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Thread: 1978 R100/7 suspected fuel/ignition problems

  1. #1

    1978 R100/7 suspected fuel/ignition problems

    New & first time BMW airhead owner here

    Firstly, I'm excited to finally become an airhead owner. After a year of sorting through craigslist I finally found a nice example!

    When first purchased it had been sitting for some time, (older owner who lost his riding buddy, thus his want to ride) and it had some carb issues that I'm assuming were related to sitting for extended periods of time, I ran it with aviation fuel mixed with the recommended amount of seafoam and that cleared up the carb issues after two tanks.

    Now I've got a two different problems

    Problem 1:
    After a nice afternoon of riding I parked it a few weeks ago, days later I tried starting it up again and I could not get it started. It's getting air and fuel but no spark

    - I pulled the plugs out and cycled the engine, no spark visible on either side
    - the coils checked out OK

    Thats as far as I've gotten thus far

    Problem 2:
    When it was running, it had no problem at speed/through the power band. It did have an issue with idling where without any intervention it will stall. This problem is greatly exacerbated if I have the bike leaned to either side

    - I pulled the bowels of each carb as well as looking at the jet, both were relatively clean and clear of any buildup
    - No issues with airflow to note

    Any sage wisdom is greatly appreciated

    This bike came with brand new in the box "luggage cases" as well as a front fairing I've since removed. Is there an interest for those types of items here?

    Lastly- Is there anywhere that has a comprehensive list of mechanics that are SME's on airheads? I live in West Texas and have found almost no resources in my city, Abilene, that have a clue as to what an airhead even is.

    Thanks!!

    Mike

  2. #2
    Liaison 20774's Avatar
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    Mike -

    Welcome to the forum and to Airhead-dom!! I kinda of like the '78 R100/7!! Sorry I don't have any idea of mechanics in your neck of the woods. I'd suggest checking the Anonymous Book, either hardcopy or you can download it. Maybe you can find some people to mentor you a bit near you.

    As for #2, I'd be sure that your valves are set to the right clearances. General values are 0.15mm intake and 0.20mm exhaust. These are set on the side that is at OT on the compression stroke.

    For #1, I wonder if you have some issues with the coils. A few step might be to use a digital ohmeter and measure the ohms from one spark plug cap to the other cap...should be 22K ohms or so.
    Kurt -- Forum Liaison ---> 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!

  3. #3
    Registered User STEVENRANKIN's Avatar
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    Hello Mike

    Hello and welcome to the forum.

    Check your wiring for any loose looking wires, or damaged wires to the coils. if nothing visibly wrong give a quick check inside the headlight where the main harness goes. Ignition switch wires and such plug into the color coded circuit board there. A loose plug for the ignition system could be a problem.

    If your bike has points, check the gap and amount of wear as well as the wires to the points and from. When you set the points you will have to set the timing. This can be done static with the bike shut down or dynamic with the bike running. Snowbum's web page will most likely cover how all of this is done.

    Kurt mentions the coils,

    Tight valves can cause problems with idle. Airhead valves tend to tighten up over time and should be checked every 15K miles. a tight valve will cause rough running and poor idle. The exhaust valves are usually the ones out of adjustment, Intake valves tend to behave themselves.

    Not sure exactly where to look but try Brook Reams's website, he restored a 77 RS which is close enough to your bike to get an idea as to the components and some of the diy stuff. Boxer two valve may have a video on how to set points, Anton Largidare is a good guy to check out.

    I am in NY so I can't give you the name of a good airhead mechanic in your area. Check the Anonymous book, check the Vintage BMW web site and the Airhead sites, they may have a shop listed or advertised.

    Airheads are not hard at all to tune up or make most repairs on, if you have good mechanical and electrical sense. Very good wiring schematics are available from EME's website, if you need to check wiring or figure out how the wiring works, they have the best diagrams I have found.

    Get a copy of the Clymer manual covering your bike. it will have oil data such as amounts to fill the forks and such along with recommend weights of various oils. It will also have settings for things such as carbs and torque specs for various parts. The help sections are not too bad either.

    Good luck. St.

  4. #4
    Thanks for the responses- where can I find "EME's" website at for the wiring?

    Is it typical for both coils to go bad simultaneously?

  5. #5
    Liaison 20774's Avatar
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    EME - Euromotoelectrics at https://www.euromotoelectrics.com/
    Kurt -- Forum Liaison ---> 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!

  6. #6
    Registered User STEVENRANKIN's Avatar
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    Coils

    No, it is pretty rare for a original BMW coil to go bad at all, let alone both at the same time. Now if you had a 85 to 95 bike, there was an issue with the coils but they are a completely different design. St.

  7. #7
    For anyone who wants to know-

    It ended up being a failure in the points- the arm was stuck in the closed position, once fixed it still didn't spark. I then filed down the points, reset the gap and got it to run for about 30 minutes. It looks like the gasket failed around wire to the condenser.

    Ordered new points, condenser and all the gaskets. This has fixed the "leaning to the side idle problem" and it now starts right up.

    I'm still suffering from stuttering at low speeds, low in the power band - less than 2k rpm. Once above 2k everything runs great- giving it about 1/4 choke also helps it sub 2k.

    I'm guessing I've still got some of that wonderful ethanol gunk in the carbs? Any sage wisdom?

    Thanks,

    MK

  8. #8
    Liaison 20774's Avatar
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    MK -

    You probably need to look into the low speed circuit on the carbs, likely needing to go through a carb synch on a fully warmed up engine. You might need to confirm that the float level is set correct. Checking the float level is relatively easy. Turn off the petcocks and carefully remove one of the bowls. There should be about 1 inch of fuel in the bowl. Then with a container to catch the fuel, raise the float up so that it is parallel to the body of the carb. Turn on the petcock, lower then raise the float, and confirm that when the float gets parallel to the body of the carb, the flow of fuel starts.
    Kurt -- Forum Liaison ---> 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!

  9. #9
    Registered User STEVENRANKIN's Avatar
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    Gunk

    Kurt is right, you may have a booger in the low side part of the carbs.

    You have done the first parts of a good tune up by taking care of the valves, points and timing. Now comes the last and not so fun part the carbs. I'm not saying they are impossible to work on but there is a bit more to do with them. As Kurt says, check the float level, while the bowls are off, look for "stuff" in the bottom of the bowl. The filters in the gas tank will catch big stuff but there may be varnish from old gas in there and in the passages of the carbs as well.

    Do simple things first like check the float level and bowl inspection and see if there is any improvement of running. Before you do any adjustments on the carbs, be sure the cables for the chokes and throttles are properly adjusted to give you true idle with the choke off when it is supposed to be off. Same goes for the idle speed screws make sure they are set so the carbs are in the idle position. It is surprising how many fellows compensate for a poor idle by raising the idle speed which only brings the carbs out of idle setting. Idle should be below 1000RPM.

    So if the cables are adjusted properly and the idle is below 1000, and the bike won' run smooth at idle, you can do minor adjustments to the idle mixture. Large adjustment of this or no change when turning means most likely dirt or gunk has things bunged up.

    Pretty much if the simple things don't work, and you are sure the cables are adjusted properly, then you may have to do a carb rebuild and cleaning. I don't mean to throw rocks at you as you are the new owner but, you really don't know how the previous owner maintained things or how he made adjustments to things. A bike sitting for a long time with gas in the carbs is a disaster waiting to happen to the new owner. At the same time, tight cables, or mis adjusted cables by the PO, can also be a problem.

    Take a look on line at Brook Reams's web site, as well as Anton Largidare, and Boxer two valve, maybe a general search on the web for a video on carb rebuild and adjustment. Snowbum, also will have something I am sure. Clymer manuals help. Just be careful not to do what I did last time I did a rebuild on carbs. I dropped a part and it rolled into the black hole in my garage only to be found a week later. If I can rebuild carbs, you will be able to do so as well. Good luck, St.

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