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Thread: Condenser or....

  1. #1
    The Raven
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    Condenser or....

    I posted this over on Boxerworks but think I will get more responses here

    First:
    So I know that there has been an ongoing debate about the reliability of the R80g/s hall sensor and how folks like me put switch them out to points.

    So yesturday I was 80miles from home and it started to intermittently backfire. Halfway home the backfiring got worse. Figured I'd stop for coffee and let things cool down. Started back up and sounded like a tommy gun. Pulled into NAPA. Figured that perhaps a condensor was going bad. Figured that I would test that therory of the benefits of a BeanCan over a Hall, in such if something goes south you can stop by any auto parts store and get the parts as we all know they should be easy to find. Well, it failed and the closest replacement was in 2000mi away. Rode the rest of the way home on 1 cyl and put in my spare, readjusted my points, and greased the lobe. Runs good now......

    So, although a simpler system, cheap to fix...but the parts are NOT easily available at any NAPA as much as I used to think. Lesson is to always carry a spare.

    ------

    Then:


    Ok...so I have a brand new coil, wires and points from Motorrad Elektric, as well as new plugs from my on the road attempt at fixing the problem. But nothing changed until I did two things....regap my points as they were too close, and change the condensor

    Here is my troubleshooting steps as well as problem progression and result

    Pre-issue: Found the bike somewhat hard to start when cold, and had a intermittent low idle.

    1. Bike misses and backfires in sweet spot doing 35-40, remedies with more or less throttle
    2. Later bike misses at higher RPMs, remedies at less throttle.
    3. Bike then starts to miss under any load...but still goes when coasting and at idle
    4. Bike badly hesitates and backfires at acceleration, remedies at coasting
    5. Bike stalls at idle, starts right back up, runs fine slow
    6. Bike starts on one cylinder and backfires very badly and loudly, check plugs, white with sediment on them...ethanol stuff?
    7. Bike backfires at idle, replaced plugs, switched coil terminal on wires, wiggled wire, and isolated it from any contact point on engine.
    8. Bike loses all power on one cylinder with occasionally hits every 5-10 minutes, wiggling wires make no difference.
    9. Stop halfway home, pull bad plug and see mild spark, replace and pull wire, bike stays running. Do with opposite plug and bike stalls. Got shocked off of good plug, but not off of bad plug. Exhaust hot but not hot enough.
    10. Get home, check plug and find it mildly wet. Put in good wire, no change.
    11. Open beancan, check points. 1/2 closed to spec...opened them to spec. Changed condenser to new Bosch. Lubed shaft.
    12. Started bike and found spark on both sides to be identical and strong.

    I did find another case in my search that a similar problem was solved by a new condenser by someone with an R90. Rick at Motorrad said the BMW condensors are crap.

    -----
    Finally:

    One other thing I need to mention. The compression on the bad side is at 90. Oil on ring test caused no change in compression. The valves are loud so I need to get in there and adjust them. The good side is at 135 and quiet valves.

    I did just go for a ride tonight and for the first time for a long time the bike started without the need for any choke....though the low Compression side was slow to catch up and start firing..but did after a few seconds.

    Bike ran good and strong.....

  2. #2
    Registered User lmo1131's Avatar
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    Lots of variables, huh? Differential compression (5-10 psi is the generally "allowable" difference) coupled with valves out or adjustment. The "low" side might be so tight that it's actually leaking (burnt valve). Or, it may be time for a top-end refurb.

    When was the last time you went through a complete all-at-one-time tune-up (valve adjust, ignition adjustment, carb rebuild/sync, oil change). Little tweeks here and there can become problematic.
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  3. #3
    The Raven
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    Ok, this is weird.

    Just went out to try to address the low comp in the right head. Found the valves slightly out of wack...but not too bad. The torque on the head was loose so once that was done the valves were a bit questionable. I got everything into spec and tested the comp cold...gained about 6 lbs over the hot compression.

    Oddly though the cyl was missing again. Out of frustration I pulled the lead on the good cyl and after about ten revolutions the cyl started to fire and then started up. I reconnected the good plug and the bike sounded great.

    I let it warm up then tested compression, at startup it was at 96, then jumped to 110, then jumped to 125, then jumped to 135, 145, 150.........Everytime I blipped the throttle the compression jumped? It did not do this yesturday......

    I'm stumped...thoughts?

  4. #4
    Registered User lmo1131's Avatar
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    Ideally the CV carburetors need to be disconnected to accurately test compression. The engine is turned over by hand (not by the starter).

    Read this > http://w6rec.com/duane/bmw/compression/
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  5. #5
    The Raven
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    Quote Originally Posted by lmo1131 View Post
    Ideally the CV carburetors need to be disconnected to accurately test compression. The engine is turned over by hand (not by the starter).

    Read this > http://w6rec.com/duane/bmw/compression/
    I did just kick it first, when cold and got 96psi. Deducting from the linked site, and not having my throttle open to restrict airflow I really should be OK in the compression end of things.

    Perhaps I should be concerned with the 130psi instant cold compression on the good side?

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