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Thread: New 75/5 and questions

  1. #1
    sneakers563
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    New 75/5 and questions

    Hi all,
    After months of lurking and scheming, I finally pulled the trigger and bought my first "proper" motorcycle, a '71 75/5. It's been sitting since 2002, so it's going to need a bit of work, but the price was right and I've been looking for a little bit of a project. Needless to say, I'm pretty happy.

    Besides wanting to announce the acquisition of my new baby, I was hoping that the good people here might be able to give me a little bit of advice/help too. The previous owner made a few modifications to the bike over the years (he owned it for 38 years, bought it new when he was 19), and I'm trying to decide which to keep. In general, I lean towards stock except in cases when there's a clear benefit, or reverting to the stock part would be overly expensive. Because this is my first BMW, I'd like to stay fairly close to what's in the Clymer manual.

    So, apart from fairing and seat (which are definitely going), these are the mods that I'd like your thoughts on:
    1) The points have been replaced with a Dyna III electronic conversion kit. This is one that I'm leaning towards keeping, but I do have one question: I'm planning on running compression tests on the cylinders. My Clymer manual says that bikes with electronic ignitions should not be turned over with the spark plugs disconnected. It recommends disconnecting the ignition unit to avoid damaging it. I assume that's the case here as well, but I have no idea what needs to be disconnected. Can anyone look at the picture below and help me out? Would disconnecting the battery be enough?

    2) Something called the "Road Rider Air Fork Kit" was added to the fork. Anyone know anything about this and whether it's worth keeping? As far as I can tell, it's simply some new fork caps, compression fittings and a gauge added to the existing fork to allow additional air damping. The rest of the fork remains more or less the same (oil, etc.), so it should be fairly easy to remove.

    3) The carbs have been swapped for 36mm Mikunis. I haven't found anything that looks like a model number anywhere on the body of the carbs, so I can't confirm the size and I don't know what model. My only real concern with these is that I don't know how to properly balance them. Again, the Clymer manual says not to run the engine with one plug disconnected if you have electronic ignition, and there doesn't seem to be a vacuum port on the carbs. Any tips?

    Thanks, looking forward to any help you all can provide.

    -Steve
    Attached Images Attached Images

  2. #2
    Manfred
    Guest
    Regarding compression test: If you disconnect the battery, you won't be able to crank the engine adequately for the test to mean anything. The best route is to securely connect the spark plugs to the frame, providing a ground. That way you won't ruin the electronics.

    Regarding balancing the carbs - I know it can be done by spark plug grounding (long, detailed explanation here: http://bmwmotorcycletech.info/synchcarbs.htm) but I've never used that method. Without vacuum ports (if that's truly the case), I don't know of another one.

  3. #3
    sneakers563
    Guest
    OK, I figured I could use the kickstarter to crank it for the compression test, but I can see where that might be tough.

    I'm attaching a pic of the carb if anyone can help me ID it. The previous owner told me there wasn't a vacuum port so I'm going by what he said. He said he balanced them by judging the tension on the throttle cable, which seemed kinda imprecise to me.
    Attached Images Attached Images

  4. #4
    dsenn
    Guest
    VM roundslide.

  5. #5
    Administrator 20774's Avatar
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    I have the Dyna III on my /7. To do the compression test, just take out the spark plugs and reinsert them into their caps. Get some rope or something non-conductive and lay the spark plug threads down so they touch the engine finds using the rope to secure them. You'll be in good shape to do the compression tests. You need to provide a path for the spark to go to ground otherwise you could damage the electronic control unit.
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    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  6. #6
    advrider.com
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    Quote Originally Posted by sneakers563 View Post
    He said he balanced them by judging the tension on the throttle cable, which seemed kinda imprecise to me.
    That's only the first step of a balance.

  7. #7
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    Without a vacuum port, your carb tuning process will be different than the 'usual' one for airheads with Bing CV carbs. But, if you google "Mikuni VM carb" you'll find lots of info on the 'net. Assuming that the PO selected the right jets and needles, you'll mostly be just making sure the float level is correct, cleaning things up, and adjusting the air screws on these, after making sure that the cables are properly adjusted. But, I am definitely not an expert on these type of Mikunis
    BMWs in my garage: 1982 R65LS, 1978 R100/7

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