Right cylinder blue smoke on an R80/7. Oh dear...

Discussion in 'Airheads' started by Blackburn Buccaneer, Feb 27, 2019.

  1. Blackburn Buccaneer

    Blackburn Buccaneer Been here awhile

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    So I was following my dad around on his '78 R80/7 fresh out of winter storage and I noticed some blue smoke when the engine started warming up. Damn, the PO said the heads had been totally redone, including new valve seats and guides and new piston rings among other things.

    It's especially noticeable when accelerating or standing still and a stoplight and it's predominantly coming from the right cylinder. It stopped a few times over the course of the ride, but then reappeared later again. When we got home I ran it for a bit to mess with the carbs and there wasn't any noticeable blue smoke until I went for another ride to store it (it started smoking a bit at the end again).

    Now I've gone over a few options going on info from threads on AV rider, forums all over the web and a call to a (not so local) friendly expert and I've compiled a list of possible culprits:

    - It's due to sitting for 3,5 months. However, I don't think this is likely given the amount of smoke and how it continued after a 30+km ride.
    - The crankcase ventilation could have gone wrong, I'm definitely checking that one.
    - Maybe it's the carbs? They've been redone completely and they're clean, but they're calibrated in a bad way and I'm waiting for new connection rubbers between the carbs and the heads and airbox. Messing with the idle mixture mixture screw on both sides seemed to spawn more smoke (with some even appearing to come out of the left jug).
    - Badly eased in after the rebuild the PO had done causing the piston rings to not seat properly, or even a cracked ring.
    - The PO lied and the guides and rings have never been done.

    My dad and I are going to do a compression test this weekend, along with some carb balancing and a check up of the crankcase vent. The guy I rang up said I shouldn't ride it any further due to the possibility of a cracked ring scraping away at the cylinders. My initial intentions were to redo the rings and see how that goes, assuming the heads were done properly. I might also bring the heads to a shop to examine them, but that'll have to be further down the line.

    What are your takes on this? I am by no means an expert, so all advice is greatly appreciated! :)
    #1
  2. bmwrench

    bmwrench Long timer

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    It's almost always the ring sealing. Seating new rings is tricky business.
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  3. Manrider218

    Manrider218 Been here awhile

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    +1
    [​IMG]
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  4. Richarde1605

    Richarde1605 Long timer

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    How many miles on the rebuild?
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  5. caferacer100s

    caferacer100s n00b

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    Take the right air tube off and check and see if you have any signs of oil coming from the breather tube. Do a cylinder leak down test also.
    #5
  6. Blackburn Buccaneer

    Blackburn Buccaneer Been here awhile

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    Thanks! That would warrant an inspection by a good BMW shop I have nearby if the leak down test comes back negative.

    Not sure exactly, I'd say about 5000km. PO had it done in the summer of 2017 and had it as a second bike.
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  7. Blackburn Buccaneer

    Blackburn Buccaneer Been here awhile

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    Thanks, there was some oil smeared in the right tube. I'm gonna check this weekend on how smoky it runs when the right tube isn't connected. Compression test is also definitely on my to do list for the weekend.
    #7
  8. bpeckm

    bpeckm Grin! Supporter

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    Sounds like you got it:
    1. Compression test, if that's ok from side to side, then go to
    2. Crankcase breather dumps into right side. Investigate.

    :D
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  9. bmwrench

    bmwrench Long timer

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    Leakdown and compression tests are not much help when diagnosing oil consumption, as the oil helps the rings seal in a static test. Your better served by shining a light into the cylinder-preferably a borescope. However, you'd just be delaying the inevitable; pull the cylinders and check everything. Some people's idea of a valve and ring job doesn't mean much.

    I had a guy come into the shop recently who'd bought his bike from another fellow. He said, "Eric tells me you've done all the work on this bike." I said, "I worked on it once, and that was a service. Since then, he's put two sets of rear seals in the engine, installed brake lines, etc. himself."
    #9
  10. Steve W.

    Steve W. Boxer Pilot

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    I had a guy bring me a r100 years ago that had been "rebuilt" by the previous owner. It was smoking on acceleration, but ran OK. Found the "rebuilder" had forgot the wrist pin clips on the left side. The wrist pin had machined a nice groove in that cylinder wall. The other side was good. He soured some parts off flea bay for the repair. You never know what's in there, I would just pull it down, it is actually easier than all the guessing. JMHO Cheers
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  11. Plaka

    Plaka Brevis illi vita est

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    If it is smoking that is because it is burning oil. So there is oil in the combustion chamber. Thus your compression check will actually be a wet compression check, and not mean a lot.


    I would be interested if you pulled a valve cover and posted up a pic of what the head looks like under that cover.
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  12. Piggdekk

    Piggdekk love speed, hate rush

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    I like Plaka's idea. If it smokes and there are not signs of soot in the valve train most likely the issue are the rings. It really takes 5 minutes to do it. Based on Your description, smoke on acceleration, rings are the most likely culprit.
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  13. Blackburn Buccaneer

    Blackburn Buccaneer Been here awhile

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    Great feedback gents, many thanks!

    bmwrench, you're absolutely right and my dad is saying the same. We're probably pulling the head when he's got some more time on his hands in a month or two. Till then a bit of diagnosing and scouting is all I can do. Oh and ordering parts and memorizing the do's and the don'ts.


    Thanks! I will home from base tomorrow, I'll be sure to post some pictures of what I find then.
    #13
  14. tsADV

    tsADV Been here awhile

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    I've seen several descriptions of "new rings, but that didn't stop the engine burning oil" on the iron-lined cylinder engines. Most likely because they forgot to hone the cylinders.

    Will new but "failing to seal" piston rings bed in properly if one just hone the barrels and reinstall the new "failed" rings? Or will a new set of rings be required?
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  15. bmwrench

    bmwrench Long timer

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    It should be possible to reuse the low-mileage rings. Cylinders need to be honed with a real hone, not one of those spring-loaded "glaze-breakers".
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  16. Plaka

    Plaka Brevis illi vita est

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    not enough crosshatch on the cylinder wall means the surface will not hold oil. The ring then scrapes the iron wearing the ring and the bore. At minimum I would say new rings.
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  17. Plaka

    Plaka Brevis illi vita est

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    I'll throw one other thing out there: mixing two different types of oil in the crankcase can cause smoking. YMMV.
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  18. Blackburn Buccaneer

    Blackburn Buccaneer Been here awhile

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    Good call. I have an oil change planned for the weekend. It's running God knows what now, the PO was a nice old dude but he didn't know too much about machinery whispering.
    #18
  19. Blackburn Buccaneer

    Blackburn Buccaneer Been here awhile

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    I'm gonna go ahead and play it safe when pulling the jugs and replace the rings.
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  20. bmwrench

    bmwrench Long timer

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    The cross-hatch serves as a single-use file, machining the rings to fit the cylinder.
    #20
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