Converting a /6 headlight assembly to /5 Nacelle + timing issues...

Discussion in 'Airheads' started by sprouty115, Sep 13, 2017 at 8:52 AM.

  1. sprouty115

    sprouty115 Long timer

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    I don't have enough time on the bike to know anything about engine vibes. I'm really still at the get-it-running-right stage. And this thread is just about understanding a specific change I would like to make to the headlamp.
    #21
  2. testa d'aria

    testa d'aria Adventurer

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    I bought a /5 bucket and cluster then went looking for a 90/6 with the transplant in mind. Had I known then what I know now ..... It's perfectly possible although you're taking the internals from an already crowded bucket (the /6) into the /5 bucket which has even less space. Time, patience and $ - it can be done. Of all the modifications though, this one is not as easily reversible if you're concerned about keeping it stock for it's resale value (which is the last thing on my mind). The purists may not like it but it is your bike to screw up any way you'd like :D DSC01461.JPG
    #22
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  3. bpeckm

    bpeckm Grin!

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    Oooh... wouldn't mind seeing more pics of that one ^ , Testa! Looking at your switchgear/perches, please elaborate on whatcha got and how you dunnit! Oh, nice tank, too!
    #23
  4. testa d'aria

    testa d'aria Adventurer

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    Thanks, bpeckm. The switch gear is pretty much stock. It's now bare aluminum where it was black. The left hand controls are a little newer ('78 I believe) so the signal switch is a little more intuitive for me. I moved the brake master cylinder to the handlebar. Bar-end signals (1 sided) in place of the stock square ones. Several modifications - but I've tried to use as many BMW parts as possible. DSC01635.JPG DSC01453.JPG
    #24
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  5. sprouty115

    sprouty115 Long timer

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    Really nicely done, testa.
    #25
  6. nobbylon

    nobbylon Been here awhile

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    That's a great 90/6, leave it as is because once converted it'll never be changed back and another one is lost :(
    #26
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  7. tlub

    tlub Long timer

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    Yep. The R90/6 was, IMHO, the peak of that era. My wife has one and I borrow it whenever I can. I have an R75/5, R69US, and R1100GS, and the R90/6 is my favorite. They got it right on that one. (Except maybe the front brake- sleeve it down.)
    #27
  8. sprouty115

    sprouty115 Long timer

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    Hmm, "lost" is a pretty strong word for a completely reversible mod. But don't worry too much, as it may never happen.

    I'm really still focussed on getting it running well and anything beyond that will be awhile later. Right now I'm wrapping my head around why it won't statically time properly...
    #28
  9. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    If the issue is there isn't enough rotation of the backing plate to get the S mark lined up in the window, try reducing the point gap to around 0.014". I've tried filing the slot wider to allow more adjustment, but I seem to remember there not being enough room for the points wire clamp afterwards...
    #29
  10. sprouty115

    sprouty115 Long timer

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    Thanks. I've reduced the gap all the way down to .004" (way out of spec) and rotated the points timing plate to the extremes of the slots - and from the closed to open transition I still have to rotate the flywheel about 20-degrees before I see the "S" in the timing hole. It's way off. I have new points (not the ones with the long contact block). All I can think of is that maybe the flywheel was removed and not oriented properly reinstalled? I've set points for years on bikes and cars and this is normally an easy process, but right now, I'm stumped.
    #30
  11. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    It is showing up too late or too early? Did you clean and lube the advance unit when you had it off? It's supposed to rotate on the cam and the old grease can make it stick and not return to idle. Also it can have a bit of slop on the D shaped portion of the cam where it's mounted. Try holding it either extreme when tightening the nut. The correct grease for the shaft is very thin, nothing like the grease used for the rubbing block. I assume you are using a static timing light and not a strobe? A strobe will show any tendency for it to stick. If it's running at all the flywheel isn't incorrectly mounted, that would put it something like 72 degrees off.
    #31
  12. sprouty115

    sprouty115 Long timer

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    Hope no one minds the thread-drift, but I appreciate the help...

    The bike was running but I wanted to verify timing before I tried to balance the carbs. All work is being done without the engine running, I'm checking the static timing with a DMM.

    As for what I'm finding:
    - the transition happens much too early. I set the gap, then rotated the engine with a hex key in the stator while I monitored the state change of the points with the DMM.
    - the points open and then I need to rotate the engine maybe 20-degrees more before the "S" shows in the timing hole.
    - no change in the gap or rotation of the points timing plates gets it anywhere close

    Here is pic of the points where the closed to open happens:
    BMW points 1.jpg

    Here is how much further I have to rotate the engine before I see the "S" in the timing hole:
    BMW points 3.jpg
    #32
  13. Brun

    Brun Been here awhile

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    Good advice from Big there ^^^. Be aware that it is fairly common for the quill - the delicate nose of the camshaft - to be slightly bent. This will cause one side to be quite differently timed vs the other. Test this by rotating the crank through another 360 degrees to check the other side. On my old dear the timing was around 30 degrees different. I understand that it is possible to straighten the quill using a dial gauge and judicious hammer work. I eventually resorted to fitting a Dyna III electronic ignition, which has individually adjustable pickups. Crank mounted electronic ignition systems are also available.
    #33
  14. disston

    disston ShadeTreeExpert

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    Number one is you appear to be missing the small nut holding the advance unit on the cam tip, called the quill. Is it just removed for the pic? There should be a nut and washer on that thread at the center of the cam.

    Careful with tightening this nut. The cam quill is sometimes called the most broken thread on an Airhead and the break easily. You only need a quarter turn past finger tight for it to stay.

    Next will be checking the flywheel orientation. But let me post this item first and I'll be right back.
    #34
  15. disston

    disston ShadeTreeExpert

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    OK, so yes a bent quill may also be what's happening.

    Let me give you the dialog on flywheel positioning. The flywheel is where the timing marks are. It is held onto the end of the crankshaft by 5 bolts. There is no indexing and it often happens that when it has been removed for some operation such as rear seal replacement it then gets installed in the wrong position. Being 1 bolt hole off will give the timing marks 72* off set, either + or -.

    Checking that the flywheel is in the correct position is easy. Remove both spark plugs and use a strong wooden stick, I use a chop stick. Insert stick into the spark plug hole so you can observe the position of the piston while you rotate the engine. Once the stick tells you that the piston is at TDC the TDC mark on the flywheel should be in the timing window. Careful to not trap your stick in the cylinder and break it off.

    It can not be off by a little bit. It is obvious when the TDC mark is in the window the piston is at TDC if it is correct. If it is off it is off by multiples of 72* (360*/5)
    #35
  16. Renner

    Renner combustophile

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    Some years ago friend of mine replaced the cam chain on his 90/6 but was off a tooth on the cam sprocket, so it never ran very well.
    He was busy with family life - couldn't find time to address the issue - so I borrowed the bike and set it right.
    Any chance that's what you're facing?
    #36
  17. sprouty115

    sprouty115 Long timer

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    Great advice, thank you all!
    - I have the quill nut and a new washer, it was just removed as I was going back an forth between setting the gap and then checking the state change with the DMM. I have been using 48inch-pounds as a max torque on the nut (http://bmwmotorcycletech.info/torquevalues.htm)
    - I have a dial test indicator so I can check the quill run-out
    - I will also pull the plugs and check the flywheel orientation via the TDC/chopstick method
    - Once I figure out what is going on, I'll find the appropriate grease and make sure the mechanical advance is lubed properly

    I will do all that tonight and report back.

    In the mean time, one more question - @Big Bamboo mentioned "If it's running at all the flywheel isn't incorrectly mounted..." How does the flywheel orientation effect the engine running, outside of allowing you to precisely set the timing by the "S" and "F" marks? Possibly I don't understand the relationship between the two, but it seems to me that if the engine was assembled correctly and the timing was set properly, if I were (in theory) to take the engine apart, remove the flywheel and then install it one bolt-hole off, then reassemble it, the engine would run just as it had before, except the timing marks on the flywheel would be off?
    #37
  18. r60man

    r60man Long timer

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    For all of the people saying not to modify your R90/6 because it is original, whatever. It is an R90/6 it will never be worth a fortune. Who buys basic bikes for investments anyway? Its not like it is an R90S, that would be different. You do whatever you want to your bike, it is your bike after all. It is not being "lost", it is being ridden and enjoyed and that is the point of riding, right?
    #38
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  19. sprouty115

    sprouty115 Long timer

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    I understand the sentiment of the people saying keep it stock, and to be truthful I very much relate to the idea that there is a "loss" when a classic bike is chopped to bits. But I'm not planning that. I have a non-functioning gauge cluster that will take real money to fix so I'm just trying to figure out what makes sense for me - without destroying anything in the process.

    I hope it's evident from my posts above that I'm being pretty methodical even in my approach to get it running. This bike won't be polished to death and become a trailer queen, it will be very close to stock and in much better condition than it is now and I think that's an honorable stewardship.
    #39
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  20. disston

    disston ShadeTreeExpert

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    You are correct. If the flywheel is off by any number of bolt holes the only thing that is disturbed are the timing marks. The engine will run just fine if another method of establishing correct timing is found. When it has happened riders have had to remove the flywheel to properly place or in some instances they have made new marks to time the engine.

    Earlier you said that you couldn't get the timing any closer to correct than something like 40*. In other words it was way off. This led me to think of the flywheel issue since I don't think you had measured the 40* but merely estimated a big error. So it's easy to check.

    If the issue is the timing gears up front it will be a lot more work to check that. As in what happened to Renner's friend. We learn from our mistakes, the wise man learns from the mistakes of his friends.

    I think BB said something about checking if the timing is concentric? Because the advance unit has 2 lobes so fires twice for each rotation this is also fairly easy to check with your dial test indicator. Take the advance unit off the quill and measure the run out of quill as you rotate the engine. I think this will tell you if the problem is a bent quill.
    #40