My RS's dark side

Discussion in 'Airheads' started by ME 109, Dec 4, 2017.

  1. cycleman2

    cycleman2 Long timer

    Joined:
    Jan 18, 2010
    Oddometer:
    1,005
    Location:
    Alberta, Canada
    One thing I've found helpful when checking oil on various bikes that I've owned, including airheads, is to always check it the same either with the oil hot or oil cold, and use the same procedure. There is a noticeable difference in volume on a dipstick when measuring hot or cold oil.

    An example. The oilheads I've owned where very tricky to get accurate readings and the only way I could ever get it right was after getting back from a ride, put the bike on the side stand, go away for about 10 minutes, then come back and put the bike on the center stand. Or leave the bike on the sidestand overnight ( when on the road ) and put on the center stand in the morning/ or hold the bike level, and check the oil. For some reason the oil stayed in the upper parts of the engine and took time to drain back to the sump.

    On my current rig if I put in the recommended amount of oil ( with filter change ) it shows about 1/4 inch overfull on the dipstick. I've taken out 1/2 litre to get it below the full mark on the dipstick. I have some ideas why and one of them is the height of the front end with the leading link and a 400x18 tire. The height at rest is about an inch higher than a stock m/c tire, not counting the height of the front suspension at rest. So I'm going to get somebody to sit on the bike and then check the oil level with the suspension compressed on the front to see if there is any difference and I am going to go back to a m/c tire on the front end. I don't know if this will change anything or not, but it will certainly change the level of the oil in the oil pan ( front to back) at rest.
  2. ME 109

    ME 109 Long timer

    Joined:
    Jun 25, 2009
    Oddometer:
    6,602
    Location:
    Albury Australia
    Thanks cycleman2, I most always check cold/cool on centre stand with front wheel touching the ground to level the engine.

    I'm riding to see my brother this weekend for a compression test and leak down test with good equipment.
    I think the engine will fail both the leak down and compression test, after my results using 'bilge rat's' basic cylinder pressurising device that I made. The heads are good as far as valve seating goes.
    My brother also has a strong engine '81 RS for comparison.

    Motobins won't have rings to suit my bike for another week. Oh well, I can still ride the bike in a gentlemanly manner.
  3. bilge rat

    bilge rat Adventurer

    Joined:
    Jul 25, 2015
    Oddometer:
    19
    Location:
    sunshine coast
    ' I'm riding to see my brother this weekend for a compression test and leak down test with good equipment'.



    Just for the hell of it, when your doing the compression test once the initial dry test is done give each pot in turn 3 or 4 squirts of oil and re test.
    Results are food for thought/confirmation of suspicions.
  4. Geezerrv

    Geezerrv Been here awhile

    Joined:
    Mar 6, 2015
    Oddometer:
    313
    Location:
    MO Ozarks
    I’ve seen good compression and still burning oil. Oil control stuck but compression rings still sealing. This doesn’t sound like your issue though
    ME 109 likes this.
  5. ME 109

    ME 109 Long timer

    Joined:
    Jun 25, 2009
    Oddometer:
    6,602
    Location:
    Albury Australia
    Although not as good as oil I did use WD40 the other day when testing compression and did get an increased reading, although small.
    Tomorrow we’ll hit it with oil as well when testing.

    When I compression tested my old cylinders before replacing them I used oil and got no increase, interestingly.
    Those old cylinders gave 90 psi dry, and no noticeable difference wet, which goes against the norm for worn rings?
  6. Ron50

    Ron50 Been here awhile

    Joined:
    Oct 12, 2008
    Oddometer:
    481
    Location:
    Canberra
    The oil trick was developed for vertical pistons so it's not surprising that it's not as effective with horizontal cylinders: the oil will only seal the bottom side of the cylinder.
    Jim K in PA and ME 109 like this.
  7. robtg

    robtg Been here awhile

    Joined:
    Jun 29, 2007
    Oddometer:
    926
    Location:
    san jose
    The added volume of oil in the cylinder will increase the compression without
    actually sealing the rings. On a horizontal cylinder you will get a false reading.
    ME 109 likes this.
  8. ME 109

    ME 109 Long timer

    Joined:
    Jun 25, 2009
    Oddometer:
    6,602
    Location:
    Albury Australia
    Good point Rob, re the increased compression reading.
    In that case, would a spritz of WD40 that covers more of the cylinder walls give a better indication for a few revolutions with a compression gauge hooked up?
  9. Ron50

    Ron50 Been here awhile

    Joined:
    Oct 12, 2008
    Oddometer:
    481
    Location:
    Canberra
    If you are keen enough and don't have a wet cell battery you could lie the bike on each side for the tests - with appropriate padding.
    Uke and ME 109 like this.
  10. nk14zp

    nk14zp Been here awhile

    Joined:
    Feb 24, 2010
    Oddometer:
    856
    Location:
    Maine
    Does the engine oil turn black sooner after an oil change than normal? Wondering if that might indicate bad ring sealing.
  11. ME 109

    ME 109 Long timer

    Joined:
    Jun 25, 2009
    Oddometer:
    6,602
    Location:
    Albury Australia
    4500 k's on the oil and it's still reasonably clean.

    I have noticed that at idle there is reasonable air flow from the 'extended to the rear' breather hose. The airflow seems to disappear as the revs rise above approx 2000. I thought that was odd, but perhaps it's normal?

    A hot engine compression test using my brother's workshop tester gave 130 psi. A leak down test gave 5% leakdown.
    The leak down tester is a type that only has a single leak down gauge.

    We also did a compression test on my brothers '81 RS which came in at 125 psi, and a 5% leakdown.
    My brother's RS has better roll on in 5th gear. His rh piston crown is wet, as was mine before removing the cv plumbing from the airbox.

    I tried to find compression specs in my bmw manual but couldn't find any information.
    Could it be that my pistons just don't produce high compression?

    At this point I think I'll build a suitable catch can and keep the cv plumbing out of the airbox.
    The next question is whether to just wait for the carbon builup to burn off the piston, or whip the cylinders off and clean it off. Either way, it doesn't address the oil consumption.
  12. tsADV

    tsADV Been here awhile

    Joined:
    May 28, 2014
    Oddometer:
    285
    Location:
    Surrey, UK
    You surely have seen these airhead engine specs: http://www-cdr.stanford.edu/~petrie/bmw/R80ST/engines.html? According to which, the above readings are "normal".

    I've seen readings of around 170, 180 and 200 (modified as in Voltaires racer) PSI with 9.5:1 R100 pistons, in worn bores, and with worn rings. With R80 heads, admittedly, but still above what I expected.

    I'm wondering if the HC R100 pistons supplied to different markets were the same or not. Cfr the CR calculations @adrenal did and put up on his skrunkwerks website (http://skrunkwerks.com/skrunk/category/teknik/) where he got a significantly lower CR than the expected 9.5 - 8.2 or 8.4 using two different methods. If your pistons should happen to be LC versions, one might expect the pressures to be lower, as those indicated for e.g. the R80G/S, which got 8.2:1 pistons.

    I think the height of the piston crown above the shoulders should give an indication of which pistons one has, for the R100s I've been using it is around 9-10 mm.
    ME 109 and Jim K in PA like this.
  13. 100RT

    100RT Long timer

    Joined:
    Nov 4, 2009
    Oddometer:
    1,497
    Location:
    N. E. OHIO
    ME 109, My USA "detuned" 81 R100RT has 8.2:1 pistons. I would think you have a higher ratio.
    ME 109 likes this.
  14. RGregor

    RGregor Been here awhile

    Joined:
    Oct 31, 2007
    Oddometer:
    645
    Location:
    Bavaria
    @ME109: stupid question, did you remove the carbs for measuring compression?

    Anyway, maybe now it's time to determine real CR.
    It just takes about 60ccm of light oil and a small syringe ....

    And again I'd suggest to buy or borrow a cheap pressure gauge and measure crank case pressure.
    Something like this (no advertising intended):
    https://www.ebay.com.au/itm/TWO-mot...862219?hash=item5b3f59a00b:g:wPcAAOSwKPdZ3qAT
    Then you would know if blow-by is the cause for that.
    ME 109 likes this.
  15. RGregor

    RGregor Been here awhile

    Joined:
    Oct 31, 2007
    Oddometer:
    645
    Location:
    Bavaria
    Just another thought: are you sure there's a stock cam in your bike?
    Cam timing affects measured compression values.
    ME 109 likes this.
  16. cycleman2

    cycleman2 Long timer

    Joined:
    Jan 18, 2010
    Oddometer:
    1,005
    Location:
    Alberta, Canada
    " I have noticed that at idle there is reasonable air flow from the 'extended to the rear' breather hose. The airflow seems to disappear as the revs rise above approx 2000. I thought that was odd, but perhaps it's normal? "

    Something doesn't seem to add up. You would think as the pressure inside the case builds up with higher revs, then the crankcase valve would open. I would think the reverse would be true. No air at idle then as the revs went up more pressure being created in the case and air coming out of the tube. That to me is the way the system should work. Maybe replace the crankcase valve and see if there is any difference.

    Your hot engine compression numbers seem fine. It would drop a bit when cold, but still should be acceptable.

    If there is a suggestion that the oil rings my be gumed up, try add a solvent like Sea Foam and take the bike for a ride. Just follow the directions on the can. You will have to change the oil & filter after you use the solvent. I've used this method to free up sprag clutches in the old gold wing starters. Worth a try if you are grasping at straws.

    It will be interesting if there is a change after you replace the rings. Hopefully there is.
    ME 109 likes this.
  17. ME 109

    ME 109 Long timer

    Joined:
    Jun 25, 2009
    Oddometer:
    6,602
    Location:
    Albury Australia
    Thanks blokes, some good info to digest. I dragged my lazy ass out of bed too late this morning and need to get cracking and go to work. I'll get back on the case when I get home.
    I've always thought my compression ratio was 9.5:1
    Stock cam as far as I know.
  18. ME 109

    ME 109 Long timer

    Joined:
    Jun 25, 2009
    Oddometer:
    6,602
    Location:
    Albury Australia
    Like all good employees, I made it to work on time. And now I’m on my phone. :D

    I did just have an aha! moment.
    I no longer have the oe pistons in my engine.
    Looking at my piston collection, I can see a slight difference in piston crown contour.
    The pistons currently in my engine came from Germany off eBay so I’ll assume they are Euro spec.......
  19. tsADV

    tsADV Been here awhile

    Joined:
    May 28, 2014
    Oddometer:
    285
    Location:
    Surrey, UK
    Remember "Euro spec" wasn't the same for all model years. AFAIK all Euro twinshock RSs got "9.5:1" pistons (whatever they actually were), whereas the Euro spec detuned mono series supposedly got 8.45:1 CR pistons (wtaw).

    And other R100 models were equipped with 9.1:1 or 8.2:1 CR pistons (R100/7 vs "R100").

    Some info on the specs of different BMW models on Phil Hawkley's "BMbikes" forum & website: http://www.bmbikes.co.uk/bmwmodels.htm
  20. bmwrench

    bmwrench Long timer

    Joined:
    Nov 10, 2006
    Oddometer:
    5,023
    I'm wondering if a worn cam is causing your low compression. Have you inspected it?
    ME 109 likes this.