'78 R100/7 Racing Idle where to begin?

Discussion in 'Airheads' started by swanker, Jul 25, 2012.

  1. swanker

    swanker Shadetree Tinkerer

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    Jul 21, 2012
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    Hey guys, first post and my first bike that requires me to get the tools out and actually maintain it but I am handy so not put off by getting my hands dirty.
    I recently got a nice '78 R100/7 from an old biker guy who seems to know his stuff and appears to have maintained it decently.
    I'm having some high idle problems and struggles at higher speeds.

    After about 20 minutes of highway riding during the 75 mile trip home after purchase (98 degree day) I noticed that the bike would struggle to get above 60-65 (per car's speedo following me not the bike's) but had great power and acceleration at lower speeds.
    I've read a ton of posts about holey diaphragms, opening the gas tank etc..
    One of the carbs was recently rebuilt by the PO and I have it on my short list to rebuild the other (Clymer book came with the bike).
    Also, after a short period of riding the engine races from between 2000 and 3000 rpm.
    I've read at http://bmwmotorcycletech.info/Ignition.htm that a bad or stuck ATU on the ignition(?).
    The symptoms of the idle problem sound like this is the problem although I don't think I have the 'canister' as this is a '78 and the post mentions it is most common with those types of points assemblies.
    Two other symptoms are turning the handlbars to the right causes the idle to increase and to the left decrease (assume I need more slack on the throttle as I don't see anything physically restricting the cables).
    The last problem, which might point to where I should begin, is that at idle if I pull on the throttle cable on the right carb that cylinder sputters and bogs down.
    I checked the plugs and the right plug seems ok, tannish tip no oil or carbon. The left plug was oily and cleaned up pretty easily.

    Clymer says to make sure timing etc. is correct before adjusting idle, is that necessary as that seems to be more of an involved procedure and I dont have a strobe yet.
    I've inspected both floats and bowls and they seemed fine to me, not full of gunk and the underside of both carbs looked relatively clean.
    The PO gave me an electronic ignition that I was thinking of installing as I'm not planning on riding this on any tours so the main benefit of points is somewhat moot (easy roadside repair).

    I'm in Columbus Ohio and plan on checking out Re-Psycle sometime soon but if there are any other buckeye airheads that would be willing to share some maintenance time with me that would be most appreciated and I'd be sure to have plenty of beer!


    swanker
    #1
  2. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Welcome to the forum swanker!

    I'm guessing a full tune up and carb clean are what's needed.

    After sitting (the bike hasn't run in a while, right?) you'll need to go through the settings and ensure everything's right. Carb o-rings could be old and cracked. The atomizer area might need cleaning.

    And then a proper carb tuning and synch will probably do it.

    All simple enough, but take your time and get to know the bike. Others will recommend a complete carb rebuild, but I don't replace anything that isn't broken, and BING charges a fortune for a tiny little package of parts. Usually it's just the o-rings.
    #2
  3. Biebs

    Biebs BMW Airhead

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    What did the PO do on cleaning Carb??? full tear down and clean?? This is a definite Carbs need to be cleaned correctly problem.
    #3
  4. swanker

    swanker Shadetree Tinkerer

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    Thanks for the welcome and quick reply.
    The PO said he used it as a somewhat daily (he also has a newer Triumph touring bike so figure this got most use) but regardless, it was supposedly ridden about weekly this season.
    As far as the BING parts, the PO gave me a bag with o-rings and the diaphragm for the carb he didn't get to.

    Noob question, but what is the difference between a good carb cleaning and rebuild?
    I would think a good cleaning means a teardown and dip (sans rubber parts). Or is it really just removing opening up and spraying it out a bit with cleaner and air?

    Noob #2, "tuning and synch".
    Tuning meaning adjustment of idle and mixture screws as well as throttle cables?
    What do you mean by 'synch'? Getting both carbs to fire at the same time?

    thanks
    #4
  5. supershaft

    supershaft because I can

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    It could be anything but chances are it isn't. Chances are all you need to do is slow down the idle speed and readjust the mixture screws. It's rarely ever the ignition advance bean can or not. In my experience is is more likely loose carb spigots versus a sticking advance but . . . .

    Struggling above 65 IS a problem. Water in the float bowls? Ignition timing? It could be a lot of things. Good luck!
    #5
  6. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    "Seems" and "appears" are meaningless when you've got running problems. A new-to-you bike needs a
    "full tune up and carb clean... and then a proper carb tuning and synch" just like 'Wire said. You need to go over everything and "zero-out" the scheduled maintenance. Start in by getting a shop manual (Haynes is the better choice) and searching this ADV forum. Asking questions is an important step, too. :)

    It's the difference between "fast" and "half-fast". Do it right, now, and it won't be an issue later on.

    Welcome to the Asylum... :D
    #6
  7. Kai Ju

    Kai Ju Long timer

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    Don't forget to make sure that the intake spigots are tight in the heads, especially since what you're describing sounds suspiciously like an air leak.

    Also, the right cylinder bogging down when you pull on the cable sounds like a torn diaphragm. Which would also explain the low top speed since it's only running on one cylinder.
    #7
  8. Padmei

    Padmei enamoured

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    Hey Swanker
    I found Infras writeup really good when I rebuilt my carbs- scroll down to it. I actually felt so ashamed of my workshop while reading his thread I ended up spending my first hour or so cleaning up:lol3

    http://www.advrider.com/forums/showthread.php?t=431153

    I was a noob & still am at the mechanicals however I can strip & rebuild a carb in about 15mins now- once you've done one they're so much easier. My only advice is to take a few pics before you open it up of the choke levers etc as I & others I know have put them back on the reverse way.
    The other thing that I found (&others) is that after cleaning the choke dial it was stiff to turn when back in position. After being used for a while it became easier to turn so maybe something happens to it when it dries out.
    #8
  9. swanker

    swanker Shadetree Tinkerer

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    Holy crap! You ain't shittin' when you say it's "really good", what a saga.
    Thanks for the link.
    #9
  10. Wirespokes

    Wirespokes Beemerholics Anonymous

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    A rebuild means replacing stuff like floats, diaphrams, needles & needle jets, gaskets and o-rings, pulling the butterflies and replacing the o-rings behind them.

    A good cleaning is usually all that's needed - the best method is disassembling and soak in an ultrasonic. Otherwise, ensure all passages are clean and flow carb cleaner when sprayed through them. Be sure to clean the area around the atomizer (that's where the needle passes through the floor of the venturi).



    If you think of these engines as two separate one cylinders siamesed at the crank, you'll get the idea what it's like tuning these things. The carbs need to be in synch with each other, otherwise one is dragging the other along which robs horsepower from the whole.

    You want both sides tuned to run on the proper mixture, and start out with the butterflies each opened the same amount. Then you want to make sure both cables pull the same so one isn't lagging.

    That's essentially it, and there are different ways to get there. But when it's done, it should idle thump thump thump pretty evenly and rev easily with little or no vibration.
    #10