BMW R80/7 - Timing way, way out

Discussion in 'Airheads' started by madoc, Nov 24, 2020.

  1. madoc

    madoc Adventurer

    Joined:
    Jan 3, 2009
    Oddometer:
    24
    Location:
    Herefordshire
    I bought a 1978 BMWr80/7, ex-plod. It's not my first air-cooled BMW...
    It had been off the road since 2006, if the roadtax and MOT certs were to be believed. I think it passed through a few hands before it got to me .. so I cannot quiz the last person to run it.
    It's pretty tidy and very original.

    I cleaned out the carbs, sorted out the perished rubber bits, evicted the electrical gremlins, rebuilt those dreadful ATE brakes, changed the alternator brushes, checked the rocker clearances and changed all the fluids and have it running, charging and lighting up.

    BUT.

    it's terribly over-advanced, if my strobe is telling the truth, and the exhausts glow red. It has the F-mark (which is the advance firing mark) up at idle and then advances way beyond that !
    It's running points and it looked that the last time it was running it had a new ATU (seems new and the receipts and the date it was last on the road give it about 800/1000 miles with that) which doesn't have too much play.
    I cannot get the S mark (the idle mark) up statically at all, with my little bulb, and I've lengthed the points plate slots as far as possible. It just wants way more retarding ..

    A quick check down the plug holes reveals the OT (TDC) mark on the flywheel seems aligned when the pistons are fully out. If I remember, it's pretty tough to get the flywheel wrong by a little bit (i.e. the timing marks by a few degrees)

    The timing pieces have been apart a few times and I cannot see anything untoward, but I've got some new points and ATU springs in transit.

    It starts really easily and sounds very mechanically quiet (I previously had an R80/7 and R100/7 for about 10 years - so I'm kinda familiar with noises).
    I don't see any receipts in the big pile, which I got with the bike, which indicate an engine rebuild before it came off the road. It looks like some servicing last up .. but it's _possible_ it did have something serious done (timing chain ?) and someone has mis-timed the crank and cam . I've misaligned the cam/crank on a CB550, in the past, by ONE tooth and it would barely run thus I'm not enitirely convinved that ripping open the timing chest is quite needed yet as this does run.

    I've read snowbum, airheads and other sites, and have a lot more background.

    Anyone got any ideas ?
    is there a way to check the cam vs crank timing without pulling the front cover ?
    #1
  2. Arktasian

    Arktasian Induction saves lives

    Joined:
    May 23, 2014
    Oddometer:
    1,810
    Location:
    BC
    I'm sure others will chime in here, your flywheel may be mounted incorrectly on the bolt circle.
    A simple measurement can confirm. Pulling the front cover is not hard, remove battery negative terminal first to avoid shorts. Then, pull your plugs and insert a curved probe to rest against the piston crown while it is close to the top on one/ either side, you can turn the engine over and chart top dead center. If you make a felt pen mark on the rod and then watch that as you turn the engine it will climb then descend. (front rotor bolt usually okay for this if your careful) Take stock of the mark descending at the plug threads on each side of rotation and mark that on either the alternator rotor, or you can utilize the attached angle wheel (print and affix through front bolt area). Measure halfway between these marks and that is accurate TDC. You can look up the angles of your F & S marks to confirm they are okay or not. If the flywheel is off its correct position it is a large error.
    Angle Wheel .jpg
    #2
  3. UnclePete

    UnclePete Long timer Supporter

    Joined:
    Nov 13, 2014
    Oddometer:
    1,022
    Location:
    Plumas County , CA
    No personal experience , but I do remember reading about a run of points sets that were manufactured with a too big rubbing block .
    Seems it would be easy to check if yours was high enough to put your timing " out of the ball park " , and an un-invasive first step .
    #3
  4. bmwrench

    bmwrench Long timer Supporter

    Joined:
    Nov 10, 2006
    Oddometer:
    8,373
    Two things: First, many /7s advance units indexing is very sloppy. Loosen the 6mm nut at the front and turn the unit counter-clockwise as far as possible.

    Secondly, no currently available points I've used will allow correct timing at the specified gap, although .013-.014" will usually get you there.

    With the price and quality of points where they are, EI is making a lot of sense.
    #4
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  5. madoc

    madoc Adventurer

    Joined:
    Jan 3, 2009
    Oddometer:
    24
    Location:
    Herefordshire
    Well.
    New points fitted and it appears that, with some faff on gap/timing pate, I can get my timing to where it should be.

    The old points have some wear, compared to the new Norris ones, but I've taken off worse ones in the past.

    Non-plussed but happy.
    #5
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  6. Dreossi

    Dreossi Been here awhile

    Joined:
    Apr 21, 2013
    Oddometer:
    521
    Location:
    London, Ontario, Canada
    Are you familiar with the practice of enlarging the slot in the backing plate in order to increase its travel? This gives you greater adjustment for timing with current points.
    #6
  7. B_C_Ries

    B_C_Ries Long timer

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    Mar 15, 2013
    Oddometer:
    11,529
    Location:
    Imperial Valley, California
    I run my points with a smaller gap so I can get my timing right.
    #7
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  8. crazydrummerdude

    crazydrummerdude Wacky Bongo Boy

    Joined:
    Nov 28, 2006
    Oddometer:
    8,512
    Location:
    Los Angeles, CA
    Same. On one set, I had to halve the usual gap to get the timing in range.
    #8
  9. vagueout

    vagueout Long timer

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    Mar 3, 2010
    Oddometer:
    4,745
    Location:
    sydney, east
    I did exactly that on my 100/7 way back when. I always used a strobe light and found quickly the unit had run out of adjustment travel so created more travel and all was well. That was my last ever bike with points. If I ever have one again I'm going solid state.:*sip*
    #9
  10. Beemerguru

    Beemerguru Beemerguru...G/S guy

    Joined:
    Nov 7, 2006
    Oddometer:
    2,178
    Location:
    Redwood City, CA
    #10