch both carbs at the same time to see their butterflys open at the same time. OK, I can't watch both carbs at once but I'm just trying to say that is the idea, that both butterflys open at the same time. There should be some slack in the throttle cable. If the part of the cable where it goes into the threaded screw on the carb top is pushed to the side or lifted out of the recess there should be the same amount of slack in the two cables. Sometimes a bit of unbalance in the cables is hard to pinpoint because they don't seem to be the same internally or one of them seems kinked in the sheath. It can take a lot of fussing to get all this right.

    There is another recent thread where I tried to cover all this recently. It may be just another telling of this story and it may be a better or a worse version but I refer you to it just for redundancy so I hope it helps.

    http://advrider.com/forums/showthread.php?t=821078

    Takes you to the start of this thread. My telling of the carb balance method is post #240. Also check #243 and #248. There is also a lot of other info in Tete's thread about timing and other stuff.

    I hope this helps to get you started. Next Spring I want you to tune my R90/6 for me.
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  14. headtube

    headtube 6 mesas de invierno!

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    Great stuff disston. Thanks.
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  15. chollo9

    chollo9 Screwed the Pooch

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    Yeah, good stuff. My little tip: I lay one finger on one butterfly arm while I watch the other, sight and feel seem to register pretty well together inside my little brain. Hell, this is probably how everybody else does it too, right?
    #15
  16. disston

    disston ShadeTreeExpert

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    Nope C9 I never heard that one before. I'll try it.

    I try to feel the twist grip moment of contact. So I watch one or the other butterfly lever and see if I can feel the moment of opening in the twist grip. Then I look at the other butterfly lever and see if I can find the same point of feelingb the opening.

    You can also just check for the same amount of free play at the adjustment screws on the carbs. Well that may just be the initial setting.
    #16
  17. Uncle Pollo

    Uncle Pollo Long timer

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    Take appart carbs and measure everything and clean everything. Set valves and clean filter .... start from Veronica. Balancing the carbs by ear is the easiest thing to do. Engine off ... see that both open at the same time. It will be pretty close when you start it. It is not the voodoo you fear. Unless you are absolutely hearing impaired they will be ok.
    #17
  18. Kt-88

    Kt-88 I like everything.

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    Mine does this when cold all the time.
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  19. Padmei

    Padmei enamoured

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    While accelerating I find my bike accelerates smoothly until I reach a threshold at about 4200revs then all hell breaks loose & it takes off. Ok bit of exaggeration there but it does feel like a powerband at above 4200rpm. Could this be what you mean sluggish - the lag in acceleration till that point?
    #19
  20. boxerboy81

    boxerboy81 Stay Horizontal

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    Snowbum describes a problem with the ICU as one of the possible causes of this symptom, but I can't remember which version of the ICU it was and which model it was fitted to. If the carb and aux starter cables are all good, then it may be worth considering the ICU.
    #20